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Cavity Drainage Systems

Cavity Drainage (often referred to as ‘egg-box’) membranes fall in to the ‘Drained Protection’ – Type C – category of structural waterproofing of basements allowing the ‘management’ of groundwater that physically penetrates below-ground structures.

Image Courtesy of Delta Membranes

What is ‘Drained Protection’ for Type C waterproofing and how does it work?

Cavity Drainage manages water that penetrates basement structures usually because the previously (as-built) waterproofing measures have failed. They are ideal for use in refurbishment situations where it is not practical or cost effective to prevent water ingress from the outside of a structure.

  • ​The cavity drainage layer is sandwiched in a new cavity that is formed between the existing (usually structural) floors, walls &/or soffits of the below-ground structure. The floor build-up would comprise the parent substrate (slab) incorporating a perimeter rebate drain, the cavity drainage system itself and then usually a new cementitious screed with . insulation. Similarly for internal walls & ceilings, the cavity drain is sandwiched  between the existing substrates and  new blockwork or studwork walls and ceilings followed by decorative finishes.  
  • ​The ‘egg-box’ profile of the drainage layer is fixed to the inner face of the perimeter floors / walls / soffits. Supplied on rolls (typically 2 metres wide), they are vertically overlapped (with waterproofing tape) providing a continuous impervious layer. Any penetrating groundwater behind flows down in to the watertight gulleys that are formed around the internal wall/floor junctions which flow in to a cavity membrane pump station. These are below-ground sumps to collect the water which is then regularly pumped out, usually activated by a float switch.


BS 8102:2022 – the LATEST version of the standard – defines four grades of waterproofing protection for below-ground structures: 1a, 1b, 2, and 3. These grades determine the level of water-tightness required for a specific performance definition.

BS8102 also defines 3 types of water-resistant construction:


Type A – Barrier (Membrane) Protection
Type B – Structurally Integral Protection
Type C – Drained Protection

Cavity Drainage System Design, Installation & Maintenance

A key pre-requisite of BS8102:2022 is that Type C ‘basement drainage’ waterproofing systems are designed and installed as a fully maintainable water management system. Water that collects in the drainage cavity falls by gravity in to the perimeter floor gulleys connected to collection sumps. These components of the system must remain accessible for regular inspection and maintenance throughout the life of the system.

An installed Type C system should always be inspected and integrity tested both on installation and final completion prior to handover and potential overlay with permanent coverings.

Detailing of Cavity Drainage Systems:

Correct detailing is an integral part to the successful performance of a cavity drainage system, including:

  • Wall / floor junctions
  • Service penetrations
  • Wall / soffit junctions
  • ​Internal / external thresholds
  • Level changes / continuity
  • Single source of system components

System Design, Workmanship & Maintenance

“The devil is in the details …”

  • ​Prior to design, the substrate(s) must be properly assessed and suitable for a Type C system
  • ​Design / construction kept simple with detailed consideration to floor / wall / ceiling junctions
  • Design out service penetrations wherever  possible
  • ​Poor design results in water ingress and both potentially expensive and disruptive remedial works
  • ALWAYS engage the services of Registered Installers of the designed system
  • Post installation inspection and servicing of sump pumps to improve efficiency and extend service life
  • Install battery-backup system so that in the event of a power failure, pumps will continue to function
  • Install active alarm system linked to mobile comms that notifies building owner / occupier in the event of pump malfunction or excessive water build up in sumps

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“It was clear from the outset that Gunite Waterproofing had the depth of knowledge & experience to deliver this demanding basement re-waterproofing scheme,”

Why choose Gunite Waterproofing for Type C ‘basement drainage’ system installation?

Gunite Waterproofing have been keeping water out of buildings  for more than 30 years. We are accredited installers of waterproofing systems from most of the UK’s leading construction materials manufacturers including Sika, Delta Membranes, RIW.

Third-Party Accreditations

Whether re-waterproofing an existing structure OR making a new one waterproof, you need to be able to find an experienced partner who you can trust to deliver your requirements to the right design. All of our clients benefit from our 30+ years of knowledge & experience in delivering innovative, bespoke and cost-effective waterproofing solutions complemented by exemplary customer service.