Ponding water on a flat roof

Two days of dry weather, the trade's benchmark

A flat roof is designed to drain. Flat roof drainage leads water to the drain through slight slopes. Right after rain, the surface stays wet until it dries.

The usual benchmark among roofers is 48 hours of dry weather. Water still there after that time, in the same spot, is called ponding water.

You have nothing to measure and you do not go up. A photo taken from a window is enough for the roofer to start.

Five possible causes

• The drain is clogged. The water then forms a pond around the drain.
• The drain sits higher than its surroundings. A re-covering or sagging nearby has left it above the low point, and the water would have to climb to reach it.
• The slopes are missing. The roof was built almost level, or an earlier roof replacement left it as it was.
• The insulation has compressed or soaked up water. A board crushed by foot traffic or full of water forms a bowl.
• The framing has sagged. Over the decades, wood joists bend. The middle of a span becomes a low point the roof did not have when it was built.

What the water does while it stays

A membrane handles rain. It is not designed to stay under water all the time.

On the surface. In winter, the pond freezes into a sheet. The ice expands and pushes on the seams, and each freeze followed by a thaw widens what was already weak. On an area that dries, a seam barely open lets a few drops through. Under a pond, that seam feeds a leak for days on end.

Underneath. Water is heavy, and a large puddle puts a load on the roof that never lets up. The joists bend a little, the dip deepens, the puddle grows, and the load grows again.

A dip visible in the roof, a ceiling bulging downward or recent cracks on the top floor call for a structural engineer. The roofer is not in a position to judge the strength of a frame.

From the simplest to the heaviest

1. Clear the drain. We clean the strainer. When the pond goes away after our visit, the matter is closed.
2. Bring the drain back to the right height. If it sticks up, we redo its connection lower or install a new drain.
3. Add an outlet. An extra drain or scupper serves a low point too far from the existing outlet.
4. Rework only the dip. The area is opened, the damaged insulation replaced, the slope rebuilt, then a new membrane closes everything up.
5. Redesign the slopes during a roof replacement. Tapered insulation installed over the whole roof corrects the defect at its source.

Frequently asked questions