Cavity Drain

A cavity drain system is a Type C approach to waterproofing below-ground structures, as classified in BS 8102. Rather than attempting to resist water at the structure itself, a Type C system accepts that water may enter and controls it by diverting it safely to a drainage point. A studded membrane is fixed to the internal faces of walls and laid across floors, creating a drainage void between the membrane and the substrate. Water that passes through the structure is captured within this void and channeled to a perimeter drainage system, from where it is diverted to a sump and pump or a gravity drain for disposal.

This approach is suitable for basements, sub-structures and retaining walls, and can be used in both new build and retrofit projects. It is particularly well suited to retrofit waterproofing of existing structures where achieving a reliable Type A tanking barrier would be difficult or impractical, as it does not rely on the structural envelope being fully watertight. When correctly specified and installed, a cavity drain system is capable of providing Grade 1, 2 and 3 levels of protection as classified in BS 8102. BS 8102 also requires that Type C cavity drain systems are maintainable, meaning the drainage channels must remain accessible for inspection and maintenance throughout the life of the structure.

Cavity Drain Membranes

Cavity drain membranes are high density polyethylene studded sheets fixed to the internal face of below-ground walls and laid across floors. The studded profile holds the membrane away from the substrate, creating the drainage void that allows water entering the structure to travel freely downward to the perimeter drainage channel at the base of the wall. The size of the stud determines the depth of the drainage void behind the membrane.

They are designed for basements, sub-structures, vaults and tunnels, and can be applied to walls and floors with minimal surface preparation. They can be installed on wet substrates without requiring a drying-out period, and internal finishes and dry lining can be installed directly after the membrane is in place. The membranes provide a complete barrier to water and water vapour, and are resistant to ground salts, roots and contaminants encountered in below-ground environments and are BBA certified and suitable for use as a Type C waterproofing system in accordance with BS 8102.

Perimeter Drainage Channels

The perimeter drainage channel is installed at the critical wall-to-floor junction, where water draining down within the void behind the wall membrane collects and enters the channel through pre-drilled drainage holes. From there it is diverted along the channel to an outlet point connected to a sump collection chamber or a gravity drain for disposal. The drainage channel range from RIW includes outlet fittings that connect the channel to the sump or discharge point, and rodding eyes that provide access points for inspection and flushing of the channel at regular intervals along its run. Maintaining access for inspection is a requirement of BS 8102 for Type C systems and must be built into the layout of the drainage channel from the outset.

A drainage board is also available for applications where both water management and membrane protection are needed. It promotes drainage of water away from the structure while providing a protective covering over an applied membrane.

Cavity Drain Ancillaries

A complete cavity drain installation requires a set of ancillary products to secure the membrane, seal laps and junctions, and maintain the integrity of the system at all vulnerable points. Fixing plugs are used to mechanically secure the membrane to the wall substrate, driving through the membrane and into the substrate below.

Each fixing point requires sealing rope placed around it as a gasket to prevent water tracking through the fixing location. Sealing rope is also applied at laps between overlapping sheets of membrane, and around any service entry pipes or penetrations that pass through the membrane, ensuring the drainage void remains continuous and sealed at all these points.

Over tape is a self-adhesive butyl rubber tape applied at wall-to-floor, wall-to-wall and wall-to-soffit junctions, around pipe penetrations, and to seal membrane laps and joints where required. Where the cavity drain system is being used as a radon barrier, all lap joints in the membrane must be sealed with over tape to ensure the system performs as a gas barrier as well as a water management system.

Installing a Cavity Drain System

Installation follows a logical sequence from drainage to membrane to finish. The perimeter drainage channel is installed first at the wall-to-floor junction, connected to the sump or discharge point before work on the membrane begins. Rodding eyes are positioned at appropriate intervals along the channel run to ensure the system remains accessible for maintenance.

Once the drainage channel is in place, the membrane can be applied to wet substrates without a drying-out period. Surfaces must be clean and free from loose material, grease and frost before sealing rope and over tape are applied. The membrane is mechanically fixed to the wall substrate using fixing plugs, with sealing rope used at each fixing point and at all sheet laps to seal the drainage void. Over tape is applied at all junctions, penetrations and membrane laps where required. All sealing must be completed before dry lining and internal finishes are installed, as these points will be inaccessible once fit-out begins. On floors, the membrane is laid across the substrate with laps sealed using sealing rope, with laps and junctions sealed to ensure water flows freely to the drainage channel.

Choosing a Cavity Drain System

A cavity drain system is typically specified when the structure cannot reliably be made watertight by a Type A or Type B approach, or where the design requires a maintainable internal waterproofing system. It is the standard solution for retrofit waterproofing of existing basements and sub-structures where the external face of the structure is inaccessible, and is regularly used in new build applications where ground conditions or programme constraints make Type A tanking impractical.

Under BS 8102, the required Grade of internal environment determines the level of protection the system must deliver. Grade 1 covers utility and storage areas, Grade 2 covers working environments and Grade 3 covers fully habitable space. A correctly specified and maintained cavity drain system can meet all three grades. The membrane specification, drainage channel layout and sump capacity should all be confirmed at design stage against the applicable Grade requirement. Where the structure is subject to ground gas or radon in addition to water ingress, all membrane laps must be sealed with over tape to ensure the system provides an effective gas barrier as well as a water management solution.

Why Choose Construction Supplies UK for Cavity Drain Systems?

Cavity drain membranes, drainage channels and ancillaries in one place: From BBA-certified HDPE studded membranes to perimeter drainage channels, outlets, rodding eyes, fixing plugs, sealing rope and over tape, we stock the full component set for a Type C cavity drain system.

Trusted industry brands: Shop cavity drain products from RIW and Wykamol, two established brands in below-ground waterproofing.

Competitive trade prices: Cavity drain products supplied at competitive trade prices with no minimum order value.

Fast UK-wide delivery: Reliable nationwide delivery with free shipping available on orders over £300.

Expert support when you need it: Our team can help you select the right membrane and system components for your project. Get in touch @ sales@constructionsuppliesuk.co.uk or call us on 03300 55 32 02.