What is 'tanking' a basement?

Tanking a basement means applying a continuous waterproof barrier to the walls and floor so that groundwater cannot pass through, effectively turning the below-ground structure into a watertight tank. It is the traditional term for Type A (barrier) waterproofing under BS 8102:2022, and it is usually achieved with a cementitious tanking slurry — a cement-based coating that becomes part of the substrate — or, historically, with bituminous coatings and self-adhesive membranes. Tanking can be applied to the inside face (internal or 'negative side' tanking) or the outside face (external or 'positive side' tanking) of the structure. External tanking is the more robust in principle because it stops water before it reaches the concrete, but on an existing building it needs excavation, which is often impractical or prohibitively expensive, so internal tanking is far more common as a retrofit. The critical challenge with internal tanking is that it has to resist negative hydrostatic pressure — the groundwater is actively trying to push the coating off the wall — so the bond to the substrate is everything. That makes surface preparation absolutely vital: all paint, plaster, render and laitance must be removed back to sound brick or concrete, junctions reinforced with fillet seals, and any live cracks or leaking joints injected with resin before the slurry goes on. Tanking is not the only option under BS 8102 — for habitable space, high water tables or structures where the substrate cannot be guaranteed, a Type C cavity drain membrane system (which manages water to a sump and pump rather than resisting it) is often more reliable, and the most dependable basements frequently combine both for redundancy. Which approach is right depends on the water table, the grade of internal environment required, and the condition of the structure, and it should be specified by a waterproofing design specialist. Costs vary widely with system and size — cementitious tanking is typically the lowest cost per square metre, with full basement projects running into several thousand pounds and up. MPS Concrete Solutions designs and installs BS 8102-compliant basement tanking and cavity drainage across the UK, from single cellars to large commercial basements.