The three types
BS 8102 sets out three forms of protection. Type A is a barrier applied to the structure — a membrane, a cementitious slurry or an asphalt tanking. Type B is integral protection, where the concrete structure itself is designed to resist water penetration. Type C is drained protection: a cavity drain membrane on the inner face that accepts water, channels it to a perimeter drain and discharges it through a sump and pump. Most London residential basements use a combination, and the Standard expects a designer to say explicitly which types are being relied on and in what combination.
Why combined systems are normal here
London ground conditions, the age of the retained structure and the value of what goes inside all push towards belt and braces. A common arrangement is Type B structure with a Type C cavity drain system inside it, so that any water passing the structure is managed rather than resisted. The grade of protection required depends on the use of the space: a plant room can tolerate what a bedroom or a home cinema cannot, and that judgement belongs to the waterproofing designer, not the contractor.
The details that fail
Flat areas of membrane almost never fail. What fails is everything that interrupts them: service penetrations, construction joints, the junction between new and retained structure, drainage entries, and the point where the membrane is terminated at the top. Each of these should be detailed on a drawing before work starts and photographed as it is formed. We keep a record of every penetration through a waterproofing system, because in five years’ time nobody will remember where they were.
Sumps, pumps and power
A drained system depends on a sump and pump, and therefore on electricity. Good practice is twin pumps, a high-level alarm, a battery backup or a second power source, and a sump that can actually be reached and cleaned without dismantling joinery. We have opened up finished basements to find a sump chamber under a fitted wardrobe. Maintenance access is a design decision that gets made, or missed, at first fix.
Handover and maintenance
A waterproofing system needs a maintenance regime: pump servicing, alarm testing, channel flushing. The manufacturer’s warranty usually depends on it. At handover we issue the waterproofing designer’s details, the installer’s certification, the as-built record of penetrations and the maintenance schedule together, so that the information survives the owner who commissioned it.