Basement Tanking Calculator — Slurry, Membrane & Sump Pump

Basement tanking is the process of waterproofing a basement to prevent water ingress. BS 8102 defines three types: Type A (barrier protection — coatings/membranes applied to walls), Type B (structurally integral — water-resistant concrete), and Type C (drained protection — cavity drain membrane). Type A cementitious slurry is common for existing basements.

A minimum of 2 coats is required, applied in opposing directions (the second at 90° to the first). Coat thickness follows the manufacturer datasheet: for damp-proofing, around 2mm total is typical; for full BS 8102 Type A waterproofing, manufacturers (e.g. Sika) specify around 4mm total, applied as 2 × 2mm coats. The second coat is applied once the first is touch-dry (typically a few hours, and within 24 hours per Sika). In areas of high water pressure, 3 coats may be recommended by the manufacturer.

Cementitious tanking slurry is specified and sold by weight (kg) in 20kg or 25kg buckets — not in litres. Coverage is around 2–3 kg/m² per coat depending on product and surface (Permagard PermaSEAL quotes 2–2.5 kg/m²/coat on smooth substrates and 2.5–3 on coarse; Newton HydroCoat is ~2.8 kg/m² as a two-coat total; Sika consumes ~2.1 kg/m² per mm of thickness). This calculator uses a representative 2.5 kg/m² per coat over 2 coats — always confirm against your chosen product datasheet and round up to whole buckets.

Corner fillets (or coves) are formed at all internal angles where walls meet floors and where walls meet other walls. They eliminate sharp 90° corners which are weak points where water can penetrate. Fillets are typically 50mm × 50mm triangular sections formed from the tanking material or a dedicated fillet compound.

A sump pump is recommended for basements below the water table or in areas with high groundwater. For Type C (cavity drain membrane) systems, a sump pump is essential as the system relies on collecting water and pumping it away. The sump should have a twin pump with battery backup for reliability.

Yes, internal tanking with cementitious slurry (Type A) or cavity drain membrane (Type C) is the most common approach for existing basements. External tanking (applied to the outside of the walls) is more effective but requires excavation. The choice depends on access, budget and water pressure.