Waterproofing systems

Any sheets need to be lapped or joined in some way – a potential weakness is an opportunity for water ingress. Sheet materials are likely to be fairly rigid, and therefore require considerable skill in application over complex or non-flat substrates. However, a sheet material is of manufactured uniform thickness and reliability.

Sheets are cold applied or heat bonded to the substrate. On the positive (i.e. outer) side of the structural wall, they may successfully withstand pressures from soil and water, but on the negative (i.e. inner) side of the structural wall, may need a loading wall for stabilisation against applied forces. Bonded sheets are better able to cope with minor movement than more brittle multi-coat render systems. The substrate needs to be free of surface water for a good bond to be achieved. This kind of waterproofing would mostly be used for new works.

Modern products include toughened polyethylene carriers with polymer modified bitumen coatings, supplied in rolls and used internally or externally. Standard sheets are between 1.5 and 3mm thick. The sheets are used in combination with standard preformed units to waterproof at corners and details. This kind of membrane is designed to cope with 'normal structural movements'. Light torching enables a continuous weld to be achieved at laps. Bonded membranes can achieve all BS 8102 basement grades 1–4. The sheets link neatly from wall to floor, if needed, to create a ‘tanked’ waterproofing.

Figure 6: Internal tanking floor wall junction. Diagram courtesy Ruberoid Building Products, www.ruberoid.co.uk

As for all waterproofing methods, preparation needs to be carefully executed. You need, for example, to flush point all exposed brickwork and then prime before the application of sheets.

The downside of using bonded sheets is the requirement for loading of sheeting by internal loading walls and screeds – where room width and ceiling height is then sacrificed. Modern systems may be protected by patent boards, rather than walls and screeds. Once loaded, or hidden under protection boards, it is difficult to repair bonded sheeting due to lack of access. We find similar issues in flat roofing, where a ballasted roof finish prevents inspection of the waterproof membrane. You need to make sure of the shelf life of sheeting rolls as the performance of the adhesives might reduce over time stored.