Tanking vs Cavity Drainage Membrane: Which Basement Waterproofing System Is Right for You?

Choosing a basement waterproofing system is one of the biggest decisions a London homeowner can make. The method you pick decides how well your cellar resists groundwater. It also decides how much mess the work causes. Finally, it decides whether you can use the space for storage, a utility room or a living area.

Drywaterproofing Ltd has advised owners across Central London basement work, Kensington & Chelsea basement work, Hampstead basement work, Islington basement work, Camden basement work, Wandsworth basement work, Wimbledon basement work and Richmond basement work for over 30 years. We carry out a free survey to check ground conditions before we recommend a solution.

What is Type A tanking?

Type A tanking, often called basement tanking, is a cementitious barrier system. We mix a specialist slurry on site and apply it to the inside faces of the walls and the floor. It forms a continuous, water-resistant layer that blocks moisture. Because the material bonds to the wall, it becomes part of the building. You can then finish it with plaster, render or a decorative coat. Our basement tanking page shows the process.

The main advantage of tanking is its simplicity. It needs no outside digging, no drainage channels and no machinery. It works well in lower-risk cellars where the water table is modest and the walls are sound. However, the system relies on the coating. Cracks, movement or poor prep can open a path for water. It also copes less well with high water pressure, which can push water through even a well-applied barrier.

What is a Type C cavity drainage membrane?

A Type C cavity drainage membrane takes a different approach. It does not try to stop water at the wall face. Instead, we fix a studded polyethylene membrane to the inside surfaces. This creates a drained cavity behind the finish. Water that gets in runs down the studs to a perimeter channel. It then flows to a sump chamber, where an automatic pump removes it.

Because the system manages water instead of blocking it, it copes far better with high water tables and shifting ground. London’s period homes often move a little. The membrane also leaves a dry, ventilated gap behind the finish, which cuts the risk of condensation and mould. The trade-off is that the pump needs power and the sump needs regular checks. Fitting is also a little more complex than a tanking coat. Our cavity drainage membrane page explains the parts.

Type B: waterproof concrete construction

Type B means waterproof concrete. The mix contains special additives, or the builder adds careful joint details, so the structure is watertight from the start. This method is most common in new-build basements, where the designer controls the concrete, the steel and the joints. It works well when done right. However, it is rarely an option when you retrofit an existing London home, so we mention it only for completeness. Our guide to Type A, B and C waterproofing under BS 8102 covers it in full.

Which system is right for your basement or cellar?

The decision hinges on a few practical factors. Here is a quick guide:

• Groundwater level and pressure — high or variable water tables favour a cavity drainage membrane; low, stable conditions may allow tanking.
• Intended use of the space — habitable rooms, home offices or gyms benefit from the dry, ventilated cavity of Type C; simple storage or utility areas can often use tanking.
• Property age and construction — Victorian and Edwardian homes with brick or stone walls and limited outside access usually suit a membrane, because it avoids invasive external work.
• Structural movement — buildings that move with the seasons or settle do better with a system that tolerates small shifts.
• Budget and disruption — tanking is generally quicker and less disruptive, but its long-term reliability must match the risk on site.

Both Type A and Type C systems meet the performance criteria in British Standard BS 8102. That standard sets the frame for design, fitting and testing. A professional assessment will confirm which grade of protection your project needs. For cost, see our guide to basement and cellar tanking cost in London.

Can tanking and cavity drainage be combined?

In some cases a combined approach gives the best of both. A common example is a tanking coat on the floor slab with a cavity drainage membrane on the walls. Another is a tanking render on the lower wall, where pressure is greatest, with a membrane above. Any water that slips past the barrier still reaches the cavity and the sump, so you get back-up. We assess each site on its own, and we only suggest a combined system when the survey shows a clear benefit.

FAQ

Is tanking or cavity drainage membrane better? Neither is better in every case. The right choice depends on groundwater, how you plan to use the basement and how the property was built. A free survey will pick the best method for your situation.

Can I convert my cellar with either system? Yes, both can support a cellar conversion. However, a habitable room usually needs the higher protection and ventilation of a Type C membrane. The survey will confirm the grade you need. See our cellar conversion service for more.

Do I need a sump pump with a cavity drainage system? Yes, because the system collects water behind the membrane and must remove it on its own. The pump sits in a sealed sump chamber and runs on mains power. We check the whole system when the install is complete.