Question 1

What is structural waterproofing?

Accepted Answer

Structural waterproofing is the design and installation of systems to protect below-ground structures - basements, underpinned cellars, below-ground extensions, and buried retaining walls - from ground water ingress. Unlike above-ground damp-proofing (which addresses minor moisture transmission), structural waterproofing must resist hydrostatic pressure from groundwater - the pressure of water in saturated soil that pushes directly against the structure. The design standard is BS 8102:2022 (Protection of Below Ground Structures Against Water Ingress). The required performance grade (Grade 1 car parking to Grade 3 habitable rooms) determines the waterproofing specification.

Question 2

What are Type A, Type B, and Type C waterproofing?

Accepted Answer

BS 8102 classifies structural waterproofing into three types: Type A (Barrier) uses applied membranes to prevent water entering the structure. Applied externally to the outside face of the structure (ideal but requires excavation), or internally as tanking (cement-based slurry systems, sheet membranes, or crystalline products applied to the inside face of walls and floor). Type B (Structurally Integral) uses waterproof concrete (concrete with low water/cement ratio, integral crystalline additives, or admixtures) as the structure itself. The concrete both provides the structural resistance and acts as the waterproof barrier. Type C (Drained) uses a cavity drain membrane (dimple sheet membrane) applied to the internal face of walls and floor, creating a cavity that allows water to enter the structure but collects it and drains it to a sump pump for discharge. Type C accepts that water will enter but manages it. For habitable use (Grade 3), BS 8102 recommends using two types in combination for redundancy.

Question 3

What is tanking?

Accepted Answer

Tanking is a Type A internal barrier waterproofing system in which the internal faces of below-ground walls and the floor are coated with a waterproof slurry, render, or membrane to prevent water penetrating through to the habitable space. Traditional tanking uses several coats of cement-based waterproof slurry (sometimes reinforced with fibreglass mesh) to create a continuous waterproof 'tank' lining the inside of the basement structure. Modern systems include polymer-modified cementitious coatings, crystalline waterproofing products (which form crystals in the concrete pores), and liquid-applied or sheet polyurethane or bituminous membranes. Tanking applied internally is working against the hydrostatic pressure (water pushes the membrane away from the structure) and must be properly fixed and lapped, with all joints, penetrations, and floor-to-wall junctions carefully detailed and sealed.

Question 4

What is a cavity drain membrane system?

Accepted Answer

A cavity drain membrane system (Type C waterproofing) uses a studded polyethylene sheet (dimple membrane) fixed to the internal faces of the basement walls and floor. The dimples create a drainage cavity between the existing structure and the new internal wall or floor finish. Any water that penetrates through the structure is collected in this cavity rather than being stopped at the outer face. The water flows down the wall cavity and across the floor membrane into a perimeter drainage channel, which drains to a sump. An electric sump pump (with a battery backup for power cut resilience) discharges the collected water to drain or to a surface watercourse. Cavity drain systems are popular for basement conversions in existing properties because they do not require external excavation, are relatively tolerant of structural movement and cracks, and can be installed more quickly than tanking.

Structural Waterproofing

Just so you know...