What is a cavity drain membrane system?

A cavity drain membrane (Type C protection to BS 8102) is a studded high-density polyethylene sheet fixed to basement walls and floors. The studs create an air gap that allows any water penetrating the structure to drain behind the membrane to a perimeter channel and sump pump. It is the most reliable method for existing basements.

How are the membranes fixed?

Wall membranes are fixed using special plug fixings driven through the membrane into the wall at approximately 330mm centres (9 plugs per m²). The plugs compress the membrane dimples against the wall. Membranes overlap by 100mm minimum at all joints. Floor membranes are loose-laid.

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

Yes, a sump pump is an essential part of a Type C system. Water that drains behind the membrane needs to be collected and pumped away. The sump chamber should contain a twin pump system with battery backup and a high-level alarm. The pump discharges to the surface water drain or soakaway.

What is the floor drainage channel?

A perimeter floor channel (typically a half-round or square profile channel) runs around the base of the walls to collect water draining down behind the wall membrane and direct it to the sump chamber. It sits at the junction of wall and floor membrane. Additional channels may run across the floor.

How long does a cavity drain membrane last?

HDPE cavity drain membranes have a design life of 50+ years. However, the pump system requires regular maintenance — pumps should be serviced annually and replaced every 5-10 years. The battery backup should be tested regularly. A maintenance contract with a waterproofing specialist is recommended.

TradeCalculator.co.uk

Cavity Drain Membrane Calculator — Membrane, Plugs & Sump

Calculate studded membrane area with lap allowances, fixing plug count at 330mm grid, floor drainage channel and sump chamber quantities.

Total Wall Length

Total linear metres of walls

Wall Height

Floor to ceiling

Floor Length

Internal floor length

Floor Width

Internal floor width

Floor Membrane Required

Studded membrane on floor

Price Per m² (optional)

Membrane material cost

How We Calculate This

This calculator determines the membrane, fixings and drainage quantities for a cavity drain waterproofing system.

The formula

Membrane Area = (Wall Area + Floor Area) × (1 + Lap/1000)

Plugs = Wall Area × (1 / Spacing²) per m²

Floor Channel = Room Perimeter

Components

Wall membrane: 8mm studded HDPE
Floor membrane: 8mm or 20mm studded HDPE
Fixing plugs: 330mm grid on walls (~9/m²)
Sump chamber: Twin pump, battery backup

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.