Drained Cavity Waterproofing System

A drained cavity arrangement begins from a different premise from the rest of this family: it assumes water will reach the inside face of the structure, and sets out to manage it rather than exclude it. A studded membrane held against the wall, and often laid over the floor, gives that water a defined space to run in; a channel at the base collects what arrives; and a removal arrangement takes it away to a discharge point.

Against a barrier-waterproofing-system the difference is visible from inside as soon as a lining is opened. A barrier approach shows a continuous applied layer whose job is to keep water out; a drained approach shows a dimpled sheet standing off the wall with an open space behind it and a channel at its foot. What is peculiar to the drained approach is that its water management is continuously operational and sits inside the building: a route that has to stay clear and reachable and, where the site offers no gravity outfall, lifting equipment that has to keep running and stay powered.

That dependency changes what counts as part of the system. Access ports, a maintenance regime, the power arrangement and a response to failure are components rather than optional extras, and the internal finishes have to be built so that they never bridge the drained space. The removal arrangement sits inside the boundary of this system rather than beside it, because the cavity has nowhere to send what it collects without the sump, and the sump has nothing to receive without the cavity.

That is a difference of kind rather than a claim that the other approaches ask nothing of anybody. A barrier and an integral structure commonly rely on drainage in the ground outside to keep the demand on them modest, and a path that silts up raises that demand without announcing itself. What sets the drained approach apart is that the part which has to keep working is inside the building and in the occupier's care rather than buried in the backfill.

The parts form a route, and each is only as useful as the next one downstream. The membrane delivers water to the channel, the channel to the sump, the sump to the discharge. Block the channel with construction debris, or discharge into something that cannot receive what arrives, and the water does not go away; it backs up and reappears at the weakest point in the route, usually a floor junction or a service entry.

The membrane has to be held to the wall, and every fixing is a hole through it on the wet side. That contradiction is what the sealed plug exists to resolve, and it is reopened every time something is fastened through the sheet later - a rail, a bracket, a lining screw. The fixing pattern for the finishes therefore has to be settled while the membrane is still visible, not once it is covered.

Keeping the linings independent is what keeps the route reachable. A stud fixed hard through the membrane, a screed poured over an upstand, or a floor finish laid across the channel all bridge the drained space and turn a maintainable arrangement into a sealed one. The finishes themselves can be as ordinary as anywhere else in the building, provided they are carried in a way that never touches across the cavity.

The removal arrangement couples the assembly to things outside it. It depends on a power supply, on level control, on a non-return device and on the site drainage being able to receive what it sends. If the discharge surcharges, that non-return device is the only thing holding water back; if power is lost, the route still exists but stops moving, and everything upstream of the sump begins to fill.

Access is the component that keeps the others honest. Ports at corners and along the channel are what allow the route to be inspected and cleared, and they stay useful only if the finish above them is arranged to be lifted. Once access disappears under a bonded floor finish, the arrangement goes on working for exactly as long as nothing goes wrong with it.