No flat roof is truly flat. It is supposed to slope, gently, toward drains, scuppers or gutters. When water sits instead of draining, the roof wears faster, carries extra weight, and in central Wisconsin, turns that water into ice that blocks the drains for the next thaw.
What counts as ponding
The common definition: water still standing on the roof 48 hours after rain, in weather that would normally dry it. Shallow puddles that disappear in a day are usually not a concern. Water that never dries, leaves dirt rings or algae, or grows each season is.
Why it matters
- Weight: water weighs about 5.2 pounds per square foot per inch of depth. A deep pond is a real load, and it deflects the deck, which lets the pond grow.
- Wear: seams, coatings and membranes age faster under standing water and its debris.
- Ice: ponds freeze. Ice next to a drain blocks it, and the next thaw backs up behind it. See ice dams on flat roofs.
- Leaks: standing water finds every weak seam and flashing.
What causes it
- Drains and strainers clogged with leaves, gravel or debris
- Drain bowls set higher than the roof around them
- A deck that has sagged between joists or over time
- Too few drains for the roof area, or drains in the wrong places
- Wet, crushed insulation creating low spots
- A roof that was simply built too flat
Roof leaking right now?
Wausau Superior Flat Roofing repairs, coats and replaces commercial and flat roofs across Wausau and central Wisconsin, with a written price before any work starts. Request a quote online and we’ll call you back.
How ponding gets fixed
Clear and reset the drains
The cheapest fix comes first: clean the drains, replace missing strainers, and reset or lower drain bowls that sit high.
Add drains or scuppers
Where a section of roof has nowhere to drain, a new drain or a scupper through the parapet, with an overflow, gives it one.
Crickets
Small built-up ridges between drains and behind curbs steer water to the outlet instead of letting it sit.
Tapered insulation
For widespread ponding, tapered insulation builds slope into the whole roof. It is most cost-effective during a recover or replacement. See drainage and ponding repair.