Digging deep for value

To create more space in a tight plot of land, the once-popular basement could be the solution, as Delta Membrane Systems’ Director Brian Davison explains

If there is a single criticism that makes a regular appearance when talking about new housing, it is space – or, more accurately, the lack of it. Current housing demands call for land to take as much housing stock as is practical, which often leaves the future occupant with little or no room for growth.

What was a common site in housing in the Victorian era seems, for some reason, to be overlooked nowadays. One area where extra space can be created, at the construction stage, is in the ground itself – the formation of a basement area. This provides a permanent, effective, extra living space without eating into the footprint of the dwelling from a planning point of view.

Our European neighbours – Germany, Belgium and the Netherlands – have all realised the benefits of basements, where they are the norm in new-build construction.

For the housebuilder and developer, the perceived value for money that a basement can bring when included in the price of a property is a valuable sales tool. The benefits to the prospective home owner can easily be emphasised, improving potential sales – particularly when comparing against other new properties in the locality.

The obvious concern for such a structure is waterproofing or tanking, to ensure the area remains damp free and allowing it to be a useful area of the house. There is no point in creating a space that is not used because it is perceived as being damp, dark, dingy and uninviting.

As you might expect, there is plenty of legislation and advice guiding us the right way to ensure the creation of a basement is to certain standards.

For example, BS8102:2009 is a Code of Practice which looks at protection of below ground structures against water from the ground. A more elaborate expansion of this standard can be found in CIRIA Report 139 Water Resisting Basements.

Other relevant British Standards include BS8110 Code of Practice for reinforced concrete design, and BS8007 Code of Practice for the design of concrete structures for retaining aqueous liquids.

The Basement Information Centre has an Approved Document – Basements for Dwellings. Reference could also be made to the Basement Waterproofing Design and Site guides published by the British Cement Association. The Property Care Association’s Code of Practice for remedial waterproofing of structures below ground is also available.

Waterproofing below ground level, or tanking, usually involves the application of a layer of cementitious waterproof render system on the walls, linked to a waterproof screed on the floor. Tanking can also be carried out using a sheet membrane, asphalt or other liquid-applied waterproofing material.

Hydrostatic pressure – the external water pressure around the basement – is also a critical factor to be considered. It is crucial that the tanking is securely fixed to the substrate, as the pressure from the water table around the basement can be significant. Hydrostatic pressure will force water through tiny gaps very quickly, so great care should be taken at this stage to ensure the waterproofing will meet the demands made of it.

Cavity membranes are a suitable alternative to tanking. Membranes with a studded profile can be used to form an inner waterproof structure. The studded side is placed against the wall, creating an area that allows water to flow down to the floor. Here it flows in a drainage channel to a sump, and is then pumped out to a suitable drainage outlet.

In other words, rather than preventing damp from entering the structure, it manages the ingress of water. With external tanking, any slight imperfection will result in damp entering the building, and it then becomes a time consuming and costly repair job that will mean extensive excavation work.

This method is also the number one choice in refurbishment work, as the amount of construction work needed is greatly reduced compared with the alternatives.

A typical cavity membrane for walls is made from high density polyethylene. While the membrane itself is just 0.6mm thick, it features 8mm studs on the wall-side and it is these studs that create the drainage channel – allowing up to 135 litres/minute/m or 8,100 litres/hour/m of incoming water to be drained which is well in excess of the likely demands.

The construction provides more than 1,800 dimples/m², giving an air volume between the studs of about 5.3 litres/m². This dimpled sheet offers a compressive strength of greater than 250kN/m². The floor must not be forgotten, and a suitable high capacity drainage membrane should be specified. Good examples feature 20mm studs which offer an impressive compressive strength figure of 150kN/m².

With a contact surface to the ground of about 130,000mm² per m², the design ensures good pressure distribution and low point loads.

Both the wall and floor membranes offer the advantage that, once fixed, they can be worked on immediately. This allows the wet trades to get started to apply plaster or screed flooring.

Basement construction does need a little more effort than the rest of the property, but the benefits for the future householder are substantial, and this has to be a valuable sales benefit for the property. Whether it be a speculative one-off property, or a site of many dwellings, developers and housebuilders have the chance to bring more to the table – with the unmistakable enhanced value for money being a major sales tool.