External Waterproofing Calculator — Excavation, Membrane & Drainage

External waterproofing involves excavating around the outside of below-ground walls and applying a waterproof barrier or coating to the external face. This is Type A (barrier) protection under BS 8102:2022, and applying it externally keeps water away from the structure entirely — generally the most robust approach, though more disruptive and costly than internal methods. BS 8102 recommends combining it with a second waterproofing type for higher-grade habitable basements.

Self-adhesive bituminous membrane, torch-on modified bitumen, or liquid-applied rubber coatings are commonly used. The membrane must be continuous, lapped at joints, and sealed at all penetrations. A protection board (such as Platon or polystyrene) is placed over the membrane to protect it from damage during backfilling.

Yes. A perforated land drain pipe should be laid at the base of the excavation, bedded in gravel, to collect any water that reaches the base of the wall and direct it to a sump, soakaway or surface water drain. This prevents hydrostatic pressure building up against the membrane.

The excavation needs to be wide enough for safe working — around 600mm clear width from the wall face is a sensible practical minimum to apply the membrane and work the base. There is no fixed legal minimum width: the HSE position is that any excavation can collapse, so the sides must be battered back or properly supported regardless of depth (the 1.2m figure is the depth at which support must be considered, not a threshold below which it is safe). Follow the Construction (Design and Management) Regulations 2015 and assess the ground conditions before digging.

The lower 300mm should be gravel or clean stone to assist drainage to the land drain. Above this, the original excavated material can be used if it is free-draining. Clay soils should not be placed directly against the protection board — use imported granular fill instead.