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Upstairs, downstairs

A century ago, basements were a common feature of housing. It's about time we started building them again

While children have always been excited by the concept of a basement, it's not so clearcut for adults. For the young, it is a great adventure playground - for parents it's a mixture of spiders, spooks and serpula lacrymans.

It is common in mainland Europe and North America for buildings to be constructed with a basement and anyone visiting properties abroad must be curious about their absence back in Blighty.

There are many ways of designing and constructing basements. They can be fully or partially buried (see diagram right), the latter allowing the provision of high-level basement windows, as per the standard European construction.

They can be internally or externally accessed with vehicular access as necessary and they can be constructed in, inter alia, insitu concrete, trench block or masonry.

Indeed, in Canada, it is not unusual to construct basements in timber! In a triedand-tested procedure, concrete rafts are cast with no damp-proof membrane, with preservative-treated softwood sole plates fixed to the perimeter and insulated studwork built up on them.

Often the external surface is no more than treated 22mm oriented strand board with a drainage filter layer before backfilling. According to the Canadian Home Builders' Association these structures have performed well for over 30 years, (see Canadian Standards Association, Construction of Preserved Wood Foundations, CAN3-S406).

Perhaps the success of this construction - in a country with a higher average annual rainfall than London - should raise queries about the belt-and-braces approach to basement construction in this country.

Stability Basements constructed from new should not cause any worries. Ground conditions must be ascertained at the outset to gauge the level and pressure of the groundwater and the bearing capacity of the surroundings. Ideally, the site should consist of well drained, dry, granular soil.

Creating a basement under an existing building is trickier, although an issue of Practical DIY 25 years ago ran a guide on how to underpin your own house. Provided that underpinning is done with professional care - ensuring that all existing structural walls are undamaged and remain shored or braced - drainage connections can be laid to falls, or with pumped connections.

With affordable internal access and egress, basements provide options for dwellings short of space. It is preferable for an existing structure to have a suspended ground floor. This will enable the bulk of the excavation to be carried out with the minimum of disruption.

In new-build plots or conversions alongside existing structures, it is likely that a party wall agreement will have to be entered into. Due to the depth of excavation necessary there is the possibility that it could undermine the stability of adjacent terraced or semi-detached properties. Given that the developer is obliged to pay for the full party wall award appointments and that neighbours will probably not wish to agree to the project without assurances, this might add considerably to the time and cost of the procedure and should be factored into the architect's programme.

Costs In urban developments, careful design of the floor plate of a domestic dwelling could double its conventional floor plan by using a roof space and a basement. Theoretically, buildings with basements could reduce the land area required, while maintaining the same-space requirements of a conventionally constructed house (see cost example left). Conversely, a bungalow with a basement can reduce its visual impact in sensitive sites, but still retain the spatial comforts of a house.

All these benefits can be translated into savings with reference to plot size (land is around 30 per cent of total construction costs), cost per square metre, or professional costs saved avoiding planning inquiries.

On a sloping site requiring a large proportion of cut and fill, the basement - whether as habitable space or as an undercroft garage - is a definite cost benefit. Given a good site, with granular, well-drained soil, even a full excavation of a basement need not be cost prohibitive, as shown in the generic cost details. Indeed, in an estate site plan, the reduced plot size can improve densities.

Terraced layouts can be even more costeffective.Depending on the size and type of construction, costs can vary between around £300/m 2for basements and £150/m 2for conventional foundations.

Depending on complexity and the range of finishes and fittings, actual build costs are fairly similar. If site layouts are arranged to take advantage of economies of scale, then the resultant reduction in land costs per plot make the choice of basements even more attractive.

Damp

Damp is the biggest fear of property owners when considering basements. But why do we have such fears about waterproofing? The principle of coating the building in a waterproof skin and draining away external water is straightforward.

There are a variety of details to address this issue (see right), which are described in BS 8102 as follows:

Types A: tanked protection systems rely totally on the application of a waterproofing membrane or layer. Whether applied externally (and protected), internally or sandwiched between layers, the waterproof layer must be able to withstand hydrostatic pressure from groundwater, plus any imposed service loads.

Type B: structurally integral protection relies on the structure itself to be sealed and water-resistant. This is often more expensive and depends on careful concrete manufacture. The design should comply with BS 8007 or 8110 and the waterproof concrete should not be relied upon to act as a vapour barrier.

Type C: drained protection relies on a drainage cavity or membrane in the structure to collect groundwater seepage and drain it away. The concrete structure should be sufficiently dense to limit water ingress to ensure that the system can cope.

It must also provide a high level of water vapour resistance.

The demands for increased air tightness in basement construction will ensure that concerns about vapour resistivity will be reduced, provided that the vapour-control layers are tied in and pressure tested.

Kel lerbau UK is patenting a process of pouring a 1200mm basement wall and slab as one, thus avoiding a joint between slab and wall - the weakest point from a waterproofing and structural point of view.

Conclusion Basements provide fantastic opportunities. We should follow the lead of the US and Canada in making basements into a games or leisure space, rather than building another storage area; the traditional function of the British garage. With increasing demands on land use, the cost of building extensions, and the tendency to create home havens away from the world of work, basements are overdue for reassessment.

The British Cement Association and Basement Development Group have four basic design guides dealing with costs, design, thermal performance and market evaluation. From Basement Development Group. Tel 01344 725737, or see its website (www.basements.org.uk). Kellerbrau, West Kirby. Tel 0151 625 2604. Costs have been provided by Jim Naylor, Gateshead, 0191 489 3771

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