Waterproofing systems

Always consider external support measures when remediating a damp basement.

Where a basement or cellar is only used for car parking, storage of non-valuable items or perhaps just as a casual work area, you might only need to implement external support measures to produce a suitably dry internal environment. You might be prepared to use the space as long as it does not flood too often, suffer too much water ingress or damp patches.

The concept of 'total waterproofing' is used in the BCA Design Guide – Basement Waterproofing. The success of waterproofing might depend not just on the applied waterproofing, but also on how we control moisture conditions inside and outside the building envelope to produce an acceptably dry space.

We have already touched on the importance BS 8102 places upon suitable measures to control the internal environment – to beat the dampness enemy within by careful control of heating and ventilation. But what about wetness from outside?

Turning to section 3.2 of BS 8102, there is a useful section on 'preventative measures'. Our total waterproofing approach can include control of surface water and ground water. The more we can assist the waterproofing, the greater the chance of success.

In section 3.2.2 'Exclusion of surface water', it is recommended that adjoining ground should slope away from the structure for 3m, and should not be open-jointed. Open-jointed pavings can trap and hold moisture and enable it to drain towards the property. But remember too that jointed pavings collect and drain off considerable volumes of surface water, which cannot be allowed to impinge on the property perimeter. Cut-off drains are advised for adverse ground slopes.

Section 3.2.3 then offers measures to improve or maintain subsurface drainage; for example, by preserving existing land drains or laying new roddable drain pipework in straight lines. The subsurface land drainage could discharge by pump or gravity, with open-jointed or perforated pipes protected from silting by graded filters.

One basic principle needs to be mentioned here. Coarse soils – for example, coarse gravels, chalk, rubble, rocks – offer good drainage and are not likely to significantly shrink or swell to cause heave or subsidence from changing moisture content. However, fine soils – for example, silts, clays, very fine sands, peat – drain poorly, and are often likely to significantly shrink or swell with changing moisture content. Soils that shrink or expand markedly can threaten waterproofing systems that are unable to cope with the resultant movement stresses. Such soils also drain inefficiently, so think about incorporating perimeter subsoil drains to reduce the water load impinging on perimeter basement walls below ground.

Water may impinge on basement walls around and below ground level where surface water drainage around a building has been ill thought out. Rainwater must be diverted away from the building perimeter, or directed efficiently into underground drains.

Hard surfacings may enable rainwater to splash against porous walling at low level to penetrate into external walls – causing damp patches in basement rooms – typically at high level where the internal floor level is well below external finished levels.

Gary Branson devotes considerable advice for home owners on reducing perimeter damp penetration at low level. In a sketch entitled 'Common Sources of Wet Basements' we see a rainwater downpipe discharging water close to an external wall, with water percolating downwards to soak into basement walls. He shows us how a 'splashblock' could help here. A splashblock is a chute located under the rainwater shoe (i.e. final bend at base of rainwater downpipe) that diverts rainwater away from the wall. An alternative is to extend the bottom section of the rainwater pipe away from the building.

These methods are unnecessary if the rainwater downpipe links properly to the gully. But too often, especially in heavy rain, water tumbles down inside the rainwater pipe to gush out in profusion, some of it missing the gully. The gully needs to be large enough to accept the rainwater discharge, and ideally the rainwater pipe should feed into the gully by a patent connector. The gully may need to perform 2 functions. It could be accepting rainwater from a vertical pipe, as well as surface water from a path or yard – in which case the gully must be seated at the correct height.