A root and branch review of Standards Chapter 4.2 An introduction to the significant changes in the Chapter rewrite.

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1 NHBC s technical newsletter April 2003 Issue 26 This edition includes: A root and branch review of Standards Chapter 4.2 An introduction to the significant changes in the Chapter rewrite. Concrete changes to Chapter 2.1 A better understanding of concrete and chemical attack leads to a Chapter update. New guidance for underfloor heating systems NHBC introduces guidance on the design and installation of these systems. Changes to Part E of the Building Regulations A summary of the changes to regulations governing sound insulation. NHBC gives support to the joist debate We evaluate the two methods used for the support of floor joists. EXTRA STANDARDS EXTRA 1

2 CONTENTS 2 A root and branch review of Chapter Concrete changes to Chapter NHBC publishes Good Craftsmanship Guides 3 NHBC launches Trade Talks 4 Adequate fixings for tiles and slates A root and branch review of Chapter 4.2 One of the major areas of claim against Buildmark is foundation failure. All too often these claims involve homes which have been constructed on shrinkable clay soil in close proximity to trees or shrubs. 4 New guidance for underfloor heating systems 5 Changes to Part E of the Building Regulations 6 NHBC gives support to the joist debate 7 Reminder - Notification of energy ratings 8 Questions and Answers Shrinkable clays change in volume as their moisture content varies. This can result in subsidence or heave to the home where the tree causes the moisture content to vary and the foundations have not been designed to take account of the possible movement. The problems associated with clay soils were highlighted in the late '60s when NHBC first published Practice Note 3 which provided guidance to avoid damage caused by trees near dwellings. This was revised in 1974 and again in This version subsequently became Chapter 4.2 of the Standards, which was itself revised in Over the years our knowledge of the way in which trees and foundations interact has increased and the time has come to review the Chapter again. This review has been based on both our claims data and up-to-date advice from engineers and arboriculturalists. The 2003 version expands the guidance to include shrubs. NHBC Claims data confirms that some claims are caused by shrubs; Pyracantha, Cotoneaster and climbers such as Ivy, Virginia Creeper and Wisteria can be particularly damaging. The guidance asks for minimum foundation depths to be increased where shrubs are planted unless they are excluded within stated distances from the foundation. Other changes include: the addition of an introduction to explain the philosophy behind Chapter 4.2 better guidance about where heave is likely to occur guidance about the Plasticity Index of shrinkable clay soils limitations on the maximum depths of trench fill foundations - where they are greater than 2.5m deep then an engineer will need to become involved. This is because of the uncertainty of such foundations at these depths. The actual foundation depths given in the Chapter remain unchanged. However, the graphical solution to finding the correct foundation depth has been improved. The alternative method by use of tables has also been improved by combining tables. A D/H table has been included so it is no longer necessary to work this out when determining foundation depths. A number of tree species have been added and two species have been changed to a higher water demand category. Hawthorn has been moved from Moderate to High and Beech has been moved from Low to Moderate. The Chapter will become effective for homes where the foundations are commenced on or after 1 July Familiarise yourself with the new Chapter. If you have used very deep trench fill foundations in the past, then these will not be acceptable in the future unless they are designed by an Engineer. 2 STANDARDS EXTRA

3 Concrete changes to Chapter 2.1 Image courtesy of RMC UK Familiarise yourself with the new Chapter and ensure that the correct grade of concrete is specified. Ready mixed suppliers are gearing up to supply the new mixes. NHBC Standards Chapter Concrete and its Reinforcement - has been revised to take account of the better understanding of concrete specification needed to resist chemical attack from the ground, as detailed in the new BRE Special Digest 1, (see page 7 of Standards Extra 23). In addition, the revisions bring the Chapter in line with the European Standard for concrete, BS EN 206-1, and its complementary British Standard, BS 8500-Parts 1 and 2. In December 2003 these Standards will replace the current British Standard for concrete, BS 5328 which was revised in September 2002 to take account of BRE Special Digest 1. The revised NHBC Chapter lists Designated mixes which are supplied ready mixed or Standard Prescribed mixes for site mixing which are suitable for non-aggressive and the lower range of ground chemical aggressiveness. Specialist advice is required for the more aggressive ground environments because a range of items needs to be considered before the concrete can be specified. These include: whether the ground water is static or mobile whether the ground water is acidic or not whether the site is brownfield or natural the size and service life of the structural elements what aggregates are to be used in the concrete whether additional protective measures are to be applied to the concrete e.g. protective coatings or sacrificial layers. In general terms a concrete that satisfied the requirements of Class 1 or Class 2 sulfate conditions under the old BRE Digest 363 is also likely to satisfy the ACEC (Aggressive Chemical Environment for Concrete) Classes 1 and 2 as shown in table 4b, Appendix B of the Chapter. The revised Standards Chapter 2.1 is effective from July NHBC publishes Good Craftsmanship Guides Enclosed with this issue of Standards Extra are Good Craftsmanship Guides for: Brickwork and Blockwork, and Carpentry and Joinery Carcassing These easy-to-use booklets are practical reference guides, which use photographs and illustrations to highlight common pitfalls and provide advice on avoiding them. The problems are those most often identified during NHBC s inspection of homes under construction. All of the photographs were taken on real building sites. Further copies of the Good Craftsmanship Guides are available from NHBC Distribution (phone , fax ) at for 50 copies (including postage & packing). Building Inspectors hold a limited number of copies and may be able to provide small quantities, on request, during scheduled visits to your sites. NHBC launches Trade Talks NHBC has recently launched its troubleshooting Trade Talks for site tradespeople. Based on the NHBC Standards and delivered by NHBC Building Inspectors, these one-hour talks are designed to highlight the most common reportable items or defects encountered by inspectors and claims investigators on residential developments. Practical guidance is then given on how to prevent these problems. The emphasis is on preventing defects through getting it right first time. This type of training will help cut costs in terms of repair work, and will also have a positive impact on customer satisfaction and the levels of aftercare service needed to manage problems. Everyone who attends a Trade Talk will receive his or her own copy of the relevant NHBC Good Craftsmanship Guide. We are currently offering Trade Talks on the following subject areas: Brickwork and Blockwork, and Carpentry and Joinery Carcassing Trade Talks can be delivered on or near your site, provided that a suitable room is available to seat the attendees. Cost is VAT per Talk for a maximum of 12 attendees. A 10% discount is offered for two or more Talks run consecutively at the same venue. Bookings can be made by calling Lisa Jones on STANDARDS EXTRA 3

4 Adequate fixings for tiles and slates Some readers may have seen press articles about the need for extra fixings for tiles and slates. These have come about because of changes to the British Standards dealing with roofing (BS Code of practice for slating and tiling) and wind loading (BS Code of practice for wind loads). The effect of these changes is that fixings arrangements are now different. This is due to the way wind loads are calculated and applied to different tiles, different roof configurations and geographical location of the site. How can builders be sure they are providing an adequate number of fixings? Tile manufacturers we have spoken to provide a free design service. They have computer programmes to determine wind pressures for a specific location. From that information, they can specify the number of fixings for the tiles. Image courtesy of Redland Roofing Ensure that your specification for fixing roofing tiles and slates meets with BS 5534 and BS 6399 for each site. It is necessary to have each location checked, as fixing requirements will vary. New guidance for underfloor heating systems Underfloor heating systems may not be widely used in mainstream house building but they are popular with certain sectors of the industry. Underfloor heating systems need to be designed and installed carefully to ensure they are capable of heating the home in accordance with NHBC s requirements. As the heat output is determined by the length of the heating loop, systems which fail to work correctly cannot be put right simply by fitting a larger radiator. Extending the heating loop is bound to involve major upheaval to the property. The Building Services Research and Information Association (BSRIA) has recently completed a project looking at underfloor heating systems. This has culminated in the publication of two guides one for designers and the other for the assessment of installations. NHBC Standards Chapter 8.1 now recognises these two publications where underfloor heating is proposed. By following the guides, it will ensure that a consistent approach is adopted by system designers and installers and systems meet home owners expectations. Image courtesy of Wavin UK If you are proposing to use an underfloor heating system, then ensure it meets with the BSRIA guides, both in terms of the design and installation. Copies are available from BSRIA, Old Bracknell Lane West, Bracknell, Berkshire, RG12 7AH. Tel: Fax: bsria@bsria.co.uk 4 STANDARDS EXTRA

5 Changes to Part E of the Building Regulations The Government has introduced changes to Part E of the Building Regulations (England & Wales), which currently sets the standard for sound insulation between dwellings. The new regulations are designed to improve the standard of sound insulation in dwellings and have been extended to encompass other building types such as 'rooms for residential purposes' (e.g. hostels and hotels), and schools. The proposals are also backed up by new requirements for sound testing. What are the changes? In summary the significant changes are: Improvement of sound insulation between dwellings and between 'rooms for residential purposes' by: setting a new performance standard for separating walls and floors that takes greater account of low frequency noise promotion of substantial constructions more likely to comply with the new standard introduction of mandatory sound testing to prove compliance of each construction type used (see sound testing ). Improvement of sound insulation within dwellings and 'rooms for residential purposes' by: controlling sound insulation between bedrooms and rooms containing wcs and other rooms controlling sound insulation of internal floors. Limiting reverberation in common parts of buildings containing flats or 'rooms for residential purposes' by: designing the common parts of buildings to prevent excessive reverberation (e.g. by providing acoustic ceiling linings). When? The new Part E comes into effect for all relevant building work from 1 July Sound testing For new houses and flats the requirements for sound testing come into effect on 1 January For converted houses and flats and new and converted 'rooms for residential purposes' the requirement for sound testing comes into effect on 1 July From then at least one set of pre-completion tests will need to be carried out for each ten units. Builders should be aware that the new provisions will not apply where work on a plot has started, or a Plans Certificate for the work has been issued by NHBC or unconditional plans approval has been given by the Local Authority prior to 1 July The following table provides additional clarification as to which regulations apply where work is commenced on individual buildings on or after 1 July Plans Certificate* not issued prior to 01/07/2003 Plans Certificate* not issued prior to 01/07/2003 New build dwellings and flats Plans certificate* for work issued prior to 1 July 2003 Individual buildings commenced prior to 1 July 2003 Individual buildings commenced on or after 1 July 2003 and prior to 1 January 2004 Conversions and rooms for residential purposes * Plans certificate issued by NHBC Building Control or unconditional plans approval issued by Local Authority Building Control. Robust Standard Details Pre-completion sound testing will not be required for new homes until 1 January This delay is to allow the industry the opportunity to develop Robust Standard Details. A major House Builders Federation (HBF) project is currently underway to develop forms of separating wall and separating floor constructions, which provide consistently Approved Document E applicable Sound testing required Individual buildings commenced on or after 1 January 2004 Approved Document E applicable Sound testing required Plans certificate* for work issued prior to 1 July 2003 Individual buildings commenced prior to 1 July 2003 Individual buildings commenced on or after 1 July 2003 good performance and so will not need testing. Given that the project is successful, builders will be able to choose between building in accordance with the Robust Standard Details or undertaking sound testing. Further information on the HBF project is available by visiting STANDARDS EXTRA 5

6 NHBC gives support to the joist debate A hot topic at the moment is the support of floor joists. Here we look at two alternative methods of support in masonry construction. Each method needs to be carefully considered for very different reasons. The first part of this article confirms the correct use of joist hangers. The second part details one method of building-in joists, which satisfies Part L1 of the Building Regulations (England & Wales). We also look briefly at the implications of the new Part E with respect to the building-in of joists. 1 Support of joists using joist hangers The Health and Safety Executive (HSE) has expressed concern that unless hangers are used correctly, there is a potential risk of floor collapse during construction. This is not a new issue and NHBC highlighted the problem in 1997 in Standards Extra 14. However, HSE has asked NHBC to remind registered builders of the need to use hangers correctly because they have had to continue investigating incidents where floors have collapsed. When using joist hangers:- use joist hangers that comply with BS 6178 Part 1, 1999* obtain and follow the manufacturer s installation requirements ensure that the hangers and blocks are compatible. Do not use hangers marked for use with a particular block strength on blocks of a lower strength, eg. don t use 3.5N/mm 2 hangers on 2.8 N/mm 2 blocks ensure that the hanger back-plates are flush to the wall and that the hangers are vertical ensure joists are cut to length with a maximum overall tolerance of 6mm do not load the joists until the required number of courses have been laid above the masonry flanges (usually 3 block courses but consult the hanger manufacturer) and the mortar joints have achieved the required strength. Alternatively install an engineer designed support system under the joists and have it checked before loading the floor. 2 Building-in of joists In England and Wales the Robust Details, which support Approved Document L1, show timber floor joists supported by joist hangers rather than being built into a masonry wall. This is to reduce air leakage at the junction of the two elements. The Building Regulations in England and Wales are performance based. Therefore, alternatives are acceptable if they meet with the performance standard. NHBC has considered a number of proposals for the building-in of joists into a cavity wall. The following alternatives are acceptable. Solid timber joists Solid timber joists may be built into the inner leaf of a cavity wall, provided that: the mortar joints around each joist perimeter are recessed or struck, and the joint between the masonry and the timber is carefully pointed with silicone mastic: (*The above guidance is based on hangers which meet with BS A new European Standard (BS EN 845-1) is available and some manufacturers may be producing hangers to this standard. If this is the case, check the manufacturer s recommendations about use.) Bead of sealant applied between wall and joist 6 STANDARDS EXTRA

7 Timber engineered joists Timber engineered joists may be built into the inner leaf of a cavity wall, provided that: proprietary filler pieces are fitted on both sides of the web between the top and bottom flanges. These filler pieces must not damage the joist flanges. The depth of the filler pieces should be slightly less than the dimension between the joist flanges to achieve a loose fit the mortar joints around each joist perimeter are recessed or struck, and the joint between the masonry and the timber and any other air paths are carefully pointed with silicone mastic Bead of sealant applied to joint between filler piece and top and bottom flanges Proprietary filler piece Bead of sealant applied between wall and joist Points to watch The sealant should be applied after the home is roofed so that the joists and masonry are reasonably dry. The workmanship needs to be good: Masonry blocks should be cut accurately (sawn, where possible) and a struck or recessed mortar joint formed to allow a good silicone mastic bead to allow for shrinkage of both timber and masonry. The use of expanding polyurethane foam is not acceptable for sealing around joist ends. Joists and separating walls Builders should be aware that the new guidance contained in Approved Document E - Resistance to the passage of sound (2003 edition)* shows joists supported on hangers to separating walls. This is considered to be the most reliable method of ensuring adequate sound insulation for separating walls. However, if builders have designs which show joists built into separating walls then the ends do require sealing, to limit air leakage. *see page 5 for more on Part E Whether you use joist hangers or build joists in, follow the above guidance carefully. Consider the implications of building joists into separating walls for buildings that need to comply with the new requirements of Part E. REMINDER - Notification of energy ratings In England and Wales the Building Regulations require that the energy rating of a new home be made known to the home owner as soon as possible. Builders may give the rating directly to the purchaser or choose the most appropriate time for posting the rating notice in the completed home. The intention of the legislation is to ensure that purchasers are aware of the energy efficiency of their new home.research by National Energy Services reveals that this notification is not being carried out. Their findings show that 98% of unsold homes checked did not display the SAP rating. Whilst the research failed to identify whether builders were making the rating known directly to purchasers, a mystery shopping exercise demonstrated that few sales staff understood or even had knowledge of the SAP rating. There is a legal obligation to give details of the SAP rating to home owners and you should consider your approach to complying with the requirement. It is often not practicable to display the energy rating notice prior to occupation of dwellings; in these cases you should ensure that home owners are given this information, perhaps as part of an information pack or via a separate communication. Sales staff are ideally positioned to help inform purchasers regarding energy efficiency and the relevance of the energy ratings for new homes. NHBC Training Services will be happy to advise further on Building Regulation training courses available for sales and other non-technical staff (contact Dave Towell on or DaveT@nhbc.co.uk). Place an easy to read notice of the energy rating in unsold properties - members of the House Builders Federation can obtain a blank notice from the HBF website at When selling off-plan, provide information on the SAP rating to purchasers and ensure sales staff are briefed about the benefits of energy efficient homes. STANDARDS EXTRA 7

8 Questions & Answers Below are the answers to a selection of questions we have recently been asked in NHBC Technical. Handrails to staircases - tapered treads/winders Question Where stairs have tapered treads/winders, is a handrail required to the outside of the stairs? Answer A safe handhold is required for the full rise of any stairs with a total rise greater than 600 mm. A suitable newel post can provide a safe handhold. Single newel In England, Wales and Scotland where the stairs have between one and four tapered treads/winders and the newel provides a safe handhold, a handrail is not required to the outside of the stairs. Double newels In England, Wales and Scotland where the newels provide a safe handhold, a handrail is not required to the outside of the stairs. Additional handrail not needed Additional handrail needed Additional handrail not needed In Northern Ireland a handrail should be fitted to the outside of the stairs in both cases (required by building regulations). In all cases where a handrail is needed to the outside of the stairs, it should be continuous for the whole rise to avoid the need to change hands. Use of nail guns for fixing trussed rafter bracing Question The guage of the nails used in nail guns is less than that quoted in the NHBC Standards. Can these be used for fixing trussed rafter bracing? Answer NHBC Standards clause 7.2 D5(c) asks for 3.35mm (10 guage) x 65mm long galvanised nails nails used in nail guns are not available to this specification. When nail guns are used, 3.1mm x 75mm long annular ring-shank nails are acceptable. They do not need to be galvanised. Roofing underlay at eaves level Question Can plastic dpc material be used at eaves level in place of a type 5U felt? Answer NHBC Standards clause 7.2 M5 (d) says where the underlay is exposed, such as at the eaves, a type 5U felt should be used. Plastic dpc materials are generally not designed for use in permanently exposed locations. The lack of suitable ultra-violet stabilisers could lead to an early breakdown of these materials in such locations. Dpc materials should not be used for the exposed sections of roof underlay at the eaves. A type 5U felt should be used for the exposed sections unless the underlay used for the remainder of the roof has a third party assessment confirming its suitability for use at exposed eaves. Alternatively, a proprietary eaves guard which extends under the unexposed section of the roofing underlay and provides adequate projection into the guttering may be used. 8 NHBC Technical Buildmark House, Chiltern Avenue, Amersham, Bucks HP6 5AP Tel: Fax: technical@nhbc.co.uk HB /03