Flat Roof Drainage

The path of water

Drainage relies on three elements working together.

• The slopes. The roof tilts slightly toward the outlet. The slope comes from the framing itself or from tapered insulation boards.
• The outlet. A drain, a scupper or a gutter receives the water.
• The downspout. A pipe carries the water to the bottom of the building.

Types of drainage

On a flat roof in Montreal, you mostly find the interior drain, the scupper and the edge gutter.

The interior drain. This is the classic plex setup. The roof forms a shallow basin, and a drain at the low point sends the water into a pipe that runs down inside the building. The pipe passes through the heated part of the building and is less likely to freeze than an outside downspout.

The scupper. This is an opening in the parapet that lets water out to a downspout fixed to the outside wall. A scupper sometimes also serves as an overflow.

The edge gutter. On low-slope roofs with no parapet on one side, water runs to an edge and falls into an ordinary gutter.

The overflow

A roof surrounded by parapets is a basin. If the drain clogs during a storm, the water rises.

An overflow is a second outlet, set a little higher than the first. It does nothing in normal times. The overflow keeps water from climbing to the top of the membrane upstands if the main drain can no longer keep up.

When water does not leave

The effects of poor drainage come in a fairly predictable order.

1. Puddles. Water stays for days after rain. Dirt settles in it and the membrane no longer dries in that spot.
2. Wear. Granules come loose, seams stay wet, and freeze and thaw work the surface.
3. Infiltration. Water ends up finding a defect. Once under the membrane, it soaks the insulation, which stops insulating and does not dry.
4. Weight. Water is heavy. A structure that sags under the load deepens the basin, which then holds even more water.
5. Overflow. In the worst case, water goes over the upstands and enters the walls directly.

The Montreal winter

Drainage is put to the test between December and April. A thaw melts the snow. Meltwater runs toward the drain and can refreeze on the way if the cold returns.

Ice then blocks the drain while the snow keeps melting, and the roof fills up. A drain cleared in the fall and a free overflow limit this risk.

Signs to watch for

You do not need to go up on the roof to suspect a problem.

• a stain on the top-floor ceiling, near the centre of the building
• water running down an outside wall where there is no downspout
• white marks or damaged brick under a parapet

Frequently asked questions