Flat roof drainage

A flat roof is never quite flat

A flat roof in Laval is built with a slight incline. Three parts work in relay to get the water off.

• Gentle slopes. They direct the water to a low point. They come from the framing or from insulation boards cut on an angle.
• An outlet. At the low point, a drain, a scupper or a gutter collects the water.
• A downspout. A pipe carries it to the foot of the building.

Water that sits on a flat roof is a defect to correct.

Drain, scupper or gutter

The interior drain. The roof is shaped like a very wide, shallow bowl. At the bottom, a drain sends the water into a pipe that runs through the building. This pipe runs through heated space. It is less exposed to frost than a downspout hung outside.

The scupper. The parapet, the low wall around the roof, has an opening in it. The water passes through and falls into a downspout fixed to the facade. The scupper sometimes serves as the overflow.

The edge gutter. When one side of a low-slope roof has no parapet, the water slides to that edge, where a gutter catches it.

Signs that drainage has failed

Do not go up to check. Several clues can be seen from the sidewalk, the yard or the top-floor dwelling.

• a stain on the top-floor ceiling, toward the middle of the building
• a trickle of water on an exterior wall, in a spot with no downspout
• whitish deposits or crumbling brick just below the parapet

From a puddle to water damage

Poor drainage damages the roof in stages, often in the same order.

1. The water lingers. Dust collects in it and that corner of the membrane never dries.
2. The surface wears. The granules come off, the seams sit in water, freezing and thawing work on the whole thing.
3. The water gets underneath. It finds a defect, then soaks the insulation. Wet insulation insulates poorly and is hard to dry.
4. The load adds up. If the structure sags under the weight of the water, the dip deepens and holds even more water.
5. The water overflows. At worst, it rises above the top of the membrane upturns and goes straight into the walls.

The thaw trap

From December to April, drainage works in the worst conditions. Mild weather melts the snow, and the meltwater can freeze on its way to the outlet if the cold returns.

An ice plug then closes the drain while the melt continues, and the roof fills up. Having a roofer clear the drain and check the overflow in the fall reduces this risk.

A second, higher outlet

Surrounded by parapets, a flat roof holds water like a basin as soon as its outlet is blocked.

The overflow is a second opening, installed a little above the first. This opening comes into play when the drain can no longer keep up, before the water reaches the top of the membrane upturns.

Frequently asked questions