M10. Cement based levelling/ wearing screeds. Scope. Contents. Reference documents. General guidance 2

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1 Scope This section deals with: Cement based levelling screeds, cast on to new or existing structural floors and roofs to form smooth and regular substrates for applied finishes. Concrete wearing screeds (sometimes called toppings ) cast onto new or existing structural floors and finished to form smooth wearing surfaces. This section does not include: Wearing screeds that are cast monolithically with in situ concrete substrates see section E41. Finishing in situ concrete slabs directly, either as a wearing surface or as a surface to receive applied floor finishes see section E41. Mastic asphalt flooring/ Floor underlays see section M11. Resin flooring see section M12. Calcium sulfate based levelling screeds see section M13. In situ terrazzo see section M41. Tolerances on levels of structural floors see section A33. Contents General guidance 2 1 Terminology and sources of guidance for screeds 2 Are screeds necessary? 3 Types of screeds 4 Properties of levelling screeds 5 Properties of wearing screeds 3 6 Tolerances on levels and flatness/ surface regularity of screeds 4 7 Health, safety and the environment 8 Contractual arrangements Specification clauses 7 ^ denotes included in the Intermediate Version. Types of screed 110^ Bonded cement:sand levelling screeds 115^ Cement:sand levelling screeds 120^ Fine concrete levelling screeds 8 130^ Proprietary quick drying levelling screeds 9 140^ Proprietary polymer modified levelling screeds Proprietary lightweight aggregate levelling screeds 155 Cement:sand surfacing to lightweight aggregate levelling screeds Proprietary surfacing to lightweight aggregate levelling screeds 160 Proprietary self smoothing levelling screeds Proprietary fibre reinforced levelling screeds ^ Concrete wearing screeds (granolithic) 185^ Proprietary polymer modified wearing screeds Proprietary self smoothing wearing screeds Generally/ Preparation Design life of screeds 210^ Suitability of substrates 215^ Surface hardness of substrates to receive polymer modified wearing screeds 220^ Proprietary levelling/ wearing screeds Control samples 250^ Conduits under floating screeds 251^ Conduits cast into or under screeds 255 Pipe ducts/ Trunking 260^ Fully bonded construction 270^ Partially bonded construction ^ Cement slurry bonding 280^ Unbonded construction 290^ Floating construction ^ Floating construction (thin sheet impact sound insulation) Batching/ Mixing 302^ Cements 305^ Aggregates 306^ Proprietary polymer modified screeds 307^ Admixtures 306^ Proprietary polymer modified screeds 310^ Batching with dense aggregates 311 Batching with lightweight aggregates ^ Mixing 335^ In situ crushing resistance (ISCR) 340^ Adverse weather Laying 345^ Level of screed surfaces 350^ Screeding to falls 351 Screeding to ramps 355^ Flatness/ surface regularity of floor screeds Flatness/ surface regularity of roof screeds 375^ Compaction of screeds 382^ Stair screeds 386 Coved in situ skirtings 392^ General reinforcement 395^ Strip reinforcement 405^ Joints in levelling screeds generally Bay joints in levelling screeds 415^ Bay joints in bonded wearing screeds Heated screeds 435^ Formed joints in wearing screeds 440 Crack inducing grooves in levelling screeds 445^ Crack inducing grooves in wearing screeds 450^ Sealant for sawn joints in wearing screeds ^ Sealing strips for sawn joints in wearing screeds 460^ Strip movement joints 468 Sealant movement joints with metal edgings Metal section movement joints Finishing/ Curing 510^ Finishing generally 520 Wood floated finish 530^ Smooth floated finish 540^ Trowelled finish to levelling screeds ^ Trowelled finish to wearing screeds 560 Dewatered trowelled finish to wearing screeds 600 Power ground finish to wearing screeds 650^ Curing Roof screeds 680 Surface sealer to wearing screeds 690^ Slip resistance testing of wearing screeds 700^ Abrasion testing of wearing screeds Reference documents For a list of documents cited in NBS, refer to the Consolidated List of reference documents in the first binder of your service. For a list of documents cited in this section, refer to All reference documents for a work section on the subscriber website. National Building Specification Ltd June 2009 Standard Version Page 1

2 To check the currency of documents cited in this section, refer to the list of New and amended reference documents on the subscriber website. Publishers of documents cited in this section include: British Cement Association (BCA) British Standards Institution (BSI) Building Research Establishment (BRE) Concrete Society (CS) Construction Industry Research and Information Association (CIRIA) Health and Safety Executive (HSE) General guidance 1 Terminology and sources of guidance for screeds BS 8204 is the code of practice that includes cement based levelling screeds (formerly referred to as just screeds ) and wearing screeds (formerly referred to as toppings ) in the following parts: BS : levelling screeds to receive applied floor finishes. BS : wearing screeds finished as the wearing surface (sometimes referred to as high strength concrete or granolithic screeds). BS : polymer modified wearing screeds. BS : pumpable self-smoothing screeds. See also the following BS EN standards on screeds: BS EN 13318: Definition of screed material and floor screeds. BS EN 13813: Screed material properties and requirements (also covers EU labelling). BS EN 13892: Parts 1 to 8 on methods of test for screed properties and conformity to EU labelling. Other useful sources of guidance include: BS (Workmanship on building sites series). BRE Report 460 Building elements: Floors and flooring. BCA Guide Floor levelling screeds. CIRIA Report 184 Screeds, flooring and finishes. Selection, construction and maintenance. 2 Are screeds necessary? Separately cast screeds are vulnerable to failure through shrinkage cracking, curling and hollowness, and it is wise to consider whether they are necessary. Other methods of providing a fine finish, good wear resistance, falls, service runs, thermal and sound insulation will often be possible. BS and -2 advise that hollowness, curling and cracking can be avoided only by eliminating the use of a screed, and that with unbonded and floating construction there is a high risk of the screed curling. For wearing screeds, BS advises that if unbonded construction is required then the screed should be designed as a concrete overslab at least 100 mm thick. For in situ concrete floor slabs, alternatives to separately cast screeds are: Direct finishing of the concrete slab to receive either applied floor coverings or to form the wearing surface (see section E41). Monolithically cast wearing screeds (see section E41). For guidance on screed failures see BRE Report 460 and CIRIA Report 184. For some thin flooring materials (e.g. plastics sheeting or tiles) applied to either screeds or directly finished concrete slabs, smoothing compounds may be necessary to make good minor imperfections (caused by poor finishing or inadequate protection) or to improve the texture of the finished surface. Where separately cast screeds are necessary they must be designed and specified with care, particularly: Water content of the mix, to control drying shrinkage. Preparation of the substrate where a bonded or partially bonded construction is required. Thickness, related to the type of construction. Thorough compaction. Curing and subsequent drying out. Good workmanship and close supervision are very important to the success of screeds. Refer for guidance to BS Types of screeds For types of screeds, including thicknesses and construction (i.e. fully bonded, partially bonded, unbonded or floating on insulation) see: Table 1 for levelling screeds (includes pumpable screeds). Table 2 for wearing screeds. Bonded construction must be used for screeds on stairs and landings. Mixes for floated/ trowelled cement based levelling screeds are normally of a stiff consistency with a low water:cement ratio (sometimes referred to as semidry ). See also guidance to clause 330. Exceptions are proprietary self smoothing ( flowing ) levelling screeds that are pumped into place. Levelling screeds and wearing screeds must be specified and laid with care if they are not to suffer from poor adhesion (bonded screeds only), cracking, curling and inadequate impact or abrasion resistance. The use of proprietary screeds or admixtures to enhance performance (e.g. to reduce drying shrinkage, speed up drying, increase strength) will not compensate for inadequate workmanship. Proprietary components are supplied as: Admixtures (typically water reducers or polymers). Special cements (to impart very fast drying). Ready-mixed material (including admixtures). Dry, pre-blended and bagged constituents. Some proprietary screeds are Agrément certified. Some are offered on a supply and lay basis through approved licensees. Self levelling screeds that are pumpable can be time and labour efficient. Refer to BS , clause 7.6 and Table 1, BS , and manufacturers data on maximum screed depth. 4 Properties of levelling screeds 4.1 Drying The concrete slab must be suitably dry and any moisture migration must be considered when choosing the type of screed. Consider whether a dpm or moisture barrier is required on new concrete substrates. Refer to BS , section Drying periods for levelling screeds can be critical for project programmes. The rate of drying depends on various factors including: Ambient environmental conditions. Moisture condition and thickness of in situ concrete substrates if screed is not isolated by a dpm. Type and thickness of levelling screed. Drying from one or two sides, e.g. ground bearing slab or cast in situ intermediate floor slabs. Levelling screeds are usually sufficiently dry to receive moisture sensitive floor coverings or adhesives when they reach a relative humidity of 75% or less (see section M50, general guidance). Examples of approximate drying times for different types of levelling screeds laid on a dry substrate or isolated from damp substrates by a dpm are: Screed type (50 mm thick) Cement:sand (unmodified by any admixtures) Proprietary quick drying (clause 130) Proprietary polymer modified (clause 140) Proprietary self smoothing (clause 160) Approximate drying times* (to reach 75% RH or less) 8 weeks *[based on ideal drying conditions] 2 to 14 days (or more depending on additive formulation) 10 to 28 days (or more depending on polymer dosage) 3 to 6 days National Building Specification Ltd June 2009 Standard Version Page 2

3 4.2 In situ crushing resistance (Soundness) BS recommends that the contractor should determine the mix proportions to suit the in situ crushing resistance category. Cement:sand and fine concrete screeds are traditionally specified with cement:aggregate proportions in the range 1:3 5, but in the absence of full time supervision leaner mixes are often incorrectly used. This, together with inadequate mixing and poor compaction, has led to many screed failures due to crushing and crumbling under furniture or traffic loads. BS , Table 3 relates floor usage category and service conditions. The BRE screed tester can be used to assess the soundness of levelling screeds in depth. In situ crushing resistance testing is included in the relevant types of screeds clauses. The test method involves dropping small weights onto the finished screed and measuring the resulting indentations. In situ crushing resistance classifications and related acceptance levels for maximum depth of indentation (after four drops of the weight) based on BS , Tables 3 and 4 are: Category Service conditions Indentation depth A Heavy use (including heavy trolleys) and/ 3 mm or where disruption, dust or pollution at a later date is unacceptable (e.g. hospital corridors, operating theatres). B Normal use (e.g. public areas, corridors, 4 mm classrooms, restaurants). C Light use (e.g. domestic). 5 mm (note 1) Note for floating screeds: 1 Floating screed test modified by reducing both maximum allowable depth of indentation to 2.5 mm and mass of dropped weight from 4 kg to 2 kg. The modification to the in situ crushing resistance test when applied to floating screeds from BS , Table 4 and Annex E is intended to give a truer indication of the strength and integrity of the screed. If the test is not modified, there is a tendency for the screed tester to knock out a truncated cone of screed material onto the insulation layer, resulting in exaggerated indentation depths. See also clause 335 and BS , clause 8.7. For the in situ crushing resistance test see BS , Annex D (bonded and unbonded) and Annex E (floating). For further guidance on the use of the BRE screed tester see BRE Report Properties of wearing screeds 5.1 Wear resistance Concrete wearing screeds (clause 180) BS , tables 3 and 4 reflect BS EN for abrasion resistance (AR) labelling and classifications for concrete wearing screeds (granolithic) bonded to concrete substrates as follows: Class AR1 AR2 * AR3 * Service conditions Very high abrasion and impact from steel or hard plastics wheeled traffic. Heavy duty industrial workshops, intensively used warehouses, etc. (see clause 180). High abrasion from steel or hard plastics wheeled traffic. Medium duty industrial and commercial. Moderate abrasion from rubber tyred traffic. Light duty industrial and commercial. For further guidance on abrasion resistance and testing see BS EN , Annex B and BRE Report 460. Specially durable aggregates are used for an AR1 class of abrasion resistance. In situ concrete substrates to receive bonded concrete wearing screeds should be at least grade C 32/40 with a minimum cement content of 325 kg/m³ Proprietary polymer modified wearing screeds (clause 185) BS , table 1 uses duty classifications for polymer modified wearing screeds bonded to concrete substrates as follows: Class Very Heavy Heavy Medium Light * Service conditions *Limited use as a wearing screed. Very heavy duty industrial use with severe abrasion and impact from steel or hard plastics wheeled traffic. Heavy duty industrial use including intensive use by hard-wheeled traffic. Medium duty industrial and commercial including use by solid rubber or pneumatic tyred traffic. Light duty industrial and commercial including use by soft-wheeled trolleys. The wearing performance of polymer modified wearing screeds is indicated by the abrasion resistance. For recommended thicknesses and mixes corresponding to these duty classes see BS , table 2. In situ concrete substrates to receive high strength bonded polymer modified wearing screeds should have at least a rebound hammer value not less than 25 (see BS , clause 7.1). Strength testing of the substrate may be necessary, for example, on existing concrete slabs (see clause 215) Proprietary self smoothing wearing screeds (clause 192) Self smoothing screeds require no floating or trowelling and are generally pumpable, see BS Screed manufacturers normally use abrasion resistance (AR) classifications from AR1, AR2 or AR3 (see BS , Table 3) to indicate wearing performance see 5.1 for interpretation. 5.2 Slip resistance In common with other hard smooth materials, trowelled concrete wearing surfaces can become slippery when water, dust, dirt, oil, grease or wax is present. The effect of traffic over time may polish the surface and increase slipperiness or, conversely, create a surface texture which reduces slipperiness. Because the degree of slip resistance provided by the floor surface is heavily influenced by conditions of use (wetting, spillages, wear, cleaning regimes, type of footwear, etc.) the specification of slip resistance by performance is not normally practicable. In wet conditions the slip resistance of wearing screeds depends on the type and depth of surface texture. Site testing for slip resistance may sometimes be necessary as a guide to performance see clause 690. Testing involves the measurement of slip resistance value (SRV) in both wet and dry states the higher the value the less slippery the floor surface. For horizontal surfaces, SRVs of at least 40 are normally considered satisfactory from purely a safety point of view. For further guidance on slip resistance and testing, see BS , clause 6.15, BS and -2, and BRE Report 460. *See BS , clause 9.2 for guidance on wearing screed mix proportions. National Building Specification Ltd June 2009 Standard Version Page 3

4 6 Tolerances on levels and flatness/ surface regularity of screeds 6.1 Tolerances on levels The screed is, in effect, a tolerance zone between the structural substrate level and the finished floor level. However, there are significant technical and cost implications and it is appropriate to specify with care: Tolerances on levels of screeds surface (see clause 345). Tolerances on levels of structural floors (see Preliminaries. It is suggested that: Finished floor levels and structural floor levels should be shown on the drawings. Minimum and, where applicable, maximum thicknesses of screeds should be specified, in preference to the designed nominal thickness (see types of screeds clauses). In the case of proprietary screeds, thicknesses will also be specified indirectly via the requirement to comply with the manufacturer s recommendations (see clause 220). If thicknesses are required to be given in bills of quantities or schedules of works, they are described as nominal or subject to tolerances. All areas requiring falls and direction of falls are shown on drawings (see clause 350). 6.2 Tolerances on flatness/ surface regularity As with tolerances on levels (see 6.1) there can be significant technical and cost implications, it being appropriate to specify with care: Flatness/ surface regularity of screeds (see clause 355). Flatness/ surface regularity of substrates to receive thin or relatively thin screeds (see section E05, section A33). 6.3 Levelling screeds to receive thin floor coverings Where thin floor coverings are specified, the smoothness and flatness/ surface regularity of levelling screeds will be of great importance. Smoothing compounds can usually be used to overcome such problems, but they are expensive see clause 540 and section M50. There is advantage in avoiding Nomination or Naming of thin flooring materials and screeds so that the two can be priced simultaneously by the main contractor. The contractor can thus make clear decisions as to the quality of screeded finish, protection thereof and whether or not a smoothing compound will be necessary, and price accordingly. 7 Health, safety and the environment 7.2 The environment Issues include: The cement industry is a major source of carbon dioxide release. Portland cement production is very energy intensive and gaseous and particulate pollution is released during manufacture. The limestone and chalk processed into Portland cements and the rock and gravel for aggregates are non-renewable resources. Recycling of concrete from substrates is likely to be more straightforward when screeds are unbonded. Recycled concrete aggregates (RCAs) are potential aggregates for fine concrete levelling screeds and wearing screeds see section E10. By-products from industrial processes are used in some lightweight levelling screeds and roof screeds. Aggregates include sintered pulverized fuel ash and foamed blast-furnace slag. Life cycle costs should consider the production and transportation of materials, and design life of the screed. 8 Contractual arrangements 8.1 Subcontracting The specifier may choose a subcontractor or influence the choice of subcontractor in several different ways. See Preliminaries section A30. However, because of problems of divided responsibility it is usually advisable to avoid Nomination or Naming in the case of screeds. Divided responsibility is likely to arise in respect of: Tolerances on levels of finished floor and structural floor see 6.1. Tolerances on flatness/ regularity of finished floor and structural floor see 6.2. Suitability of screeds to receive floor finishes, particularly thin finishes see 6.3. Underfloor heating systems. Some proprietary screeds are subject to licensing or approval of subcontractors by the manufacturers, so that a degree of quality control will exist anyway. 8.2 Requirements for submission of information The specifier may require the contractor or subcontractor to submit drawings or other technical information either at tender stage or during the course of the contract. See Preliminaries section A Health and safety Concerns include: Control of dust and debris (including cement and silicas) generated by preparation of concrete substrates for fully bonded levelling or wearing screeds, or by ground finishes to wearing screeds. Normal precautions (including vacuum recovery equipment and safety clothing) must be taken to prevent inhalation, ingestion and contact with eyes. See BS , section 9. Slip resistance of floors in use see 5.2. Measures to reduce slipperiness include adequate matting at entrances and proper cleaning methods and regimes. Health and safety risks should be listed in the pre-tender health and safety plan (see Preliminaries section A34) and method statements obtained from the contractor in the outline construction phase health and safety plan (see Preliminaries section A30). For legislation and other publications relating to general health and safety requirements, see Preliminaries section A34. For safe handling and storage of materials associated with screeds (admixtures, bonding agents, etc.) see manufacturers data sheets. For cements see also HSE Construction sheet 26. National Building Specification Ltd June 2009 Standard Version Page 4

5 TABLE 1: TYPES OF LEVELLING SCREEDS (see also 3) Type and construction Screed thickness (See also 6) Comments (See also 4.1 for drying periods) Clause 110 Bonded cement: sand levelling screeds Fully bonded (clause 260) Clause 115 Cement:sand levelling screeds. Partially bonded (clause 270). Unbonded (clause 280). Floating (clause 290, 295). Clause 120 Fine concrete levelling screeds. Partially bonded (clause 270) Unbonded (clause 280). Floating (clauses 290, 295). Clause 130 Proprietary quick drying levelling screeds. Fully bonded (clause 260). Partially bonded (clause 270). Unbonded (clause 280). Floating (clauses 290, 295). 25 mm minimum and 40 mm maximum at any point. Suggested nominal thickness 35 mm to allow for deviation in the level of concrete substrate. Partially bonded and Unbonded: 50 mm minimum at any point. Suggested nominal thickness 70 mm to allow for deviation in the level of substrate slab. Floating: 75 mm minimum at any point (excluding thickness of insulation). Suggested nominal thickness 95 mm to allow for deviation in the level of concrete substrate. For lightly loaded domestic applications these dimensions can be reduce by 10 mm. Partially bonded and Unbonded: 50 mm minimum at any point. Suggested nominal thickness 70 mm to allow for deviation in the level of substrate slab. Floating: 75 mm minimum at any point (excluding thickness of insulation). Suggested nominal thickness 95 mm to allow for deviation in the level of concrete substrate. For lightly loaded domestic applications these dimensions can be reduced by 10 mm. Refer to manufacturer s recommendations, but generally as for clauses 110 and 115. Water reducing admixture included to help reduce drying shrinkage. The upper limit on thickness is to reduce the tendency of screeds to curl and debond during drying. Water reducing admixture included to help reduce drying shrinkage. BS does not refer to partial bonding because of the increased risk of failure compared to fully bonded construction (clause 260). Fine concrete (clause 120) is a nonproprietary alternative. See guidance to clause 428 where using heated screeds. A low water:cement ratio fine concrete laid using screeding techniques. Single size coarse aggregate helps reduce drying shrinkage. BS prefers their use for floating construction and suggests they be considered for screeds 50 mm thick or more. However, they are not always suitable to receive thin flooring materials (see guidance to clause 120). See guidance to clause 428 where using heated screeds. Drying times vary according to proprietary mix constituents water reducing admixtures (with or without other drying enhancers) or special cements. Reduced drying shrinkage and increased strength. Clause 140 Proprietary polymer modified levelling screeds. Fully bonded (clause 260). Clause 150 Proprietary lightweight aggregate levelling screeds. Partially bonded (clause 270). Unbonded (clause 280). Floating (clause 290, 295). Use clause 150 in conjunction with clauses for surfacing layers clause 155 or clause 157. Refer to manufacturer s recommendations. Typical thicknesses are: 6 mm minimum. 40 mm maximum for a single layer application. Refer to manufacturer s recommendations and BS , Annex A5. Typical thicknesses for lightweight aggregate base layer: Partially bonded: 25 mm minimum. Unbonded: 50 mm minimum. Floating: Refer to manufacturer. Thicknesses of surfacing layer (clause 155): 15 mm minimum and 20 mm maximum. Typical thicknesses of proprietary surfacing layers (clause 157): See guidance for quick drying and polymer modified screeds above. Total thickness of screed is base layer plus surfacing layer. See guidance to clause 428 where using heated screeds. Polymers (e.g. styrene butadiene rubber SBR) impart a strong adhesive property giving strength in tension and flexure and allowing thin applications. Reduced drying times and increased strength (depending on polymer dosage). For floors and roofs. Lightweight aggregate screeds provide increased thermal insulation, reduced weight and reduced drying shrinkage. Aggregates include sintered pulverized fuel ash, expanded clay and foamed slag. Full bonding not usually feasible with no fines lightweight aggregate. Surfacing layers (clauses 155 or 157) are used to provide closed and even surfaces to lightweight aggregate base layers. National Building Specification Ltd June 2009 Standard Version Page 5

6 TABLE 1: TYPES OF LEVELLING SCREEDS continued Clause 160 Proprietary self smoothing levelling screeds. Fully bonded (clause 260). For unbonded, partially bonded or floating construction check with screed manufacturer. Refer to manufacturer s recommendations. Typical thicknesses for bonded screeds: 6 mm minimum. 25 mm maximum. See BS , Table 1. Sometimes referred to as flowing or self levelling. Pumped into place and without the need for finishing by floating or trowelling. See BS Normally bonded to substrate to reduce thickness and allow economic use of material. Reduced drying shrinkage and increased strength. Clause 165 Proprietary fibre reinforced levelling screeds. Fully bonded (clause 260). Partially bonded (clause 270). Unbonded (clause 280). Floating (clauses 290, 295) Refer to manufacturer s recommendations, but generally as for clauses 110 and 115. Not suitable laid to falls. Polypropylene fibres control shrinkage cracking and can be an alternative to steel fabric crack control reinforcement. However, steel fabric reinforcement may still be needed in strategic locations. Increased strength also claimed. Fibre reinforcement material can also be specified for some other proprietary screeds mixes (see clause 130). Can be self smoothing pumped screed. See guidance to clause 428 where using heated screeds. Table 2: types of wearing screeds (see also 3) Types and construction Screed thickness (See also 6) Comments (See also 5) Clause 180 Concrete wearing screeds (granolithic). Fully bonded (clause 260). 20 mm minimum and 40 mm maximum at any point. Suggested nominal thickness 30 mm to allow for deviation in level of concrete substrate. See BS , clause for stairs. Alternatives to separately cast wearing screeds (and associated risks of curling and hollowness) are directly finished concrete slabs including thick overslabs (specify in section E10) or monolithic wearing screeds (specify in section E41). Clause 185 Proprietary polymer modified wearing screeds. Fully bonded (clause 260). Clause 192 Proprietary self smoothing wearing screeds. Fully bonded (clause 260). Refer to manufacturer s recommendations. Typical thicknesses are: 6 mm minimum. 40 mm maximum (single layer application). This depends on duty class, see BS , Table 1. Refer to manufacturer s recommendations. Typical thicknesses are: 6 mm minimum. 25 mm maximum. This depends on duty class, see BS , Table 1. On some precast concrete substrates the risk of cracking at joints is significant. Polymers (e.g. styrene butadiene rubber SBR) impart a strong adhesive property, giving strength in tension and flexure and allowing thin applications. Reduced drying times, increased strength and enhanced water, chemical and wear resistance (depending on polymer dosage). Sometimes referred to as flowing or self levelling. Pumped into place and without the need for finishing by floating or trowelling. See BS Normally bonded to substrate to reduce thickness and allow economic use of material. Reduced drying shrinkage and increased strength. National Building Specification Ltd June 2009 Standard Version Page 6

7 Guidance notes Specification clauses CEMENT BASED LEVELLING/ WEARING SCREEDS To be read with Preliminaries/ General conditions. TYPES OF SCREED 110 See general guidance 3 and table 1. Cement:sand screeds, which are fully bonded to the substrate and kept within the recommended thickness limits, are less prone to shrinkage and the attendant problems of curling, hollowness and cracking. Bonded screeds are generally not suitable for floor slab cambers beyond the minimum and maximum thickness limits. Clause heading: Insert, e.g. TO CLASSROOMS Substrate: See BS , clause 6.3. Insert, e.g. In situ concrete slab. Thickness: See general guidance 3 and table 1, see also BS , clause Mix: See guidance to clauses Proportions (cement:sand): See general guidance 4.2 and BS , Annex B. The contractor will adjust sand within the range to meet the specified performance for in situ crushing resistance (soundness). Insert, e.g. 1:3 4. To BS In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C ( 5 mm maximum indentation). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor. Insert, e.g. SR1. SR2. SR3. SR2 generally, SR1 to IT suites. Finish: See guidance to clauses Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. 2.5 mm sheet flooring. Other requirements: Use for accessories and fixtures that will affect the laying of the screed. Insert, e.g. Movement joints. Pipe ducts, as section P See general guidance 3 and table 1. Clause heading: Insert, e.g. TO UPPER FLOORS GENERALLY Substrate: See BS , clause 6.3. Insert, e.g. Precast concrete planks. In situ concrete slab. Screed construction: See guidance to clauses Select from: Partially bonded, as clause 270. Unbonded, as clause 280. Unbonded on dpm. Floating, as clause 290. Floating, as clause 295. Reinforcement for crack control: See guidance to clauses 392 and 395. Insert, e.g. Not required. Steel fabric, as clause 392. Thickness Nominal/ Minimum: See general guidance 3 and table 1. Mix: Proportions: See BS , clause The contractor will adjust sand within the range to meet the specified performance for in situ crushing resistance. Insert, e.g. 1:3 4.5 To BS BONDED CEMENT:SAND LEVELLING SCREEDS Substrate: Screed construction: Fully bonded. Thickness: Nominal: Minimum: 25 mm. Maximum: 40mm. Mix: Proportions (cement:sand): In situ crushing resistance (ISCR) category: Flatness/ Surface regularity class: Finish:..... To receive: Other requirements: CEMENT:SAND LEVELLING SCREEDS Substrate: Screed construction: Reinforcement for crack control: Thickness Nominal: Minimum: Mix: Proportions (cement:sand): In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 7

8 See also guidance to clause 540. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2.5 mm maximum indentation). C (5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor. Insert, e.g. SR1. SR2. SR3. SR2 generally, SR1 to dance studios. Finish: See guidance to clauses Insert, e.g. Floated, as clause 520. To receive: Insert floor covering, e.g. Varies, ceramic tiles and timber sprung floors. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. Pipe ducts, as section P31. Heated screed, as clause See general guidance 3 and table 1. For fine concrete screeds there is a high risk that the coarse aggregate will prevent a fine closed trowelled finish from being achieved and thus make the screed unsuitable to receive some thin floor coverings (e.g. plastics tile or sheet) without the use of remedial smoothing compounds. See guidance to Mix proportions following and also guidance to clause 540 and BS , clause and Annex B. Clause heading: Insert, e.g. TO UPPER FLOORS GENERALLY Substrate: See BS , clause 6.3. Insert, e.g. Precast concrete planks. In situ concrete slab. Screed construction: See guidance to clauses Select from: Partially bonded, as clause 270. Unbonded, as clause 280. Unbonded on dpm. Floating, as clause 290. Floating, as clause 295. Reinforcement for crack control: See guidance to clauses 392 and 395. Insert, e.g. Not required. Steel fabric, as clause 392. Thickness Nominal/ Minimum: See general guidance 3 and table 1. Mix: Proportions: BS , clause and Annex B gives examples of sand:coarse aggregate proportions which translate to: 75:25 for a fine closed texture suitable for thin flooring materials (e.g. plastics sheeting and tiles). 50:50 where a coarse texture is acceptable for thick flooring materials (e.g. ceramic tiles and bedding). An upper limit of 40% for the proportion of coarse aggregate is normally more practicable. Insert, e.g. 1:3 4.5 To BS See also guidance to clause 540. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2.5 mm maximum indentation). C (5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction 120 FINE CONCRETE LEVELLING SCREEDS Substrate: Screed construction: Reinforcement for crack control: Thickness: Nominal: Minimum: Mix: Proportions (cement: total aggregate): Sand to coarse aggregate: In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 8

9 required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor. Insert, e.g. SR1. SR2. SR3. SR2 generally, SR1 to dance studios. Finish: See guidance to clauses Insert, e.g. Floated, as clause 520. To receive: Insert floor covering, e.g. Varies, ceramic tiles and timber sprung floors. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. Pipe ducts, as section P31. Heated screed, as clause See general guidance 3 and table 1. Proprietary quick drying levelling screeds use different constituents to achieve a range of improved drying times. Clause heading: Insert, e.g. TO CORRIDORS Substrate: Insert, e.g. In situ concrete slab. Screed manufacturer: Insert, e.g. XYZ Screedy Ltd. Product reference: Insert, e.g. J-Screed (Normal duty). Screed construction: See guidance to clauses Insert, e.g. Fully bonded, as clause 260. Unbonded, as clause 280. Unbonded on dpm. Floating, as clause 290. Reinforcement for crack control: Check screed manufacturer s recommendations and see guidance to clauses 392 and 395. Insert, e.g. Not required. Steel fabric, as clause 392. Some screed manufacturers offer polypropylene fibres as an alternative to steel. However, steel strip reinforcement (clause 395) may still be needed across, e.g. joints. Insert, e.g. Steel fabric, as clause 392, and with polypropylene fibres recommended by screed manufacturer. Thickness Nominal/ Minimum: Insert manufacturer s standard. See also general guidance 6. Mix: See guidance to clauses Cement: For screeds that incorporate special cements, insert, Special. For other screeds insert, e.g. As clause 302. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2. 5 mm maximum indentation). C ( 5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor. Insert, e.g. SR1. SR2. SR3. SR2 generally, SR3 to plant rooms/ utility rooms. Finish: See guidance to clauses Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. Cork tiles. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. Pipe ducts, as section P PROPRIETARY QUICK DRYING LEVELLING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Reinforcement for crack control: Thickness: Nominal: Minimum: Mix: Cement: Proportions: To manufacturer's recommendations. In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 9

10 140 See general guidance 3 and table 1. Clause heading: Insert, e.g. TO UPPER FLOORS Substrate: Insert, e.g. In situ concrete slab. Screed manufacturer: Insert, e.g. XYZ Screedy Ltd. Product reference: Insert, e.g. PolyThin SB. Thickness: Follow screed manufacturer s recommendations. BS (clause 6.2) advises: For each class of duty the design thickness selected should relate to the service conditions and be compatible with achieving the required levels. The design thickness should exceed the minimum thickness given in Table 1 for that class, even when covering any high spots in the base. Nominal: Insert, e.g. 20 mm. Minimum: Insert minimum recommended by screed manufacturer, e.g. 6 mm. See also general guidance 6. Maximum: Insert maximum thicknesses recommended by screed manufacturer, e.g. 40 mm. See also general guidance 6. Mix: Normally, insert To BS , Table 2. The mix proportions in this table are described as typical, but should be checked with the screed manufacturer, especially for heavily loaded screeds. Proportions (cement:sand): Insert, e.g. 1: In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Indentation depths of only 1 mm may result, depending on the proportion of polymer used in the mix. For screeds which are 15 mm or less, surface hardness may be more appropriate as a test. See also BS , clause 10.7 and table 6 Rebound hammer values. Depending on usage, insert, e.g. A (1 mm maximum indentation). A (3 mm maximum indentation). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor, e.g. SR1. SR2. SR3. SR2 generally, SR1 to operating theatres. Finish: Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. Linoleum. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. 150 See general guidance 3 and table 1. Lightweight aggregate levelling screeds are usually laid with a cement:sand surfacing layer (clause 155 or 157) to provide a closed even surface to receive floor or roof coverings. Surfacing layers can be omitted for some thick floor coverings (e.g. tiles laid on a mortar bed). However, check suitability with screed manufacturer problems can arise during the application of coverings to no fines lightweight aggregate mixes (typically, loosening of aggregate particles). Clause heading: Insert, e.g. TO ROOFS TO CENTRAL SERVICE CORE FLOORS Substrate: Insert, e.g. Precast concrete planks. Lightweight aggregate roof screeds will usually be used in a warm deck design either under the vapour barrier (if required) or over board insulation. In the latter case insert, e.g. Lightweight aggregate screed, as clause /150 in the Overlay item of the appropriate warm deck roofing clause of section J21, J41 or J42. Careful consideration should be given to the use of a screed as an overlay to roofing insulation. Laying by a specialist contractor may involve a complex sequence of trades. Most serious of all, the wet laid screed would be between two vapour proof layers, with all the potential problems arising from entrapped moisture. Screed manufacturer: Insert, e.g. XYZ Aggs Ltd. Product reference: Insert, e.g. Blend 2. Screed construction: See guidance to clauses Full bonding as clause 260 is not normally achievable with no fines mixes. Insert, e.g. Unbonded. Partially bonded, as clause 270. Floating, as clause PROPRIETARY POLYMER MODIFIED LEVELLING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Fully bonded. Thickness: Nominal: Minimum: Maximum: Mix: In situ crushing resistance (ISCR) category: Flatness/ Surface regularity class: Finish: To receive: Other requirements: PROPRIETARY LIGHTWEIGHT AGGREGATE LEVELLING SCREEDS Substrate: Screed manufacturer: Product reference: Standard: To BS , Annex A. Screed construction: Reinforcement for crack control: Thickness: Nominal: Minimum: Mix: Proportions: Surfacing layer: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 10

11 Reinforcement (for crack control): Steel fabric is normally recommended for floating construction. See guidance to clause 392. Insert, e.g. Not required. Steel fabric, as clause 392. Thickness: Refer to screed manufacturer s recommendations. Nominal: Insert, e.g. 70 mm. Minimum: Insert manufacturer s standard. See also general guidance 6.2. Mix: See guidance to clause Proportions: Check manufacturer s recommendations and where mix is standard insert Preblended. Proportions vary depending on type of aggregate and conditions of use. Where required, insert size and grading of aggregates. For a no fines mix insert, e.g. 1:8 cement:aggregate. Surfacing layer: Insert, either As clause 155 or As clause 157 or Not required. Other requirements: Use for accessories or fixtures that will affect screed laying. Insert, e.g. Movement joints. Pipe ducts, as section P Alternative to clause 157. See general guidance 3 and table 1. Use in conjunction with clause 150 (or repeated renumbered clause based on 150). Use clause 157 for proprietary surfacing screed layers. Clause heading: Insert, e.g. /150 Thickness: Nominal: As screed manufacturer s recommendation for usage. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2. 5 mm maximum indentation). C ( 5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: For floors, see guidance to clause 355. Insert clause reference and permissible deviations to suit type of floor covering/ use of floor, e.g. SR1. SR2. SR3. For roofs, insert Measure as clause 365. Finish: Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. Linoleum. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. Pipe ducts, as section P Alternative to clause 155. See general guidance 3 and table 1. Use in conjunction with clause 150 (or repeated renumbered clause based on 150). Use clause 155 for generic cement/ sand surfacing. Clause heading: Insert, e.g. /150 Surfacing layer manufacturer: Insert, e.g. XYZ Screedy Ltd. Product reference: Check suitability with manufacturer and insert, e.g. PolyThin. Thickness: Nominal: As screed manufacturer s recommendation for usage. Thickness (minimum): Insert thickness recommended by screed manufacturer. Surfacing layer mix: Insert proportions recommended by screed manufacturer to meet required in situ crushing resistance, e.g. 1:4 (cement:sand). In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). 155 CEMENT SAND SURFACING TO LIGHTWEIGHT AGGREGATE LEVELLING SCREEDS / Surfacing layer: Bonded to lightweight aggregate base layer: Method: Either lay and compact within 24 hours of laying the base layer or, apply a suitable bonding agent. Thickness: Nominal: Minimum: 15 mm. Maximum: 25 mm. Surfacing layer mix: Proportions (cement:sand): 1: In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: PROPRIETARY SURFACING TO LIGHTWEIGHT AGGREGATE LEVELLING SCREEDS / Surfacing layer manufacturer: Product reference: Surfacing layer: Bonded to lightweight aggregate base layer: Method: Either lay and compact within 24 hours of laying the base layer or, apply a suitable bonding agent. Thickness: Nominal: Minimum: Surfacing layer mix: Proportions: In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 11

12 C (2. 5 mm maximum indentation). C ( 5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: For floors, see guidance to clause 355. Insert clause reference and permissible deviations to suit type of floor covering/ use of floor, e.g. SR1. SR2. SR3. For roofs, insert Measure as clause 365. Finish: Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. Linoleum. Other requirements: Use for accessories and fixtures that will affect the laying of the screed, e.g. movement joints, preformed access ducts. Insert, e.g. Movement joints. Pipe ducts, as section P See general guidance 3 and table 1. Also known as flowing or self levelling screeds. Refer to BS Pumpable self levelling screeds. Code of practice for guidance. This type of screed is not generally suitable for laying services in. Clause heading: Insert, e.g. TO MAIN HALL Substrate: Insert, e.g. Existing screed. Screed manufacturer: Insert, e.g. XYZ Cement Flo Ltd. Product reference: Insert, e.g. FlowThin PDQ. Screed construction: See guidance to clauses Bonded construction is normally required for unbonded or floating construction check with manufacturer. Insert, e.g. Fully bonded, as clause 260. Bonded and reinforced with glass fibre mesh. Manufacturer s standard. Thickness: Nominal: Refer to screed manufacturer s advice for nominal thickness. For further guidance see BS , table 1 for minimum thickness of bonded screeds. Minimum: Insert recommendation by screed manufacturer e.g. 6 mm. See also general guidance 6. Maximum: Insert recommendation by screed manufacturer e.g. 25 mm. See also general guidance 6. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2.5 mm maximum indentation). Mass of test weight: Insert either 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: See guidance to clause 355 and BS , Table 4. Insert permissible deviations to suit type of floor covering/ use of floor, e.g. SR1. SR2. SR3. SR2 generally, SR1 to IT suite. Finish to receive: Insert, e.g. Carpet. Other requirements: Use for accessories or fixtures that will affect screed laying. Insert, e.g. Movement joints. 160 PROPRIETARY SELF SMOOTHING LEVELLING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Thickness: Nominal: Minimum: Maximum: Standards: Comply with BS Pumpable self levelling screeds. Code of practice. In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish to receive: Other requirements: National Building Specification Ltd June 2009 Standard Version Page 12

13 165 See general guidance 3 and table 1. Clause heading: Insert, e.g. TO FIRST FLOORS Substrate: Insert, e.g. In situ concrete slab. Screed manufacturer: Insert, e.g. XYZ Lazycrete Ltd. Product reference: Insert, e.g. FibreScreed. Screed construction: See guidance to clauses Insert, e.g. Fully bonded, as clause 260. Floating, as clause 290. Unbonded on dpm. Thickness: Nominal: Refer to screed manufacturer s advice. See also general guidance 6. Minimum: Insert recommendation by screed manufacturer, e.g. 25 mm. Mix: See guidance to clause Proportions (cement:sand): Insert proportions recommended by manufacturer to achieve required in situ crushing resistance, e.g. 1:4. In situ crushing resistance (ISCR) category: See general guidance 4.2 and guidance to clause 335. Depending on usage, insert, e.g. A (3 mm maximum indentation). B (4 mm maximum indentation). C (2. 5 mm maximum indentation). C (5 mm maximum indentation). Mass of test weight: Insert either, 2 kg (for floating construction required to satisfy a class C crushing resistance) or 4 kg (for all other situations). Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit type of floor covering/ use of floor, e.g. SR1. SR2. SR3. SR2 generally, SR1 to operating theatres. Finish: Insert, e.g. Trowelled, as clause 540. To receive: Insert floor covering, e.g. Linoleum sheet. Other requirements: Use for accessories and fixtures that will affect the laying of the screed. Insert, e.g. Pipe ducts, as section P31. Movement joints. 180 See general guidance and table 2. Clause heading: Insert, e.g. TO WORKSHOPS Substrate: The concrete should be minimum grade C32/ 40. Thickness: Nominal: See general guidance 6 and table 2 for types of wearing screeds. Insert, e.g. 35 mm. Mix: This is the traditional granolithic mix. It is at least equivalent to an AR1 class of abrasion resistance to BS Proportions (cement:sand:coarse aggregate): See BS , clause Insert, e.g. 1:1:2. To BS Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit use of floor. Insert, e.g. SR1. SR2. SR3. Abrasion resistance: See general guidance Classification: Insert, e.g. AR0.5/WS Very Heavy Duty. AR1/ WS Heavy Duty. AR2/ WS Medium Duty. AR3/ WS Moderate Duty. Finish: See guidance to clauses Insert, e.g. Trowelled, as clause 550. Dewatered trowelled, as clause 560. Ground, as clause 600. If surface sealers are required insert, e.g. Sealer as clause 680. Slip resistance value (minimum SRV): See clause 690 for 165 PROPRIETARY FIBRE REINFORCED LEVELLING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Thickness: Nominal: Minimum: Mix: Proportions (cement:sand): In situ crushing resistance (ISCR) category: Mass of test weight: Flatness/ Surface regularity class: Finish: To receive: Other requirements: CONCRETE WEARING SCREEDS (GRANOLITHIC) Substrate: In situ concrete slab. Screed construction: Fully bonded. Thickness: Nominal: Minimum: Maximum: Mix: Proportions (cement:sand:coarse aggregate): Flatness/ Surface regularity class: Abrasion resistance: Standard: To BS , Table 4. Classification: Finish: Slip resistance value (minimum SRV): Other requirements: National Building Specification Ltd June 2009 Standard Version Page 13

14 testing standards (see also general guidance 5.2). Where slip resistance is likely to be a safety issue, insert value required, e.g. SRV 40. Otherwise insert Not required. Other requirements: Use for accessories and fixtures that will affect laying of the wearing screed. Insert, e.g. Movement joints. 185 See general guidance and table 2. Clause heading: Insert, e.g. TO STORAGE AREAS Substrate: Concrete substrates must be strong enough to withstand shrinkage stresses as the wearing screed dries. Insert, e.g. In situ concrete slab. Screed manufacturer: Insert, e.g. XYZ Tiptopcrete Ltd. Product reference: Insert, e.g. PolyHD. Thickness: BS (clause 6.2) advises: For each class of duty the design thickness selected should relate to the service conditions and be compatible with achieving the required levels. The design thickness should exceed the minimum thickness given in Table 1 for that class, even when covering any high spots in the base. Nominal: Refer to manufacturer s advice for nominal thickness. See general guidance 6 and table 2 for types of wearing screeds. Insert, e.g. 15 mm. 30 mm. 40 mm. Minimum/ Maximum: Insert thicknesses recommended by screed manufacturer. For different thicknesses of medium and heavy duty wearing screeds refer to BS Mix: Normally, insert To BS , Table 2. These mix proportions are described as typical, but should be checked with the screed manufacturer, especially for heavily loaded screeds. Flatness/ Surface regularity class: See guidance to clause 355 and insert permissible deviations to suit use of floor, e.g. SR1. SR2. SR3. Abrasion resistance: See general guidance Classification: Insert, e.g. AR0.5/WS Very Heavy Duty. AR1/ WS Heavy Duty. AR2/ WS Medium Duty. AR3/ WS Moderate Duty. Finish: See guidance to clauses Insert, e.g. Trowelled, as clause 550. If surface sealers are required insert, e.g. Sealer, as clause 680. Slip resistance value (minimum SRV): See clause 690 for testing standards (see also general guidance 5.2). Where slip resistance is likely to be a safety issue, insert value required e.g. SRV 40. Otherwise insert Not required. Other requirements: Use for accessories and fixtures that will affect the laying of the screed. Insert, e.g. Coved skirtings. Movement joints. Abrasion test. 192 See general guidance and table 2. Clause heading: Insert, e.g. TO STUDIOS Substrate: Insert, e.g. In situ concrete slabs. Screed manufacturer: Insert, e.g. XYZ CementFlo Ltd. Product reference: Insert, e.g. Toppit PDQ. Thickness: Nominal: Refer to manufacturer s advice. See also general guidance 6. Minimum/ Maximum: Insert thicknesses recommended by screed manufacturer. Flatness/ Surface regularity class: See guidance to clause 355 and BS Table 4. Insert permissible deviations to suit use of floor, e.g. SR1. SR2. SR3. Abrasion resistance: See general guidance PROPRIETARY POLYMER MODIFIED WEARING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Fully bonded. Thickness: Nominal: Minimum: Maximum: Mix: To BS , Table 2. Flatness/ Surface regularity class: Abrasion resistance: Standard: To BS , Table 4. Classification: Finish: Slip resistance value (minimum SRV): Other requirements: PROPRIETARY SELF SMOOTHING WEARING SCREEDS Substrate: Screed manufacturer: Product reference: Screed construction: Fully bonded. Thickness: Nominal: Minimum: Maximum: Flatness/ Surface regularity class: Abrasion resistance: Standard: To BS , Table 4. Classification: Finish: Slip resistance value (minimum SRV): Other requirements: National Building Specification Ltd June 2009 Standard Version Page 14

15 Classification: Insert, e.g. AR0.5/WS Very Heavy Duty. AR1/ WS Heavy Duty. AR2/ WS Medium Duty. AR3/ WS Moderate Duty. Finish: Insert, e.g. Self finished. Slip resistance value (SRV): See clause 690 for testing standards (see also general guidance 5.2). Where slip resistance is likely to be a safety issue, insert value required, e.g. SRV 40. Otherwise insert Not required. Other requirements: Use for accessories or fixtures that will affect screed laying. Insert, e.g. Movement joints. GENERALLY/ PREPARATION 205 Design life or durability of constructions are now required on many projects. Overall life expectancy will normally exceed the period of stated warranty. Confirm the client s requirement for screeds, and whether the duration is achievable. On proprietary products, check with the manufacturer. Premature failure of screeds could be as a result of many factors, refer to BRE Report 460. Duration: Insert, e.g. 30 years. 50 years. 70 years. Location: Define areas this duration applies to. Where necessary repeat the clause for other areas requiring a different duration, or refer to a schedule. Insert, e.g. General areas. Classrooms. 210 Tolerances on level and flatness/ surface regularity of substrates must be coordinated with those required for finished screed surfaces. See general guidance 6. For levels of finished surfaces see clause 345. For flatness/ surface regularity of finished surfaces see clauses 355 (floor screeds) and 365 (roof screeds). For roofs, the exact method, sequence and timing will depend on the particular roof construction details, e.g. type of weatherproof membrane, venting of screed if any, finish to soffit, etc. Moisture content: The time required to dry out new slabs (particularly solid ground floor slabs and composite slabs) can not be underestimated and can seriously affect the construction programme. The additional expense of a dpm bonding agent or separating layer may need to be considered. Refer to general guidance 4.1, and guidance to clauses 270 and See general guidance and BS Use this clause where it is necessary to check that the substrate has adequate strength to withstand the tensile shrinkage stresses imposed by screeds. Test for surface hardness: The BS EN test allows rapid evaluation of large areas of flooring. The rebound hammer values for different thicknesses of wearing screed are those recommended in BS , section 7. Lower readings are usually acceptable if the surface tensile strength of the substrate, is greater than 0.8 N/mm². The significance of the test for finished surface hardness is limited for toppings of 15 mm thickness or less because the rebound hammer values will be influenced by the strength and density of the substrate. Where testing of toppings is necessary, BS suggests the in situ crushing resistance test (see general guidance 4.2) as an alternative to the rebound hammer, with an acceptance value for maximum depth of indentation of 1 mm. However, this will damage the wearing surface. 205 DESIGN LIFE OF SCREEDS Duration: Subject to reasonable wear and tear. Location: Condition of use: Subject to correct loading and traffic usage throughout duration. 210 SUITABILITY OF SUBSTRATES General: Suitable for specified levels and flatness/ regularity of finished surfaces. Consider permissible minimum and maximum thicknesses of screeds. Sound and free from significant cracks and gaps. Concrete strength: To BS , Table 2. Cleanliness: Remove plaster, debris and dirt. Moisture content: To suit screed type. New concrete slabs to receive fully or partially bonded construction must be dried out by exposure to the air for minimum six weeks. 215 SURFACE HARDNESS OF SUBSTRATES TO RECEIVE POLYMER MODIFIED WEARING SCREEDS General: Substrates must restrain stresses that occur during setting and hardening of wearing screeds. Test for surface hardness: To BS EN using a rebound hammer with compliance values selected from the following: Screed thickness Rebound hammer value 15 mm or less Greater than 25 Greater than 15 mm Greater than 30 Report: Submit details of areas where substrate surface hardness does not comply with these values. National Building Specification Ltd June 2009 Standard Version Page 15

16 220 Many proprietary binders, admixtures and aggregates are marketed as proprietary screed systems even though some or even most of the materials, e.g. cement and sand, are obtained from other sources. Where proprietary screeds are specified ensure that there is no conflict with the clauses in this section, and that any special requirements are covered by additional items or clauses. 230 Control samples are normally more applicable to wearing screeds than levelling screeds. The specification of control samples should be related to the size of the project. The area of the screed is best related to the size of a room or bay. See also Preliminaries section A33. Screed type: Insert, e.g. Wearing screed, as clause 185. Location/ Size: Insert, e.g. Workshop 03, including movement joints. 250 Pipes or conduits are best avoided in levelling screeds and should never be buried in wearing screeds because of the risk of cracking. With buried pipes there is the difficulty of detecting and rectifying leaks and preformed access ducts should be used see BRE Defect Action Sheet 121. Specify preformed access ducts in section P31. The use of conduits is only practicable where the insulation is a flexible type, i.e. not boards. See BS , clause For lightweight aggregate levelling screeds see BS , Annex A. 251 Pipes or conduits are best avoided in levelling screeds and should never be buried in wearing screeds because of the risk of cracking. With buried pipes there is the difficulty of detecting and rectifying leaks and preformed access ducts should be used see BRE Defect Action Sheet 121. Specify preformed access ducts in section P31. The inclusion of conduit will not be practicable for thin screeds. Reinforcement will help to minimize width of cracks along the line of the pipe or conduit. Placing reinforcement: Screed cover: See BS , clause and BRE Report 460, Chapter mm cover should be regarded as a minimum and used with care. For lightweight aggregate levelling screeds see BS , Annex A Pipes or conduits are best avoided in levelling screeds and should never be buried in wearing screeds because of the risk of cracking. With buried pipes there is the difficulty of detecting and rectifying leaks and preformed access ducts should be used see BRE Defect Action Sheet 121. Specify preformed access ducts in section P31. See BS , clause For lightweight aggregate levelling screeds see to BS , Annex A. 260 Full bonding of screeds to precast units is often difficult to achieve and partially bonded or unbonded construction may be preferable. For proprietary screeds check requirements with screed manufacturer. Methods for exposing coarse aggregate include planing and scabbling (in situ slabs only) and more commonly, abrasive blasting. Good supervision is important. Not all contaminants can be removed satisfactorily however. For example, it will not normally be possible to achieve a full bond where the concrete substrate contains a water repellent admixture. Some penetrating surface hardeners and sealers are also tenacious contaminants. For assessing the adequacy of bonding of completed, matured screeds see BS , section 8 (levelling screeds), BS , section 12 (wearing screeds) and BS , section 10 (polymer modified screeds). They emphasize that even with correct preparation of substrates, complete adhesion cannot be guaranteed. Also, that hollowness (indicating no adhesion) does not necessarily 220 PROPRIETARY LEVELLING/ WEARING SCREEDS General: Materials, mix proportions, mixing methods, minimum/ maximum thicknesses and workmanship must be in accordance with recommendations of screed manufacturer. Standard: To BS CONTROL SAMPLES General: Complete areas of finished work and obtain approval of appearance before proceeding. Screed type: Location/ Size: CONDUITS UNDER FLOATING SCREEDS Haunching: Before laying insulation for floating screeds, haunch up in 1:4 cement:sand on both sides of conduits. 251 CONDUITS CAST INTO OR UNDER SCREEDS Reinforcement: Overlay with reinforcement selected from: 500 mm wide strip of steel fabric to BS 4483, reference D49, or Welded mesh manufactured in rolls from mild steel wire minimum 1.5 mm diameter to BS 1052, mesh size 50 x 50 mm. Placing reinforcement: Mid depth between top of conduit and the screed surface. Screed cover over conduit (minimum): PIPE DUCTS/ TRUNKING Preformed access ducts: Before laying screed, fix securely to substrate and level accurately in relation to finished floor surface. 260 FULLY BONDED CONSTRUCTION Preparation: Generally in accordance with BS Removing mortar matrix: Shortly before laying screed, expose coarse aggregate over entire area of hardened substrate. Texture of surface: Suitable to accept screed and achieve a full bond over complete area. Bonding coat: National Building Specification Ltd June 2009 Standard Version Page 16

17 mean that the screed is unsatisfactory unless accompanied by lifting (e.g. at edges/ corners of bays or along cracks). Failure of bonding can be a result of inadequate preparation of the substrate, refer to BS , table 1 for guidance. Bonding coat: Neat cement slurries or polymer modified cement slurries (clause 275) are commonly used. Proprietary bonding agents are an alternative (either polymer dispersions or epoxy resins) including those that have a dual bonding/ dpm function. Slurry bonding coats (clause 275) are not suitable for some proprietary screeds (e.g. self smoothing screeds). Insert, e.g. Slurry, as clause 275. XYZ Ltd: ResGoo. Manufacturer s standard. Where more than one type of bonding coat is required insert, e.g. /110: Slurry, as clause 275. /160: As recommended by the screed manufacturer. 270 Fully bonded construction as specified in clause 260 involves removal of the mortar matrix from the surface of the hardened concrete. Partially bonded construction may be substituted, but it will significantly increase the risk of failure it should not be used for concrete wearing screeds. Partial bonding should be contemplated only where the areas are relatively small, the loading is light, thickness is a minimum of 50 mm and where some cracking and curling can be accepted. However, partial bonding is suitable for use with some proprietary levelling screeds, subject to suitable minimum thickness and use of the screed manufacturer s recommended bonding agent. Applications include: Lightweight aggregate roof screeds (where these are not separated by a vapour barrier). Quick drying screeds laid on beam and block floors. Bonding coat: Neat cement slurries or polymer modified cement slurries (clause 275) are commonly used. Proprietary bonding agents are an alternative, including those that have a dual bonding/ dpm function. Slurry bonding coats (clause 275) are not suitable for some proprietary screeds (e.g. self smoothing screeds). Insert, e.g. Slurry, as clause 275. XYZ Ltd: ResGoo. Manufacturer s standard. Where more than one type of bonding coat is required insert, e.g. /110: Slurry, as clause 275. /130: As recommended by the screed manufacturer. 275 Use in conjunction with clauses 260 and 270 and see guidance thereto. Slurry type: Polymer modified cement slurries develop strength faster and so should provide better resistance to shrinkage stresses from the screed. Styrene-butadiene rubber (SBR) polymer dispersions are commonly used. Insert, e.g. SBR polymer modified cement. Neat cement. Screeding: If the slurry or bonding agent is allowed to dry out or set it will form a weak layer between the screed and substrate, preventing effective bonding. 280 There is a greater risk of curling and associated hollowness and cracking with unbonded screeds. Unbonded construction is used where a reliable bond cannot be achieved, e.g. where the substrate is contaminated, contains a water repellent admixture, is overlaid by a dpm or where, for any other reason, the substrate is unsuitable for bonding. See also guidance to clause 260. A separating layer is recommended for unbonded construction but will not be necessary if the screed is to be laid over a sheet dpm. Sheet damp proof membranes should be specified in section J40. Type: Insert, e.g. Polyethylene sheet, minimum 125 micrometres thick (500 gauge). Building paper to BS 1521, Grade B1F. Polyethylene dpm, as section J PARTIALLY BONDED CONSTRUCTION Preparation: Generally in accordance with BS Substrate surface: Brushed finish with no surface laitance. Texture of surface: Suitable to accept screed and achieve a bond over complete area. Bonding coat: CEMENT SLURRY BONDING Slurry type: Application: Shortly before laying screed, thoroughly wash clean the surface and keep well wetted for several hours. Remove free water then brush in cement slurry bonding coat of creamy consistency. Screeding: While slurry is still wet. 280 UNBONDED CONSTRUCTION Separation: Lay screed over a suitable sheet dpm or a separating layer. Type: Installation of separating layer: Lay on clean substrate. Turn up for full depth of screed at abutments with walls, columns, etc. Lap 100 mm at joints. National Building Specification Ltd June 2009 Standard Version Page 17

18 290, 295 Clauses 290 and 295 share guidance. Insulation may be against heat loss or sound transmission (referred to as a resilient layer in Building Regulations) or both. For guidance on floating screeds see BRE Report 460. For plastics or mineral fibre insulation boards/ slabs/ quilts use clause 290. For thin plastics sheet insulation (typically closed cell polyethylene foam) for impact sound control, use clause 295. For guidance on resilient layer materials for impact sound insulation see Building Regulations: E&W Approved Document E, Sections 2 and 3. IRL Technical Document E, Section 3. NI Technical Booklet G, Section 1. Scot Technical Handbook 5, Domestic or Non-domestic. For impact sound insulation, it is essential that there are no bridging elements to transmit sound from screed to structure (via, e.g. screed penetrating joints in insulation, fixings for pipes or screed battens). The problem with screed battens is avoided by the use of wet screeds see clause 405. Wire mesh/ netting is sometimes laid on top of compressible mineral wool insulation to provide a degree of protection during screeding. It does not have a crack control function (see clause 392). The use of protective wire mesh is included as part of a technical solution for impact sound resisting construction in Building Regulations. However, the need for the wire netting in this position is questionable and is not included in clauses 290 or 295. Insulation (clause 290): Insert type, e.g. 25 mm mineral wool slabs, density 36 kg/m³. 20 mm expanded polystyrene boards to BS EN 13163, grade EPS mm polyurethane (PUR) foam boards to BS EN mm XYZ Ltd: Kozeeflor polystyrene board. Insulation (clause 295): Insert, e.g. 5 mm extruded (closed cell) polyethylene foam, density kg/m³. 5 mm XYZ Ltd: Zoundsfoam polyethylene foam sheet. Separating layer (clause 290): To prevent wet screed material penetrating through to the substrate or abutments. Insert type, e.g. Polyethylene sheet minimum 125 micrometres thick (500 gauge). Building paper to BS 1521, Grade B1F. 290 FLOATING CONSTRUCTION Insulation: Type: Installation: Lay with tight butt joints. Continue up at perimeter abutments for full depth of screed. Separating layer: Type: Installation: Lay over insulation and turn up at perimeter abutments. Lap 100 mm at joints. 295 FLOATING CONSTRUCTION (THIN SHEET IMPACT SOUND INSULATION) Substrate: Remove projections that may puncture the insulation. Insulation: Type: Installation: Lay on substrate. Turn up for full depth of screed at perimeter abutments. Lap 100 mm at joints and seal with tape. Perimeter: Maintain isolation of screed. BATCHING/ MIXING 302 CEMENTS Cement types: In accordance with BS , clause See: BS , clauses 5.3, 7.6 and Annex B. BS EN for sand, see guidance in BS , Table B.1. BS EN for coarse aggregate, see guidance in BS , Table B.2. Note that the term sand is used throughout this section as this is the common terminology in the UK screeding industry, although the European Standards use the term fine aggregate. 306 Use this clause if clauses 140 or 185 are included. Sand: Grading limits: Normally, insert 0/4 mm (MP) category 1. For screeds of 20 mm or less, 0/2 mm (MP) category 1 should be considered. See BS , clause BS 8204, Table 1 lists air entraining and set retarding admixtures for screeds; however, water reducing admixtures are commonly used to reduce the water:cement ratio (whilst maintaining workability), thereby reducing drying shrinkage and increasing strength. Calcium chloride: This will increase drying shrinkage and danger of corrosion of embedded metals. 305 AGGREGATES Sand: To BS EN Grading limits: In accordance with BS , Table B.1. Coarse aggregates for fine concrete levelling screeds: Standard: To BS EN Designation: 4/10. Lightweight aggregates: To BS , Annex A. 306 PROPRIETARY POLYMER MODIFIED SCREEDS Cement types: In accordance with BS Sand: To BS EN 13139: Grading limits: Aggregates: In accordance with BS ADMIXTURES Standard: In accordance with BS , Table 1. Calcium chloride: Do not use in admixtures. 310 BATCHING WITH DENSE AGGREGATES Mix proportions: Specified by weight. Batching: Select from: Batch by weight. Batch by volume: Permitted on the basis of previously established weight:volume relationships of the particular materials. Use accurate gauge boxes. Allow for bulking of damp sand. National Building Specification Ltd June 2009 Standard Version Page 18

19 311 BATCHING WITH LIGHTWEIGHT AGGREGATES Standard: To BS , Annex A. Mix proportions: Specified by volume. Batching: Use accurate gauge boxes. 330 This clause is not applicable to proprietary self smoothing screeds clauses 160 and 192). See also clause 220. Water content: A low water:cement ratio is desirable to control drying shrinkage, but full compaction is more important. The correct water content of a cement:sand levelling screed mix can be judged on site by pressing a ball of the mixed material in the hand. Only a little water perhaps a drop or two should be squeezed out. When the hand is opened the material should be just moist enough to hold the ball together. Mixing: Free fall drum mixers are not recommended for screed material with low water content. Forced action trough paddle mixers, pan mixers or, alternatively, ready mixed screeds are preferred. 335 Use this clause for levelling screeds (other than lightweight clause 150). In situ crushing resistance was originally defined as soundness see BS , sections 7.5 and 8. For mass of test weight for floating screeds, refer to general guidance 4.2. Service usage and indentation (in mm) should be stated in each levelling screed type clause. 340 Frost precautions involve keeping temperature at an acceptable level and/ or increasing the rate of hardening of the mix. Methods include: Heating the mixing water and/ or aggregate. Increasing the cement content of the mix or using a cement which has a more rapid gain of strength. Covering the screed with an adequate insulating material. Controlling ventilation and providing some heat. Refer to the guidance in BS for weather conditions at the time of mixing and curing. 345 Use this clause for level screeds, do not use for screeds laid to falls. To achieve accurate finished surface levels of screeds it will usually be necessary to allow additional thickness and also specify the tolerances for structural slab levels. The extent of deviation from datum will also depend on the floor area generally, and whether there are partitions and door thresholds. See general guidance 6. BS , clause states maximum 15 mm deviation for bonded screeds. Insert, e.g. ±7 mm from datum. ±10 mm from datum. 350 Use this clause for screeds requiring falls, e.g. wet areas with internal gullies and roofs. Minimum screed cover should be specified in clauses Falls: Gradient (minimum): Insert, e.g. 1:60 to gully position. 1:60 to channel position. As drawing L(04) Use this clause for screeds on ramps. For proprietary screeds refer to manufacturer. Screed cover: Insert, e.g. Maintain consistent screed depth to graduated substrate (where the falls are created within the concrete substrate). Thickness varies, screed to falls of 1 in 12 (where falls are created by the screed consult screed manufacturer). 330 MIXING Water content: Minimum necessary to achieve full compaction, low enough to prevent excessive water being brought to surface during compaction. Mixing: Mix materials thoroughly to uniform consistency. Mixes other than no-fines must be mixed in a suitable forced action mechanical mixer. Do not use a free fall drum type mixer. Consistency: Use while sufficiently plastic for full compaction. Ready mixed retarded screed mortar: Use within working time and site temperatures recommended by manufacturer. Do not retemper. 335 IN SITU CRUSHING RESISTANCE (ISCR) Standards and category: To BS , table 4. Testing of bonded and unbonded screeds: To Annex D. Testing of floating levelling screeds: To Annex E. 340 ADVERSE WEATHER Screeds surface temperature: Maintain above 5 C for a minimum of four days after laying. Hot weather: Prevent premature setting or drying out. LAYING 345 LEVEL OF SCREED SURFACES Permissible deviation: (allowing for thickness of coverings) SCREEDING TO FALLS Minimum screed cover: Maintain at the lowest point. Falls: Gradual and consistent. Gradient (minimum): SCREEDING TO RAMPS Screed cover: Falls: Gradual and consistent. National Building Specification Ltd June 2009 Standard Version Page 19

20 355 Specify the surface regularity required (maximum deviation) in the types of levelling screed or wearing screed clauses (110 et seq). See also general guidance 6. BS , -2, -3, and -7 measure surface regularity of finished screeds in terms of maximum deviation from the underside of a 2 m straightedge placed anywhere on the floor. These translate into the classes SR1 SR3 used in the type clauses in this section. The standard of regularity depends on: The type of floor finish, e.g. adhesive-fixed thin flexible vinyl sheet requires better regularity than stone slabs laid in mortar (only affects levelling screeds). The usage of the space, e.g. a television studio requires a higher standard than carpeted offices (mostly relevant to wearing screeds). The standards mentioned above give basic guidance. Also consult floor finish manufacturers for their specific requirements, and see the relevant standards for floor finishes, e.g. BS , -4 and -5. Tighter tolerances will naturally attract higher cost. The straightedge method of measuring flatness may not be practicable or adequate for large floor areas see Concrete Society Technical Report TR 34 for alternative methods. 375 To achieve thorough compaction of levelling screed mixes with low water:cement ratios, plate vibrators are preferred. However, in small or awkwardly shaped areas, hand tamping or a weighted roller will be required. Simply levelling off with a screed board or straightedge is not sufficient. For concrete wearing screeds, mechanical methods of compaction include double vibrating beams. The soundness test specified in the relevant type of levelling screed clause should provide reasonable assurance that the screed has been adequately compacted. See also general guidance 4.2. Where crack control reinforcement is used (clause 392) screeds must be laid in two layers. Screeds over 50 mm thick: Further advice is given in BS , clause 7.8, for reinforcement, etc. 382 BS , clause recommends the following thicknesses for stair wearing screeds: Treads: 20 mm (minimum) and 40 mm (maximum). Risers: Between 15 and 25 mm. 386 Use for skirtings formed in association with polymer modified wearing screeds (clause 185). Background: Insert, e.g. Dense concrete blockwork. Mix: Wearing screed mixes incorporating a 10 mm single sized aggregate will usually require a finer aggregate for skirtings. Dimensions: Height: Heights greater than 150 mm are not normally recommended. Top edge: Insert either Rounded or Square. 392, 395 Clauses 392 and 394 share guidance. Use clause 392 with unbonded (clause 280) and floating (clauses 290, 295) screeds, where reinforcement is considered necessary to control shrinkage induced cracking. Use clause 395 for localized crack control reinforcement strips (e.g. at beam/ wall bearings, across day joints). Use clause 251 for strips of reinforcement across conduit in levelling screeds. Screeds must be sufficiently thick to accommodate the reinforcement (including up to three overlaps) within about the middle third of their depth. Screeds are laid in two layers see clause 375. Reinforcement does not prevent curling of the screed or any subsequent deflection under loads, but can limit the width of 355 FLATNESS/ SURFACE REGULARITY OF FLOOR SCREEDS Standard: To BS , Table 5. Test: To BS , Annex C. Sudden irregularities: Not permitted. 365 FLATNESS/ SURFACE REGULARITY OF ROOF SCREEDS Sudden irregularities: Not permitted. Deviation of surface: Measure from underside of a 2 m straightedge (between points of contact), placed anywhere on surface. Permissible deviation (maximum): 6 mm. 375 COMPACTION OF SCREEDS General: Compact thoroughly over entire area. Screeds over 50 mm thick: Lay in two layers of approximately equal thickness. Roughen surface of compacted lower layer then immediately lay upper layer. 382 STAIR SCREEDS Construction: Fully bonded to treads, risers and landings. Risers: Form using fine finish formwork. Wearing screed surfaces: Make good with compatible cement:sand mix. Wood float. When hardened remove laitance. 386 COVED IN SITU SKIRTINGS Background: Mix: To BS , Table 2. Aggregate size adjusted as necessary to suit skirting thickness. Construction joint: Form at base. Rendering: Apply bonding agent. Render skirting while bonding agent is still wet. Achieve a good bond. Allow each coat to set before applying subsequent coats. Render to true lines with a fine finish and an even consistent appearance. Dimensions: Thickness of each coat (maximum): 10 mm. Height: Cove radius: 30 mm. Top edge: GENERAL REINFORCEMENT Steel fabric: To BS Type: Installation: In accordance with BS STRIP REINFORCEMENT Location: Steel fabric: To BS Type Width of strips: Installation: In accordance with BS National Building Specification Ltd June 2009 Standard Version Page 20