Cavity Drain Membranes Explained: The Gold Standard for Basement Waterproofing

When it comes to waterproofing a London basement, there are several systems available. But for existing basements and basement conversions, one system consistently outperforms the others: the cavity drain membrane. As Delta-approved contractors, we install cavity drain systems on the vast majority of our waterproofing projects. Here is why.

A cavity drain membrane — also called a Type C waterproofing system under BS 8102 — is a studded plastic sheet fixed to the walls and floor of a basement. The studs hold the membrane away from the wall surface, creating a void or cavity between the membrane and the structure.

Any water that penetrates the concrete or masonry structure flows into this cavity rather than into the living space. The water is then channelled by gravity to a drainage channel at the base of the walls, which directs it into a sump pit. A sump pump removes the collected water from the basement.

Tanking — a Type A waterproofing system — works by applying a waterproof coating or membrane directly to the wall surface, attempting to stop water from entering at all. Cementitious tanking, bituminous coatings and crystalline waterproofing products all work on this principle.

Tanking can work well in certain conditions, but it has a fundamental weakness: it relies on the bond between the coating and the wall remaining intact under hydrostatic pressure. In London, where water tables are high and clay soils exert significant pressure against basement walls, this bond can fail — and when it does, water gets behind the coating and the system fails.

A cavity drain membrane does not try to stop water. It manages water. Even if water penetrates the structure — which it will, eventually, in most London basements — the system channels it safely away without affecting the living space. This is why cavity drain systems are considered more reliable in the long term.

Delta is the UK's market-leading manufacturer of cavity drain waterproofing products. Their range includes membranes for walls and floors, drainage channels, sump systems and accessories designed to work together as a complete waterproofing solution.

As Delta-approved contractors, we are trained and assessed to install their systems correctly. This means you receive a manufacturer-backed guarantee on the waterproofing system, the installation meets Delta's quality standards, and you have recourse to Delta if any issues arise with the system.

Wall membrane The studded membrane is fixed to the basement walls using plugs that penetrate the membrane at the stud positions. The membrane is lapped and sealed at joints to ensure continuity.

Floor membrane A floor membrane is laid over the concrete slab before the screed is poured. This prevents moisture rising through the floor slab.

Drainage channel A perforated drainage channel is installed at the junction of the wall and floor, collecting water from both the wall and floor membranes and directing it to the sump.

Sump and pump The sump is a pit in the floor into which the drainage channels discharge. A submersible pump removes the collected water. We always install dual pump systems with battery backup.

Ventilation Cavity drain systems require ventilation to prevent condensation building up within the cavity. This is achieved through ventilation plugs in the wall membrane and, where necessary, mechanical ventilation.

Cavity drain systems are suitable for most London basement waterproofing projects, including converting an existing cellar, waterproofing a new basement construction, remedial waterproofing where a previous system has failed, and basements with active water ingress.

They are not suitable for basements that are completely dry and have no history of water ingress — in these cases, a simpler system may be appropriate.

Contact our team for a free waterproofing survey. We will assess your basement and recommend the most appropriate system. Call 07405 046475 or 020 8087 4829.