Flat roofs aren't supposed to be perfectly flat. They are built with a slight slope so water runs to a drain, a scupper or a gutter. When water stays put instead, it is called ponding, and it is one of the most common reasons a flat roof wears out early. Here is how to tell normal puddles from a problem, and what fixes it.
What counts as ponding
The common rule of thumb in the roofing industry: water still on the roof 48 hours after it stopped raining, in weather that would normally dry it, is ponding. A few small, shallow puddles that disappear in a day are not worth worrying about. A large area of standing water, a spot that is always wet, or water deeper than about half an inch is.
Why ponding shortens a roof's life
- It finds every weak seam. A seam or flashing that would shed running water can leak when it sits underwater for days.
- It speeds up wear. Wet-dry cycles, sun magnified through water, and dirt that settles out of the puddle all age the membrane faster.
- It freezes. In a Pittsburgh winter, ponded water turns to ice that expands into seams and flashings, then melts and refreezes again.
- It adds weight. Each inch of water weighs about 5.2 pounds per square foot. On an older deck or joists that already sag, more weight means more sag and a deeper pond.
- It can void warranties. Some membrane warranties exclude damage from ponding water.
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What causes ponding
Clogged drains, scuppers and gutters
The most common and cheapest cause. Leaves, roof grit, tennis balls and bird nests block an internal drain or a rear scupper, and the roof fills up. On row houses, a full rear gutter does the same thing.
Sagging deck or joists
Older framing deflects over time, and wood that got wet from past leaks sags faster. The low spot collects water, which adds weight, which makes it sag more.
Not enough slope to begin with
Many older flat roofs were built dead flat or nearly so. Building codes generally call for at least 1/4 inch per foot on new low-slope membrane roofs.
Layers and patches
Stacked old roofs and thick patches create ridges and dams that hold water behind them.
How ponding gets fixed
- Clear and maintain the drainage. Clean drains, strainers, scuppers and gutters, and keep them clean every spring and fall.
- Add or enlarge drains and scuppers where water has nowhere to go.
- Tapered insulation. Boards cut at a slope, installed under a new membrane, steer water to the drains.
- Crickets. Small sloped sections between drains or behind chimneys that split the water flow.
- Re-frame low spots. On houses, sistering or furring sagging joists during a tear-off restores slope.
Most of these happen during a replacement, when the roof is open. If the roof is otherwise in good shape, clearing the drainage and patching the worst area can buy time.
A note on internal drains
Commercial buildings and some larger homes drain through internal roof drains piped down inside the building. Check that the strainer basket is in place and clear, and that the drain bowl is clamped tight to the roof. A cracked drain bowl or loose clamping ring is a leak that shows up directly below.
The bottom line
If water is still on your flat roof two days after rain, check the drains, scuppers and gutters first. If the water keeps coming back in the same spots, plan to correct the slope when the roof is replaced. See flat roof replacement and what ponding does in winter.