Guide to Flat Roof Drainage Systems for Disposing of Roof Runoff
InspectAPedia^®  -      

• Flat Roof Drainage System specifications, designs, troubleshooting
• Problems & Solutions for Interior Drains on Flat Building Roofs
• Flat roof drains, scuppers, and screens
• Questions & answers about keeping the flat roof drainage system working or about installing new drains in a flat roof

Also see DOWNSPOUT LEAKS or start this topic at DOWNSPOUT / LEADER DEFECTS; also see GUTTER DEFECTS LIST for other sources of wet basements or crawl spaces caused by problems with the roof drainage system.

The drainage requirements for flat roofs on buildings present some special problems that we describe and illustrate here.

The sketch shows common leak points at flat roof drainage systems:

• Scupper drains become blocked by leaves or other debris
• Through building drains (upper right in the sketch) become clogged internally and burst, flooding the building, or leaks occur around the drain itself where it penetrates the roof
• On flat or low slope roofs that drain to one end (lower right in the sketch), especially if the building rear wall does not include a projecting eave or soffit, a clogged gutter or downspout send water down the building rear wall, flooding the structure through its wall or through windows.

Sketch courtesy of Carson Dunlop Associates.

Carson Dunlop explain in their sketch (left) that some flat roof designs relying on scupper drains and downspouts provide a set of secondary roof drain scuppers located 2" above the roof membrane.

These emergency drains will prevent deep flooding (and risk of collapse or catastrophic building interior flooding) should the main scupper drains become obstructed by debris.

Sketch courtesy of Carson Dunlop Associates.

Details about roof drains that flow through the building interior are provided at Problems & Solutions for Interior Drains on Flat Building Roofs.

Interior drains on flat roofs are commonly used in urban areas. The drain runs through the building and connects to a storm drain or in some communities where it is still allowed, to the sewer system.

(Many communities have made it illegal to connect roof drains or surface water runoff drains into the sewer system because during heavy rainfall the sewer system will be flooded, resulting in discharge of raw sewage into the environment).

Watch out: debris entering the roof drain system lead to clogging and leaks into the building interior. In freezing climates a clogged interior building drain eventually freezes and bursts near the roof surface.

When the roof has been flooded due to a clogged interior building drain and the drain subsequently bursts, a huge volume of water (from the flooded roof) enters and floods the building interior.

Inspect and clean roof drain screens and drain piping at least annually to avoid this building flood catastrophe.

Sketch courtesy of Carson Dunlop Associates.

Here we illustrate just how interior drains on flat roofs become clogged, causing rising ponds of water on the roof surface.

The combination of ponding water on the roof around the roof drain and the mechanical disturbance caused by freeze-thaw cycling act to loosen the drain connections, break the roof drain seal, and cause leaks into the building below.

Sketch courtesy of Carson Dunlop Associates.

When the through-building roof drain system has become clogged, even if it does not burst and lead to building flooding, it causes other building problems.

The roof structure may sag due to the added weight of ponding water.

Watch out: in severe cases of deep ponding water (or water and ice and wet snow) on a roof with blocked drainage, the weight of the water and ice may conspire with hidden rot or insect damage (such as from prior roof leaks) to cause catastrophic roof collapse - a very unsafe condition.

Sketch courtesy of Carson Dunlop Associates.

It is possible to add new roof drains to relieve ponding on a sagged flat roof, as shown at left.

Notice that the new drain extends (through the shortest possible route) to the building exterior where it is conducted to ground or to a storm drain.

Using this method permits adding a drain at the roof's low point.

When the building is re-roofed, the roofer may install tapered foam insulation to direct water to roof scuppers or to a main or new-main roof drain to prevent future ponding.

Sketch courtesy of Carson Dunlop Associates.

• The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.