Basement Waterproofing Calculator

BS 8102:2022 defines three types: Type A (barrier or tanking) applies a waterproof coating to the structure; Type B (structural integral) uses specially designed reinforced concrete that is inherently waterproof; Type C (drained protection) uses a cavity drain membrane system that manages water ingress by directing it to a sump pump. Each has different applications, and combining systems provides the highest protection grade.

Type C (cavity drain membrane) is the most commonly used system for basement conversions in the UK because it works with water pressure rather than against it, is forgiving of minor structural defects, and can be installed without external excavation. BS 8102 recommends combining two systems (e.g. Type A + Type C) for habitable basements to achieve Grade 3 (dry) protection.

A cementitious tanking slurry (Type A) is applied in at least 2 coats, but the coverage rate is product-specific. Low-pressure and Sika Brushcoat-class products are around 1.0–1.5 kg/m² per coat, whereas standard grey tanking slurries (e.g. Permagard PermaSEAL 2–3 kg/m², Sika Damp Proofing Slurry ~2.1 kg/m²/mm) use roughly double. For 3 coats at 1.5 kg/m² over 100m² of wall and floor you would need 450 kg; the same area in a 2-coat grey slurry at 2.5 kg/m² needs 500 kg. Always follow the manufacturer's specific application rate from the product data sheet. The substrate must be clean, sound, and dampened before application.

Yes, Type C cavity drain membrane systems always require at least one sump chamber with a submersible pump to discharge collected water. BS 8102:2022 does not set a fixed sump volume — the sump and pump must be sized for the worst-case water ingress and be fully maintainable. For habitable (Grade 3) basements it advises a dual-pump arrangement with battery backup and a high-water-level alarm, which in practice is treated as a requirement. For larger basements, additional sumps may be needed. Sizing should always follow the waterproofing designer's calculation and the pump manufacturer's data.

A properly installed waterproofing system should last the lifetime of the building. Type A tanking has a typical design life of 30+ years. Type B structural concrete is permanent but relies on the concrete remaining uncracked. Type C membrane systems have mechanical components (pumps) that need replacing every 5-10 years, but the membrane itself is lifetime-rated.