Cavity Drain Membrane Calculator — Membrane, Plugs & Sump

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.

Wall membranes are fixed with special sealed plug fixings driven through the membrane into the wall. Fixing density depends on the membrane type and finish: meshed or dot-and-dab membrane needs a minimum of about 15 plugs per m² at roughly 250mm staggered centres (Newton/Delta/PCA guidance), while a battened or independent-wall non-mesh install uses far fewer — around 5 per m², which Permagard quote as 600mm vertical × 400mm horizontal centres. The Plug Spacing field here is a single equal (square) grid, so enter 250mm for a meshed/dab finish (~16/m²) or about 450mm to reproduce the ~5/m² of a battened build — entering 600mm as a square grid would give only ~2.8/m² and under-order. Always follow the membrane manufacturer’s fixing instructions. Membranes overlap by 100mm minimum at all joints. Floor membranes are loose-laid but lapped ~100mm and taped at sheet joints.

Yes, a sump pump is an essential part of a Type C system — water draining behind the membrane must be collected and pumped away. Most domestic basements need just one sump chamber: PCA best-practice guidance lets a single groundwater pumping station serve a basement up to roughly 150 m² (with the sump no more than ~25m from the furthest drainage point, and about one sump per 50 linear metres of perimeter channel). Larger or split basements may need an additional sump. The chamber should hold a twin-pump system with battery backup and a high-level alarm, discharging to a surface-water drain or soakaway.

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.

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.