Tanking a Basement or Cellar: What’s Involved and What to Ask
4 August 2026 · 2 min read
“Tanking” refers to making a below-ground room watertight against groundwater pressure, using either a cementitious waterproof render system or a cavity drain membrane (a dimpled plastic sheet fixed to the walls and floor that channels any water that gets through down to a sump and pump, rather than trying to block it entirely).
Cementitious tanking vs cavity drainage
- Cementitious (render) tanking forms a rigid waterproof barrier directly on the wall and floor surface. It works well where water pressure is moderate but can fail if the structure moves or cracks, since a rigid barrier has no flexibility.
- Cavity drain membrane systems don’t try to stop water reaching the wall at all — they manage it, channelling any water that does get through down to a sump pump. This is generally considered more reliable for habitable spaces, particularly where water pressure or structural movement is a concern, but it takes up more internal space and needs a working sump pump (with a backup power supply ideally) to function.
The British Standard
Basement waterproofing in the UK is generally specified against BS 8102, which defines different “grades” of waterproofing depending on how the space will be used — a grade suitable for storage is not the same as one suitable for a habitable bedroom or living room. Ask any contractor which grade they’re specifying and why it matches your intended use of the space.
Before committing
- Rule out cheaper causes first — blocked external drainage, ground level built up against the wall, or a leaking gutter can sometimes explain the whole problem.
- Get an independent structural/damp survey rather than relying only on a quote from the company that will also do the work.
- Ask about warranty length and whether it’s insurance-backed.
- If a sump pump is part of the system, ask what happens during a power cut, and whether a battery backup is included.
Because tanking is structural work with real consequences if done badly, this is one of the few areas where paying for an independent professional opinion before commissioning the work is genuinely worth the cost.