Cavity drain membrane systems
A cavity drain membrane is the most widely used way to keep a habitable basement dry in the UK. Rather than trying to block water, it lets water drain away in a controlled way.
How it works
A studded plastic membrane is fixed to the walls and laid on the floor, creating a narrow air gap. Any water coming through the structure runs down behind the membrane into a drainage channel and on to a sump, where a pump discharges it. BS 8102:2022 calls this Type C drained protection.
Why it is popular
- It copes with wetter conditions and imperfect walls
- It does not rely on a perfect seal against water pressure
- Channels and sumps can be inspected and maintained
Things to plan for
- Pump maintenance: pumps need regular servicing, and a battery back-up or second pump is commonly advised.
- Space: the membrane and wall finishes take a few centimetres from each wall.
- Fixings: plugs through the membrane must be sealed correctly.
Specialists sometimes combine a cavity drain with tanking. Costs are summarised in our cost guide, and the wider picture is on basement waterproofing.