Type A: Barrier waterproofing (tanking)
Type A is a barrier system — a continuous waterproof membrane applied to the substrate to prevent water ingress. Applied externally (to the outside face of the wall before backfilling), it is the most effective form of Type A because water pressure drives the membrane against the structure. Applied internally (to the inside face of existing walls), it is the standard retrofit option for existing cellars, but is vulnerable to hydrostatic pressure, which can cause delamination if the membrane is imperfectly bonded. Materials: cementitious slurry (brush-applied), cavity drainage boards with sealed laps, crystalline materials (Xypex-type), bonded sheet membranes. Cost: £80–£150/m² of treated surface area. Suitable for: cellar conversions where water ingress is minimal; structures with no significant hydrostatic head; where a sump pump is not viable. Limitation: a single point of failure — any breach or bond failure allows water through. Type A alone is not acceptable on full basement excavations in London — BS 8102:2022 requires a multi-type approach for Grade 3 habitable use.
Type B: Structurally integral waterproofing
Type B uses the structure itself as the waterproofing element — typically reinforced concrete designed to BS EN 1992 (Eurocode 2) with controlled crack widths, low water-cement ratio and the addition of a crystalline admixture (Pudlo, Caltite, Kryton) to densify the concrete matrix and seal any cracks. Type B is the standard system for new-build basements where the concrete structure is poured on site — it is built into the mix design, not added as a separate layer. Cost: included in structural concrete cost — no separate waterproofing layer. Suitability: full basement excavations (new poured concrete structures), temporary basement retaining structures, underpinned mass concrete walls given a crystalline slurry treatment. Limitation: relies on quality of workmanship — segregation, honeycombing or inadequate cover can compromise the system. For this reason, BS 8102:2022 recommends supplementing Type B with Type A or Type C in high water table conditions.
Type C: Cavity drain waterproofing
Type C is a drained protection system — rather than stopping water at the structure, it manages water that enters, channelling it to a sump and pump for removal. A studded HDPE cavity drain membrane (Newton, Delta, Oldroyd) is fixed to the walls and floor, creating a 8–20mm drainage void between the membrane and the internal finish. Water tracking through the structure collects in the void, drains to a perimeter channel at floor level, then discharges to a sump and twin submersible pump with battery backup. Type C is the most robust and most widely specified system for London basement conversions because: it accommodates structural movement and crack formation without failure; the sump pump provides active water management; it can be retrofit to any existing structure regardless of age or condition; a breach in the membrane is self-draining, not catastrophic. Cost: £150–£250/m² including sump installation. Builderr specifies Newton specialist cavity drain systems as standard on all full basement excavations.
Combined Type A + C: the London standard
BS 8102:2022 recommends a multi-type approach for Grade 3 (habitable) use — the highest level. In practice, most London basement specialists specify a combined Type A + Type C system: a cementitious tanking slurry is applied to the walls and floor as a first barrier (Type A), then a cavity drain membrane is overlaid as the second line of defence (Type C). The sump and pump provide active management of any water that defeats both layers. This belt-and-braces approach is why properly specified London basements remain dry over decades despite the aggressive clay groundwater environment. Combined system cost: £250–£400/m² of treated surface area. For a 50m² basement with 80m² of walls and floor to treat, expect £25,000–£40,000 for waterproofing alone. This cost is non-negotiable — skimping on waterproofing is the single most common cause of failed basement conversions.
