Flat Roof Drainage & Ponding Repair

Water standing on a flat roof after rain is a drainage problem before it is a waterproofing problem. If the roof cannot shed water, the membrane is asked to hold a pond under hydrostatic pressure — and no membrane performs well when it is permanently submerged and exposed to sun.

Ponding usually comes from one of four things: falls that were never built correctly, outlets that are blocked or undersized, a structural deflection that creates a low point, or a parapet or gutter that stops water reaching the outlet. Diagnosing which one applies is the first job.

Standing water accelerates everything. It ages the membrane, feeds algae and moss, adds dead load to the structure, and drives water through the smallest defect in a lap or detail. A roof that ponds after every storm will fail early even if the membrane was installed correctly.

The repair is a drainage correction first: survey the falls, clear and resize the outlets, add secondary drains or scuppers where water cannot reach an outlet, and re-screed to positive falls where the deck allows. Only then is the waterproofing renewed over a surface that can actually drain.

We hand over the roof with the outlets and drainage path documented, and we say what to check before each rainy season. Ponding usually returns because leaves and silt were never cleared, not because the membrane failed.