How does basement tanking work and what does it cost?

Basement tanking involves applying a continuous waterproof barrier to the internal or external faces of basement walls and floors. The most common method in the UK is internal tanking using cementitious slurries (e.g., Sika-1 Pre-Bagged, Vandex BB75) applied in multiple coats directly onto prepared masonry or concrete. The surface must be clean, sound, and free of paint or plaster. Tanking must resist negative hydrostatic pressure — the water pushing the coating off the wall — so surface preparation and correct application are critical. Costs typically range from £60–£120/m² for internal cementitious tanking, or £100–£200/m² for external membrane systems. A typical single-room basement (20m²) might cost £3,000–£8,000 depending on the system, access, and condition. Tanking is classified as Type A waterproofing under BS 8102:2022.