Basements for housing

Transcription

1 Benefits and solutions for sustainable housing

2 Contents Types of basements 4 Benefits of basements 6 Optimising development potential 9 Construction techniques 11 Design issues 15 Costs 19 Building legislation 20 Appendix and further reading 22 Introduction This document considers the role of basements as a viable part of the solution to sustainable housing in the UK, through consideration of their specific attributes in relation to planning policy and the Code for Sustainable Homes. Outlining the key issues to be considered at each stage of the design and construction process for single-storey basements in domestic housing, guidance is additionally offered on sources of more detailed advice. The Code of Practice for protection of below-ground structures against water from the ground (BS 8102) was revised and re-issued in December About this publication Domestic basements can aid in the creation of desirable, sustainable homes, providing greater flexibility and adaptability of space; thus extending the design life of the building. Including a basement maximises usage of available land, provides a stable construction base and improves thermal efficiency. Modern basements offer the possibility of additional, alternative living spaces in dry, warm, day-lit rooms with good ceiling heights and ventilation levels. Usage potential ranges from ideal quiet areas for home working or leisure to additional space for storage and parking. The use of full or partial basements in housing can play a significant role in meeting current and future needs for new homes in the UK. Basement design supports basic sustainability principles such as longevity, durability and adaptability, as well as providing useful space for many of the additional requirements needed to comply with the Code for Sustainable Homes. Definition: Basement Throughout the document, the term basement refers to a usable part of a building that is situated partly or entirely below ground level, as defined by the British Standards Institute [1]. Building regulations in England and Wales define a basement storey as at least 1.2m below adjoining ground level [2]. Occasional reference is made in this document to partial- or semibasements to remind readers that usable domestic basements are likely to include windows and doors for natural lighting and ventilation. All walls enclosing a basement may not, therefore, be fully below ground level; for example on sloping sites or with lowered courtyards. This document is principally concerned with the issues associated with new basement construction rather than existing cellars since the latter is, by definition, limited to space for storage below ground with less requirement for daylighting and ventilation. Much of the guidance is, however, relevant to the conversion of old cellars into habitable basements. An example of a new build development with a basement. 2

3 The case for basements today Changing requirements for housing Houses with below-ground space used to be common in the UK but their construction has declined over the last century. In mainland Europe and North America, however, basements have continued to be incorporated into new dwellings. In Germany, they are included in almost 98% of housing stock. The decline in the UK was due, in part, to changes in living requirements. Improved transport, convenience food and the use of electric fridges diminished the need for naturally cool areas to store food and, together with changes in social attitude, the need for a space below stairs for domestic help. Until recently, reliance on cheap gas and electricity also reduced the need for areas in the home to store fuel. However, housing in the UK is currently undergoing a radical reevaluation of its design and performance, as a result of evolving legislation and attitudes towards sustainability and energy efficiency. Established construction techniques and space-planning solutions for housing are being challenged and tested against a new set of sustainable performance criteria, and basements are once more considered relevant and attractive additions to housing. In addition to the drive for low energy housing, there is a movement towards increasing the size of new housing, in response to reports by various organisations, including the Commission for Architecture and the Built Environment (CABE), Homes and Communities Agency and Mayor of London Office. The provision of homes that have sufficient space to develop and grow with the requirements of a family is a significant sustainability issue for housing. The sustainability benefits of basements are described in the benefits of basements section of this document, and a summary of scoring credits applying to basements under the Code for Sustainable Homes is given in the Appendix. In Germany, and much of continental Europe, the basement provides the solution to the problem. Over there, asking for a house without a basement is like asking for a car without wheels. Government targets for housing and limited land availability A combination of Government targets for new housing and limited availability of land for new construction have led to greater consideration of increased densities and the development of difficult plots of land, such as sloping sites or those with poor soil stability. Planning Policy Statement 3: Housing (PPS3), underpinning delivery of the Government s strategic housing policy objectives, encourages increased density of housing developments. In addition, high land prices mean housebuilders are under increasing pressure to maximise potential returns through efficient use of land. The use of full or semi-basements can be a cost effective means of increasing the density of housing developments, without reducing amenity levels; particularly if planning requirements restrict the building footprint or height. Strategies for optimising residential development are explored on page 9 and 10 of this document. Improved construction techniques and contractor warranties The reliability and design of construction and waterproofing techniques has significantly improved over the years; backed by many successful examples both in the UK and abroad. Advice regarding the detail, design and construction of basements is well documented and available from The Basement Information Centre (TBIC), The Concrete Centre and National House-Building Council (NHBC), amongst others. NHBC arrangements with builders and developers provide cover under their standard building assurance system for the construction of basements [3]. Accreditation and indemnity schemes exist for specialist basement contractors. Details of accredited contractors (construction and waterproofing) may be found on The Basement Information Centre website. In addition, various manufacturers of specialist water-resisting concrete offer guarantees. An accreditation and insurance scheme, administered by the Association of Underpinning Contractors (ASUC), is available for underpinning of existing structures; most commonly required for refurbishment or retrofit basements. The correct procedures for design, soil investigation and construction are key to achieving robust and reliable basement construction. The various methods of construction on offer, and design issues, are summarised in the construction techniques and design issues sections of this document. Chris Drury - Weber House, Germany, commenting on the lack of storage space in UK housing. 3

4 Types of basements Basements can be broadly sub-divided into five categories, depending upon their location, time of construction relative to the main property and depth. A brief summary of the differences, and the key issues related to each, is outlined below. Table 1: Types of basements. Type of basement Description Details Benefits Other comments Refurbishment Alterations to existing space below ground. May require lowering the floor to increase head room; underpinning adjacent walls; improving ventilation and lighting. Typically involves improvements to, or new, waterproofing system; and new finishes and fixtures. Adds value and desirability of property Additional usable space Addresses potential existing damp problems Provides opportunities for new activities in property e.g. games room/gym/storage Building regulations approval will be required for any works. Planning permission may be required, depending on extent of works and intended use. * New garden basement New basement adjacent to existing property, usually in garden space. New structure below ground, with planted green roof or terrace at garden level. Access from main house via new external covered staircase. Adds value and desirability of property Additional usable space Provides opportunities for new activities in property Potential for natural daylight and ventilation through roof lights Planning and building regulations approval required. Likelihood of need for structural support to existing house is reduced as distance from house increases. New basement under new housing Basement space built as part of a new build development. Arrangement of windows and internal and external access vary. Designed to suit current and future use requirements, site conditions, cost and constructability. Adds value and desirability of property Future adaptability Usable space for sustainable technologies and recycling Potentially reduces footprint of house Increases thermal performance No additional building or planning regulation requirements provided it is included in initial application.* Retrofit New space created through excavation below ground floor of an existing property. Allows creation of additional space below business or homes, which benefit from staying in same location. Preservation of existing building possible (e.g. listed building) Underpinning works required. Adds value to property Additional space for business to develop in same location or family to expand Releases potential of empty property Planning and building regulations approval required. Specialist work. Generally only economically viable for high land value properties. Deep basements Spaces below one storey deep. Frequently used for car parking, plant/services space and storage below larger residential development and other uses including commercial, retail or mixed use schemes in urban areas. Building footprint and development potential optimised above ground Parking and space for deliveries possible Refer to Design and Construction of Concrete Basements [10]. * At time of writing works may not be allowable under Permitted Development Rights, depending on individual Local Authority. This may be revised in the near future. The situation should be confirmed with the Local Authority Planning Department. Note: The Party Wall Act could apply to each type of basement. See page 21. 4

5 Some examples of basement arrangements Figure 1: Basement garage and storage. External access via the driveway. Figure 2: Basement with gym, shower and sauna. External and internal access with light well for daylight and additional ventilation. Figure 3: Basement with games room, wine cellar and storage. With external courtyard for daylight and additional ventilation. Access via courtyard and separate internal stair. Figure 4: Basement with additional bedroom or annex to the property (granny flat or similar) with internal and external access.extra light provided via conservatory. Figure 5: Split level design with separate external access and light well. Potential for office or workshop. Figure 6: Garden basement with internal access and roof light above. 5

6 Benefits of Basements There are many reasons for the provision of basements in a housing development including: added value; increased development potential; occupant or purchaser attraction and sustainability. This section explains the key benefits. Desirable and adaptable spaces There is an evident desire for the provision of basements as part of our housing solution in the UK, illustrated by the significant number of basements constructed in the self-build market. In addition, studies by the Traditional Housing Bureau [4] indicate significant demand from home owners for more space. In the 2005 CABE report What home buyers want: Attitudes and decision making among consumers, basements are cited as particularly valuable in this regard. One of the major benefits of basements is allowing the creation of a single large space. Due to economies and method of construction, the floor above the basement level can be created in a single span, providing flexibility in the location of internal walls and allowing simple future alterations. Pre-subdivision, the resultant space is typically the single largest area in the house and, due to its location on a different level from the rest of the house, provides opportunity for uses not always possible to accommodate in more basic dwellings. In North America, Canada and continental Europe, it is common for basements to serve as multipurpose areas, for example games or utility rooms or storage areas. Basements can provide comfortable day-lit rooms, with natural ventilation and external access, as an extension to the living spaces above. They also provide the opportunity for more unique uses, such as gyms, music rooms and swimming pools. Alternatively, basements can simply provide practical space for games or hobby rooms, home offices parking or storage. Good sound insulation Good acoustic attenuation is provided by the concrete walls surrounding basement rooms by the earth itself and the ground floor if it is built from concrete. Basement spaces are therefore inherently well insulated for sound and ideal for locating noisy activities such as music practice, home cinemas or other loud equipment that could disturb neighbours or the rest of the house. Conversely, the quiet nature of the space provides a peaceful place for reading, relaxing or working; away from ground-level noise, in and outside of the house. Martin Grant Homes - Riverview Court development. Developers in the UK are now looking at basements as a solution for creating spacious homes whilst using the land available efficiently. Riverview Court development was built on a former water treatment works on a flat site, beside the River Cam, Cambridgeshire. 35 homes were built with sunken patios which provide natural daylight and ventilation to the basement kitchen and dining room. 6

7 Added value and space The provision of additional floor space in a home clearly increases the value of a property and the addition of a basement adds space with little or no effect on the garden area. The costs associated with constructing a basement are explored in the Costs section of this document. Sustainability benefits Including a basement in the design of domestic properties helps developers to address many of the fundamental principles of sustainable design, including improved energy efficiency, longevity and adaptability and support of local employment. Energy efficiency Basements benefit from the surrounding ground improving their energy efficiency. As a consequence, the amount of insulation needed to reduce heat loss through a basement wall is less than that required on upper floor levels [5]. The simple construction methods and minimum wall penetrations, associated with basement construction, also lead to minimal heat loss through cold bridging. Studies by TBIC and Building Research Establishment (BRE) highlight a potential 10 per cent saving in space heating for a two-storey house with a full ground basement compared with its threestorey equivalent above ground (both having the same amount of added insulation). The potential space heating saving rises to around 14 per cent for a single storey property with full basement, compared to its two-storey equivalent above ground [6]. Thermal mass The heavyweight nature of basement construction can be utilised to naturally regulate the internal temperature of a home and can be part of an energy efficient strategy for controlling the temperature of the whole house. The Met Office has projected average daily temperature rises throughout the UK, indicating the increasing need for low energy solutions to cool homes, which heavyweight construction - including basements - is well placed to provide. For further information refer to The Concrete Centre publications Thermal Mass Explained and Thermal Mass for Housing. The thermal mass properties of concrete are optimised by omitting insulating internal surface finishes. If insulated and waterproofed externally, basement concrete walls will offer greater thermal mass. This could be achieved with a fair-faced or painted finish, or alternatively a wet plaster finish. Fair-faced concrete offers potential cost and programming benefits, by omitting subsequent use of finishing materials and trades and associated waste produced on site. Air tightness and mechanical ventilation The construction of sustainable dwellings using low air permeability and mechanical ventilation with heat recovery, such as the Passiv Haus technique, is a means of improving the energy efficiency of the building fabric of dwellings. This is simply provided by basements, since the structure below ground is inherently more air tight. The addition of a basement increases the living space in a property, without compromising the garden. Courtesy of The London Basement Company. 7

8 Basements and the Code for Sustainable Homes Since 2008, all new homes require rating under The Code for Sustainable Homes (CSH) as part of Home Improvement Packs (HIPs). The inclusion of a full or partial basement can provide additional credits under the assessment criteria of the Code; for example, storage of waste, recycling, alternative fuel supplies such as wood pellets, other equipment associated with renewable energy or water recycling and even bikes. In addition, basements can be used to reduce the overall size of the building footprint, relative to the number of storeys, scoring points in the Ecology Section of the Code. A full list of how basements can assist with obtaining credits for Code for Sustainable Homes is found in the appendix of this document. Space for working from home Basements offer the possibility of quiet, private spaces for working from home, with the potential for direct access from the outside, and therefore designated work-based visits or deliveries. The solid nature of basement construction provides excellent conditions for workshop spaces and activities that create noise, require support for heavy equipment or require robust, cleanable surfaces. Changing uses for basement spaces during the life of a family home Extra storage (recycling/chest freezers) DIY work shop Utility room Hobby room Home Gym/Sauna Wet room outdoor gear All weather play room Band practice/music room Teenage den/bedsit Home office/studio Wine cellar Home cinema Granny annex Living room Basements provide flexible multipurpose spaces through the lifetime of a home. Courtesy of The London Basement Company. 8

9 Optimising development potential Higher density development - minimal extra height The use of a basement provides increased floor area in a development without significantly increasing the apparent size of the building; thereby improving the viability of a development, particularly in areas where building height or size of footprint is restricted. The potential to increase the number of proposed dwellings in one property is boosted if both a basement and habitable roof space are proposed; thereby potentially creating two additional storeys in a similar building envelope. Maximise site layout - build up to street boundary Utilise existing slope Partial basements on sloping sites become viable compared to alternative substructure construction options to make up ground levels. By lifting the ground floor level above street level to create upper and lower ground floors, properties can be constructed close to site boundaries while maintaining privacy for occupants. Consideration of Building Regulation Part M requirements for access are required. 9

10 Protect amenity space The use of a fully below-ground basement or partially below-ground basement gives increased usable space within the building footprint. Larger dwellings can therefore be built on small sites without losing amenity space around the buildings. Better use of poor sites with poor soil Where poor ground conditions necessitate deep foundations, the additional cost can be mitigated by including a basement to add space and therefore value to the proposed new properties. Where large areas of contaminated soil are removed from site, the viability of including a basement level is increased. If constructed before original ground levels are reinstated, the amount of replacement ground material is reduced and further excavation is unlikely. Back fill will need to be compacted around the walls, but in general the programme of construction is likely to benefit from improved access conditions. Stable building stock Basements create a good stable structural base, capable of supporting heavy loads above. By combining foundation design with the provision of habitable space, the extra depth of structure provides the building with greater ability to cope with climate change effects in the soil, such as shrinkage or tree roots. This means buildings with basements are less prone to movement and cracking as a result of potential future changes in soil conditions. Shading indicates equivalent accommodation areas located beside or below a dwelling, impacting on available garden space. Increased number of plots per hectare By incorporating facilities such as garages, utility rooms or habitable space at basement level, it is possible to reduce the footprint of a proposed property; thereby increasing the number of houses on a given site or along a fixed street frontage. Housing using a basement garage requires less street frontage, compared to houses with garages located alongside at ground level. 7.6m 6.0m 7.8m 9.0m 10.0m 9.0m 7.8m 6.0m 9 plots possible with basement garages, compared to 8 plots on the same site. 10

11 Basement construction Water resistant reinforced concrete wall and slab External waterproofing Sandwiched waterproofing Internal waterproofing Type A: External barrier or internal protection Water-resistent reinforced concrete or blockwork with waterproofing located either externally, internally waterstop as required reinforced concrete wall External Sandwiched Internal or sandwiched. and slab waterproofing A non-integral kicker should be avoided as it will require one waterstop where it adjoins the slab and another at the intersection with the wall waterproofing Waterstop required at junction between wall waterproofing and slab and at all construction joints. e.g Crystallisation, hydrophilic or injected waterstop Types of waterproofing protection There are three main methods of providing protection against ground water for residential basements. These are defined in BS 8102 as types A, B and C. Their application is influenced by the ground conditions and proposed building use. The definition of terms and guidance related to their appropriate usage, are highlighted in the 2009 revision. Alternative approaches The traditional method of waterproofing domestic basements in Britain was a single barrier method (Type A) or drained protection (Type C). Earlier materials used in basement construction have since been developed into more durable waterproofing membranes. External or internal Water-resistent Slab with integral waterstop kicker as required Slab with kickerless construction External reinforced Sandwiched Waterstop Internal required waterproofing concrete waterproofing wall at junction waterproofing between A non-integral kicker and slab wall and slab and at Water resistant should be avoided as it all construction joints. reinforced Drained cavity concrete wall will require one waterstop where it adjoins the Internal block e.g Crystallisation, wall hydrophilic or and slab Inner skin slab and another at the Wall cavity intersection with the wall Concrete/steel piled wall injected Access waterstop point(s) to drainage External or internal Drainage channel Water-resistent waterstop as required Waterstop at junction reinforced Waterstop required Slab with integral kicker to follow wall profile concrete Slab with wall kickerless construction at junction between A non-integral kicker and slab wall and slab and at Water resistant Sump should formed be avoided in situ as it all construction joints. reinforced Type B: structural or will separate integral require drain one waterstop where may be it adjoins solid the Internal block hydrophilic wall or protection - reinforced or e.g Crystallisation, concrete Drained wall cavity which and prestressed slab concrete or perforated designed through Floor composite slab with integral slab and another at the and injected waterstop protection and/or added May incorporate integrated details, intersection Pump such as with water the Wall wall bars, cavity to membrane be water (internal resistant. Access or drainage channel Inner skin external) point(s) to with pipe connection Concrete/steel piled wall drainage to setup Drainage channel Slab with integral kicker Slab with kickerless construction Waterstop at junction to follow wall profile Drained cavity Sump formed in situ or separate drain which may be solid Internal block wall or perforated Floor slab with integral Wall cavity protection and/or added Access May incorporate Inner skin Pump membrane (internal or point(s) to drainage channel Concrete/steel piled wall external) drainage with pipe connection to setup Drainage channel May incorporate drainage channel with pipe connection to setup Pump Waterstop at junction to follow wall profile Sump formed in situ or separate drain which may be solid or perforated Floor slab with integral protection and/or added membrane (internal or external) Further alternative approaches have been developed, whereby hydrostatic pressure on the wall structure is eliminated. These methods incorporate a drainage blanket around the perimeter of the basement, allied to effective drainage below the floor slab and around the building. Suitability depends on the drainage characteristics of the ground and topography. A combination of systems can also be an appropriate design solution. For example, the application of additional waterproofing systems to a Type B structure will improve water vapour control or provide further protection against water ingress. Each of these methods is viable for domestic basements in Britain, depending upon the specifier s preference, site conditions, the type of development and perceived risk. Table 2 on page 12 provides a summary of appropriate waterproofing protection for varying risks associated with water table levels and useful additional measures to reduce risk dependant on project particulars [7]. BS 8102 should be consulted for further details. Type C: drained protection any water seeping through external walls and floor is drained to a sump via an internal cavity, typically created by a proprietary cavity system and pumped or drained away. Diagrams from The Design Guide, courtesy of TBIC, Double height concrete basement extension to existing property. Courtesy of ph+ architects. 11

12 Table 2: Types of waterproofing protection Risk associated with water table Water table classification* Waterproofing protection Type A Type B Type C Piled Wall Reinforced concrete wall to BS EN 1992 Low Low Acceptable Acceptable Acceptable Acceptable High Variable High Acceptable if the variable classification is due to surface water. The manufacturer s advice should be sought. Acceptable where: a) an appropriate cementitious multi-coat render or cementitious coatings are used; b) the wall is of concrete to BS EN Acceptable where: a) the piled wall is directly accessible for repair and maintenance from inside the structure; or b) the piled wall is combined with a fully bonded waterproofing barrier; or c) the piled wall is faced internally with a concrete wall to BS EN Acceptable Acceptable Acceptable Acceptable * The water table classifications are defined as follows: Low - where the water table or perched water table is assessed to be permanently below the underside of the base slab, this only applies to free-draining strata. Variable - where the water table fluctuates High - where the water table or perched water table is assessed to be permanently above the underside of the base slab. Ground permeability might affect risk under a low or variable water table Measures to reduce risk Use combined protection Incorporate appropriately designed sub-surface drainage and ensure that this is maintained Use a fully bonded waterproofing barrier Lower the permeability of the main structural wall Use concrete with a waterproofing admixture, e.g to BS EN 934 Ensure that discharge systems, e.g pumps, are maintained so that the system remains effective Waterproofing barriers This section describes in broad terms some of the options and issues associated with the choice of waterproofing system for domestic basements using Type A barrier protection. For more detailed guidance refer to BS 8102 and the Waterproofing Design Guide, by The Basement Information Centre. There are six categories of waterproofing barrier materials available. The following table shows where they can be located, subject to the form of supporting structure. In addition to those noted below, there are waterproofing membranes used in Type C construction. Table 3: Categories of barrier protection Categories External Sandwich Internal Bonded sheet membranes / / Liquid applied membranes / / Geosynthetic (bentonite) clay liners / / Mastic asphalt membranes / / Cementitious crystallisation slurries and powders Cementitious multi-coat renders, toppings and coatings / / / / Water stops Water stops are an essential part of the waterproofing design solution; for Type B protection used at the junction of structural panels, between walls and floors or along day-work joints for cast in situ concrete, the principle types can be classified as: a) Passive sections e.g. PCV water bars, located outside or within the structure to obstruct water transmission. b) Active strips or slurries (hydrophilic or crystallization) that react with water to prevent its further progression. These are set within the section of the structure, or post-injected. c) Specialist sealing resin injected into pre-positioned permeable hoses or similar. Design issues Particular attention should be paid to the specification of waterproofing systems - particularly for deep basements - relating to areas of high water table and in soils with aggressive chemicals. An appropriate specialist should be contacted for early advice and help on waterproofing design. Good design and workmanship are primary factors in achieving waterproof construction. Key considerations are compatibility of waterproofing systems, sealing around joints and junctions of the waterproof membrane and, for integral structural waterproofing systems, attention to the construction joints. Structural design may affect the choice of waterproofing and compatibility between the two is essential. For example, the stress and permissible crack width of a structure is controlled by reinforcement. In plain wall structures (i.e. not reinforced) the applied waterproof membrane needs to be appropriate to the anticipated movement of the structure, as the allowable movement or cracking may exceed the strain capacity of some waterproofing membranes. This is also a key consideration when refurbishing or extending basements, since movement between existing and new structures must also be anticipated. Details and construction profiles should be simple, avoiding nibs and thickening of structure wherever possible to prevent complicated junctions. Adequate details must be provided for each junction and considered in three dimensions (3D) for thoroughness. Although discontinuity with respect to waterproofing might be acceptable - subject to careful detailing and an appropriate assessment of risk - in practice this may not be allowed due to the need to manage radon, methane and other ground gases and contaminants. 12

13 Concrete construction Concrete is the most common and appropriate material used in the construction of new basement walls and floors. This is due in part to cost and availability but also its inherent resistance to water, durability under ground and ability to provide a stable structural surface for the support of waterproofing membranes. The method of construction chosen will depend upon consideration of various factors including: potential repetition of construction elements; accessibility for labour and cranes; cost; and fundamentally, the type of construction system permitted according to water table and use, as described in Table 2. Most forms of concrete construction can provide a variety of wall thicknesses to suit the particular structural requirements of each basement. Masonry construction or concrete blockwork Masonry construction or concrete blockwork is a traditional form of basement construction in the UK. It can be used with Type A waterproofing protection, for cases in which it is recommended that render or a similar smooth, continuous layer is applied to the blockwork face to provide continuous support to the waterproof membrane. Walls are typically reinforced and particular care is required at corner details and the wall slab junctions to cope with ground pressure. Masonry walls can also be effective as internal lining to create a drained cavity basement wall (Type C). Cast in situ concrete Cast in situ concrete is appropriate for all types of basement construction. It is a common form of basement construction for residential use, due to its relatively simple application, adaptability and cost. In-situ concrete is often the only appropriate form of construction for retrofit basements under existing properties, due to its relative ease of placement on site. As with masonry, in-situ walls are most commonly installed as reinforced structures but can be used plain (without reinforcement) following guidance provided in Addendum 1- Plain masonry and plain in-situ concrete retaining walls by TBIC. Typically, cast in situ walls are constructed with steel reinforcement bars to control cracking in the structure, with particular attention given to reinforcement of the corner junctions. Plain concrete walls are not generally specified as Type B construction due to the more critical need to control crack dimensions. Workmanship is a key issue for successful implementation of Type B protection. Water stops are included in the construction joints and particular attention is required with regards to day-working joints and the constituents of the concrete mix. Cast in situ concrete requires time to dry out before water sensitive finishes can be applied. Water-resisting concrete Concrete is inherently water-resistant and robust, making it suitable for subterranean construction. Its water resistance can be further enhanced by the introduction of admixtures. These admixtures (hydrophobic and pore blocking) act to reverse the capillary or sucking action of the tiny capillaries on the concrete surface and to effectively block the pores within the concrete when subjected to hydrostatic pressure. The result is a dry concrete that protects from water ingress. Such proprietary concrete mixes are available for this purpose from a number of specialist suppliers. Warranties can be obtained for products and workmanship on site. It is still possible for small levels of water vapour to pass through these types of concrete but they are generally very low and so unlikely to cause a problem. Additional membranes or ventilation may be considered, depending upon site conditions, proposed use and client or designers assessment of, and attitude to, risk. Insulating concrete formwork (ICF) ICF systems use either lightweight twin-walled expanded polystyrene (EPS) or extruded polystyrene (XPS) in panels or blocks to create formwork walls, for in-situ concrete walls, typically 100 or 150mm thick. Once in place, the formwork is filled with ready mixed concrete and, unlike conventional formwork, is left in place to act as insulation. For basement construction, polystyrene provides good background for waterproofing barriers. Care should be taken to ensure that the specification of the waterproofing membrane and its fixing methods are appropriate for application to polystyrene. ICF provides a cost effective, simple and inexpensive means for placing cast in situ walls; most appropriate for new build, rather than retrofit, basements. A new build basement using ICF. An example of concrete twin wall construction. 13

14 Precast concrete modular units Precast concrete units are increasingly used in Britain and elsewhere as a form of basement construction and provide an excellent support for waterproof membranes, either as a tanked membrane system or as the outer wall of a drained cavity or even as proprietary Type B system. Precast sections can be fabricated to specific design requirements for just-intime delivery, providing rapid on-site construction, integrated water bars, low site waste and high quality finishes. They are particularly appropriate for developments potentially benefitting from a high number of repeated standardised elements and the use of a crane on site. The waterproofing detail should be designed to suit the manufacturer s established method of joining panels. Twin wall This construction method is a hybrid of precast and cast in situ concrete walls and floors. Each wall unit comprises of two plates of precast concrete with a cavity between, linked by a lattice of steel reinforcement and placed on site; effectively as permanent concrete formwork. Once units and water bars are in place the cavity is filled with ready mixed concrete to complete the structural wall. Twin wall systems offer all the benefits of precast concrete described above but with the added benefit of continuous cast in situ concrete across the whole wall and, potentially, floor above. Concrete piles Piles are more commonly used for deep basement construction, rather than domestic situations and come in various forms, but can be useful for the creation of retaining walls to facilitate excavation in areas of restricted access or close to site boundaries. Secant or fair-faced contiguous piles can effectively become the outer wall of a Type C construction, or be faced with concrete or waterproofed to provide Type B or Type A protection. Further information on this and other forms of retaining structures can be found in The Concrete Centre technical publication Design and Construction of Concrete Basements. Concrete floors At basement level, floors are typically cast in situ concrete. The choice of system will be driven in part by coordination with the wall construction. Floors at ground floor level in housing can be constructed using a variety of different concrete construction techniques, including in-situ, block and beam, hollow core precast units or hybrid systems. Typically, it is possible and beneficial to span the full width of the basement space with the floor structure. Concrete easily exceeds the minimum building regulations requirements for fire and imposed loads and provides excellent sound insulation between the spaces. 14

15 Design issues Design principles The appropriate design of basements is well established and achievable, provided design and construction guidance is implemented. The general principle is to assess the risk of water reaching the below ground structure and to select an appropriate form of construction, structure and system of waterproofing to achieve the required internal environment. To do this the designer needs to understand the expectations of the client, the proposed and likely future use of the basement space and its associated performance requirements in terms of building regulations. It is essential that an appropriate site investigation is carried out to establish the soil and ground water conditions. Evaluation of these factors provides the basis for selection of an appropriate construction method, structural solution and system of waterproofing. It is strongly advised that a three dimensional (3D) review of structure and waterproofing is undertaken to identify and avoid any complex geometries, which will not be readily identified from normal twodimensional details. Basement design process (simplified) 1 Establish basement use; current and future flexibility 2 Site survey and exploratory works 3 Design proposals to define type of construction, water tight class and thermal performance 4 Detailed structural design integrated with design of waterproofing Roles and responsibilities Aspects of the design process are inter-related and there are likely to be a number of options available; particularly for straightforward residential properties. Minimising risk in basement design: Initial design should consider: Anticipated current and future use of basement Anticipated current and future ground water conditions Orientation of building relative to ground water Current and future daylighting and ventilation requirements Simplifying shape to facilitate waterproofing Location and access on site to facilitate construction Avoiding penetration of waterproof membrane for services where possible Site investigations should include: Appropriate qualitative assessment to appropriate depth Geotechnical investigation to indicate current and anticipated future ground water regime Tests to indicate soil properties and surface loading to establish lateral earth pressures Detailed design should consider: Correct choice of construction and waterproofing to suit ground conditions and use Integration of structural and waterproofing design to best practice recommendations Three-dimensional structural loads of building, ground and water pressure with attention to corners Access for future maintenance and alterations Obtaining specialist advice particularly for high water tables Construction should include: Supervision and checking (both essential) Experienced and skilled operatives Instigation of construction warranties In use: Maintenance and operation of drainage, pumps and ventilation systems Of particular importance for new-build basements is a unified approach to establishing an appropriate design solution and defining the roles and responsibilities of the design team from the outset. It was common for the design of the waterproofing system to be the responsibility of the architect however, in BS 8102: 2009 there is emphasis on including a specialist waterproofing advisor as part of the design team so that an integrated waterproofing system is created. This can be an architect or another consultant, manufacturer or supplier, provided they have the relevant expertise. An exception to this is when the construction method is classified as structurally integral protection ; when it may form part of the structural engineer s brief, a specialist waterproofing advisor may still be required. The client should be advised of any implications related to choice of construction and waterproofing with regards to the expected building use, future flexibility and associated maintenance requirements. 15

16 Basement use - current and future It is essential that the current and proposed use of a basement space is established early in design development, in order to provide the relevant performance criteria for the subsequent choice of waterproofing system, construction method and structural design. BS 8102 designates building uses against three grades of water tightness. These range from car parking areas, where some seepage and damp patches are tolerated, to ventilated residential and commercial areas where no water penetration is acceptable. Standards and forms of construction and waterproofing suitable for each grade of usage are provided. The previous edition of the British Standard (still referenced in the Approved Document - Basements for Dwellings) referred to Grade 4 environments. This was omitted in the later version since the only difference from Grade 3 is the performance level related to ventilation, dehumidification or air conditions. BS 5454 provides specific guidance related to the storage of exhibition or archival documents. Typical factors to be assessed in site investigation Existence of watercourse or seasonal position of water table Topography of land and direction of ground water movement Location of drains and land drains Soil type and conditions Movement risks - potential subsidence Presence of natural gases e.g. radon/methane Evidence of ground contaminants Boundary conditions A Grade 2 environment may be acceptable for permanent workshops or garages. However, since usage may change, it is better to construct a basement to a Grade 3 environment than to upgrade it later. In a high risk situation, the client and designer may wish to opt for additional waterproofing or vapour control. Site investigation The location and potential fluctuation of the water table is the key factor effecting basement design and construction. High water tables present the greatest risk for a basement and must therefore be identified at an early stage in the design. A watercourse or water table that rises and falls, and the potential for a perched water table, must also be identified. A high water table refers to, by definition, groundwater level consistently above the level of the basement floor. A permanently low water table involves a water table consistently below the level of the basement floor. A variable water table refers to levels varying between the two extremes. The installation of drainage systems can artificially lower the water table but is not always beneficial due to potential detrimental effects on neighbouring properties. Mirrors facing and adjacent to, window openings can significantly increase the perceived light levels. Courtesy of The London Basement Company. The draining ability of the soil and existence of contaminants can effect the choice of concrete construction and waterproofing method, as will the location of nearby drains and an assessment of the likelihood of their flooding. Table 4: Grades of basements Grade Basement Usage Performance Level 1 Car parking; plant rooms (excluding electrical equipment); workshops 2 Workshops and plant rooms requiring drier environment (than grade 1); storage areas 3 Ventilated residential and commercial areas including offices, restaurants etc; leisure centres Some seepage and damp areas tolerable, depending on the intended use* Local drainage might be necessary to deal with seepage No water penetration acceptable Damp areas tolerable; ventilation might be required No water penetration acceptable Ventilation, dehumidification or air conditioning necessary, appropriate to the intended use * Seepage and damp areas for some forms of construction can be quantified by reference to industry standards, such as the ICS s Specification for piling and embedded retaining walls. 16

17 Orientation and site layout The shape and orientation of a building should be considered because of the potential to dam the flow of ground water and the resultant build up of hydrostatic pressure. If unavoidable, additional subground drainage may need to be provided to discharge the water elsewhere. The form of construction of a basement and its cost will be influenced by the proximity of its walls to existing boundaries and adjacent buildings. The installation of external waterproofing and insulation, for example, requires sufficient space around the outside of the basement walls to provide a safe working area and may require temporary shoring. For new build constructions in tight - usually urban - plots, permanent underpinning of adjacent boundary walls or properties can allow valuable additional basement floor areas, but is expensive. An alternative is to install sheet piling to contain the ground supporting the structure while the new basement is under construction. Daylight The need to provide daylighting and comply with building regulation requirements for ventilation of habitable rooms will generally be met by incorporating openable windows in the same manner as above ground. This may entail adjusting the external ground levels in partially belowground basements, and would mean forming open areas for windows in fully below-ground basements. A primary factor in improving the quality of a room in a basement is the provision of natural light. Inclusion of glazed windows or doors provides greater possibility of future adaptation and uses, as well as sustainability benefits by reducing dependence upon artificial lighting. There are many techniques for improving the level of natural daylight and ventilation in basement spaces; determined by various factors including the proposed use of the space, proximity to boundary and plot size. Effect of building orientation on flow of ground water Flow of ground water Solutions for habitable spaces include simple direct lighting through windows, glazed doors or roof lights. Other supplementary solutions include the use of sun pipes or use of borrowed light with mirrors, glazed floors or stairwells. Daylighting techniques Partially sunken light wells and windows. Plan form of building avoids possibility of damming the flow of ground water Flow of ground water Full depth external spaces with glazed doors providing separate private amenity space and potential access to the garden or alternative entrance from the street. Sunpipes and pavement lights are suitable for basement spaces extending beyond the footprint of the building above. They provide permanent natural lighting with additional security but limited views. Glazed roof lights can wash spaces with natural light and provide sky views and natural ventilation if openable. Provide subground drain discharging to a suitable outfall to alleviate hydrostatic pressure where necessary Mirrors facing and adjacent to, window openings can significantly increase the perceived light levels and provide depth of field. Light and/or polished surfaces will generally improve the sense of space and daylight levels in a room. Glazed floors, particularly below upper floor roof lights or windows, can be useful additional sources of light but will require fire-rated glazing to maintain fire compartmentation between floors. Light from upstairs rooms can brighten lower ground floor spaces via the stairwell. This arrangement will depend upon the specific fire arrangements of individual properties and may require an upgraded fire resistance or detection system. Light from the upstairs room can brighten lower ground floor spaces via the stairwell. Image courtesy of Loates Taylor Shannon architects, Paul Avis photography. 17