The Ceiling Below Tells the Truth

A plaza deck looks perfect and the ceiling of the space below is stained and spalling. The membrane failed years ago, water has been tracking through the slab since, and none of it is visible from the surface. By the time it appears below, the structure has usually been affected.

A plaza deck is a roof carrying a traffic load, and treating it as paving rather than as roofing is the source of most plaza deck failures. There is occupied space underneath, a waterproofing membrane in the assembly, and a wearing surface on top that hides the membrane completely. By the time a leak appears below, the membrane has usually been compromised for a long time.

The complications are structural as much as material. Drainage has to work under the wearing surface as well as on it, and a blocked drain at that level ponds water directly on the membrane. Movement joints have to accommodate a deck that expands and contracts while remaining watertight. And any repair means removing and reinstating the surface above, which is why investigation before demolition matters more here than almost anywhere else on a building.

1. 01Survey from below as well as above.
2. 02Remove finishes and assess the slab properly.
3. 03Repair structure before waterproofing, never the reverse.
4. 04Install and detail the membrane as a system.
5. 05Correct slope, then reinstate.

Judge any deck waterproofing proposal by how it details the terminations — at drains, upstands, doors and posts. The flat field of a membrane almost never fails; the edges always do.

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