Is cavity drain membrane better than tanking?

For existing structures - especially those with irregular or cracked masonry - cavity drain membrane (Type C) is generally more reliable than tanking (Type A). Tanking relies on a perfect bond to the substrate; any defect or crack allows water through and the whole system can fail. Type C accepts that water will get in and manages it, so minor substrate defects do not compromise the system. BS 8102 recommends combining two complementary types for habitable spaces.

Do I need a sump pump with a cavity drain membrane?

Usually yes. The drainage cavity directs water to a sump chamber, which requires an electric pump to discharge the collected water. For resilience, a dual pump system (one mains, one battery backup) is strongly recommended - a single pump failure during heavy rainfall can allow the sump to overflow. The drainage design should ensure the sump has sufficient capacity and the pump adequate flow rate for the anticipated water ingress.

Damp & Waterproofing noun

Cavity Drain Membrane

Q: What is a cavity drain membrane?

A cavity drain membrane is a studded plastic sheet fixed to the walls and floor of a below-ground structure. The studs hold the membrane away from the wall, creating a drainage cavity behind it. Any water that penetrates the structure flows down through this cavity to a perimeter drain channel and sump, where it is pumped away rather than entering the habitable space. It is Type C (drainage) waterproofing under BS 8102.

/ ˈkavɪti dreɪn ˈmembreɪn /

Also known as: Type C waterproofing, studded membrane, delta membrane, sump and pump system

Definition

A cavity drain membrane is a studded plastic sheet fixed to the internal walls and floor of a below-ground structure. The studs hold the membrane away from the surface, creating a drainage void behind it. Any water that penetrates the structure flows down through this void to a perimeter drain channel, collects in a sump, and is pumped away - keeping the habitable space dry without trying to stop the water at the wall. Under BS 8102 this is classified as Type C (drainage) waterproofing.

In practice

Cavity drain membranes are the preferred solution for converting existing cellars and basements because they tolerate substrate imperfections that would cause tanking systems to fail. The membrane is fixed to walls using proprietary plugs, lapped at joints, and sealed at the floor-wall junction. A perimeter channel collects water from both the wall membrane and floor, directing it to a sump chamber.

The sump houses one or more submersible pumps discharging through a small-bore pipe to a drain or soakaway. For any habitable basement, BS 8102 strongly recommends a twin-pump arrangement - one mains-powered, one battery-backed - to guard against pump failure during heavy rainfall. The finished wall surface is typically plasterboard on timber battens fixed over the membrane, leaving a serviceable cavity that can be accessed if needed.

Building Regulations

Relevant PartPart C - Resistance to moisture

Key standardBS 8102:2022 - Protection of below-ground structures

ClassificationType C - Drainage waterproofing

Pump redundancyTwin pumps recommended for habitable spaces

BS 8102:2022 recommends that habitable below-ground spaces use at least two complementary waterproofing types. Type C cavity drain is often combined with Type A tanking or Type B integral waterproofing for a dual-protection approach. Building Control will assess compliance with Part C for any basement conversion creating habitable accommodation.

Full Building Regulations guidance

Cavity Drain Membrane

Just so you know...