Delta Membrane : What is a Cavity Drain System?
April 2025

What is a ‘Cavity Drain System’?

A “cavity drain system” is a structural waterproofing solution that isolates internal wall and floor finishes from water, water vapour and dampness. This article explores cavity drain systems and cavity drain products.

Structural waterproofing is a term which covers materials and/or techniques to stop water from entering a building, especially below-ground, by creating a barrier against groundwater or other water sources. Buildings globally depend on structural waterproofing systems.

A cavity drain system, or Type C, waterproofing system, protects below-ground structures (or any structure that is wholly or partially earth retained from water ingress by allowing water into enter into the structure (via ‘a cavity’) and safely draining it away. A cavity drain system manages water entering a structure through external walls or floors, directing it behind a membrane into a controlled drainage system. This system safely diverts water to a disposal point, like a package pumping station.

Cavity drain systems are also known as Cavity Drained, CDM, Type C, Cavity Drainage and Cavity Protection. BS 8102:2022 ‘Protection of below ground structures against water ingress. Code of practice’ classifies Type C as “Drained cavity construction” and “Cavity Drained Protection.”
Key Components

Key components of a ‘cavity drain system’ are high-density polyethylene internal drainage membranes. These membranes are loose laid in flooring application, whilst being attached to the wall with surface plugs in vertical installations.

Water entering the structure is channelled into drainage systems (made up from basement drainage channels and collected in a submersible pumping station before being discharged at a suitable point. Additional key elements of a basement drainage system are high water level alarms and battery backups.
Cavity Drain Systems

A cavity drain system is fully maintainable, and an ideal solution for both new-builds, refurbishments, and historic buildings and structures. A cavity drain system offers a host of benefits, from easy and fast installation to ability to accommodate minor structural movements.

Cavity drain systems are highly effective and considered one of the safest and most reliable waterproofing methods.
Crossover

Certain cavity drain membranes can also provide protection in other fields. Such as AT 800, AT 800 is a cavity drain membrane that meets BS 8102:2022 and BS 8485:2015+A1:2019 standards, offering waterproofing and ground gas protection in one application. This membrane provides drainage protection in Type C waterproofing (Grade 3 environment) and acts as a barrier against Carbon Dioxide, Radon, and Methane gases. It also features high adhesion for applying flame retardants.

MS 500 Fire Retardant is an 8mm studded cavity drain membrane that achieves a Euroclass rating of B-S2, d0 (EN 13501-1:2018) and is suitable for internal walls and floors, protecting below ground structures from groundwater ingress with increased Fire Safety.
Need help with cavity drain systems?

For all your project requirements, Delta’s Technical Team is a dependable resource. Offering expert advice, comprehensive support, and timely practical assistance, we cater to your specific needs. Reach us at 01992 523 523 or drop us an email at info@deltamembranes.com.