Waterproofing systems

At some point you may have discovered 'newtonite lathing' when stripping out an old building. The lathing comprises a black corrugated bituminous impregnated sheet, usually fixed with galvanised nails. It is the forerunner of the modern plastic dimpled sheet waterproofing membranes we use today. Most surveyors talk positively about the original newtonite lathing, which used 'air gap technology' to manage dampness in traditional walls. Newtonite lathing was last used in 1991. The original company, John Newton and Company, pioneered the modern cavity membrane systems used today to damp-proof walls, both above and below ground, which can incidentally also be used for external render systems in new and existing buildings. This category of waterproofing falls within Type C in BS 8102.

Things have moved on here. There are now a range of patent cavity drain systems – and this type of waterproofing seems to be growing considerably in popularity. In the BRE Good Repair Guide 23 Treating Dampness in Basements you will see mention of the 'drained cavity' method, and in figure 2 you will find a traditional detail of a drained system – where an inner block wall, tied to the main outer wall by wall ties, creates the cavity. The cavity is shown 50mm wide – so we lose 150mm for each perimeter wall around the basement space using this method, in addition to the thickness of the solid plasters. The BRE advises 2 alternative drained floor options:

• the use of triangular drainage tiles (which may be difficult to obtain from mainstream builders' merchants); or
• the use of dimpled sheet (obtainable from all the manufacturers of drained cavity waterproofing systems).

The drain at the base of the wall cavity is formed using a screed to falls – not easy to execute, and shown linked to a drain outlet. The bottom course of the inner wall is built of engineering bricks. In principle this system should work and be serviceable for many years, especially if the cavity drain can be rodded out periodically.

Nowadays there are cheaper ways of forming a cavity drain system, using dimpled sheets for the walls as well as the floor. Such sheets incorporate studs to create an air gap between substrate and sheet of typically 8mm. The outer face of sheeting is meshed so a wet plaster finish can be applied if desired. The concept is simple. By placing a dimpled sheet against the external earth retaining walls, an air gap is created between external wall and the lining. This kills off at a stroke hydrostatic pressure that in other systems acts to push waterproofing off a substrate. Any water ingress is dissipated over an area, rather than impinging on waterproofing at a single pressure point.

Water ingress is therefore dealt with easily. Any physical water trickles down the inner face of the external wall, and is drained away by perimeter drainage channels, either directly out of the building, where levels allow, or to sumps where it is pumped away. Servicing of pumping equipment is an ongoing maintenance outlay, as is rodding out of perimeter drains.

Modern systems using plastic dimpled sheeting take up little room space – as the sheets are quite thin – saving width and height of space internally. In some respects, cavity drain waterproofing systems for basements perform like masonry cavity walls: any water ingress runs down the inner face of the outer leaf to either drain out of the cavity via weep holes, or to evaporate away by ventilated cavity.

There are strong arguments for continuing the membrane for full height up subject walls, even if the floor level is not far below external ground level. First, you have the opportunity to take the membrane up into the ceiling void, which then offers a top ventilation opportunity; and second, you will not face the difficult problem of merging new plaster to existing plaster thicknesses. Original plasters may be no more than 20mm thick, and the finished thickness of the plastered membrane a little more – meaning you could find perfect merging of new to old a problem, possibly requiring a dado rail to mask it. It is also difficult to avoid a tiny shrinkage crack at the join of new to existing work – which could create a decoration problem.

The BRE Guide also shows a 'ventilated dry lining' option in figure 1, where a dimpled sheet is shown affixed to an external wall in conjunction with a cement/sand screed. However, no drainage provision is provided at low level. The BRE rightly comment that such a system will cope only 'if the dampness is slight'. Where no perimeter drain exists, in conjunction with a solid floor as per figure 2, there is nowhere for any water ingress to drain. The BRE detailing shown is potentially risky, as quite a high external ground level is actually shown. Risk of water ingress could be reduced by installing an external land drain – which is not shown or mentioned by BRE.

Beware: cavity drain systems need to be well maintained so that any water ingress drains away efficiently. The dimpled lapped sheets are not designed in themselves to cope with direct water penetration and, should perimeter drains silt up, water ingress could occur. The sheets themselves cannot on their own cope with hydrostatic pressure. Silting can build up under dimpled floor sheets, damming up any draining water.

There have been failures of cavity drain systems when, for example, the subfloor has enabled ponding of draining water, or deposits of lime have built up. While a perimeter drain should incorporate rodding points, the gap under a ventilated membrane would be difficult to access. It is common for back-up pumps to be installed, sometimes powered by a separate power supply. This aspect of drainage design needs to be carefully thought out.

The exclusion of penetrating dampness from external ground to habitable space depends not just on the cavity drainage on the inside wall face. The BCA Design Guide reminds us that an external basement wall should resist water ingress sufficiently to ensure that the cavity only has to cope with a controlled amount of water or dampness.

Figure 3: Basement plastering in action. Brick vaulting is waterproofed using ventilated cavity membranes, linked to perimeter 'base drains' with sumps. This system can be used below or above ground. In an above-ground scenario, the damp threat could be from above, e.g. a converted railway arch. Detail sheet courtesy of J. Newton and Co., www.newton-membranes.co.uk