Ponding Water on
Flat Roofs: Causes,
Weight and Fixes

Puddle or pond: how to tell

Every low-slope roof holds some water right after a storm. NRCA's line is simple: there should be no ponding water on the roof 48 hours after a rain, during conditions conducive to drying. Water that is gone by then is a puddle. Water that is still there is a pond, and most manufacturer warranties treat it as a drainage defect.

1. Wait for the right weather

   Check two days after a real rain, with sun or wind and no fresh showers. A cold, cloudy, still week can leave water that would be gone in July.

2. Outline the water

   Walk the perimeter of each pond with marking paint or chalk so the footprint is still visible after it dries. Dirt rings tell the same story on roofs that have ponded for years.

3. Measure the deepest point

   A tape or ruler at the low spot is enough. Note the length and width of each pond too; the calculator below turns those into gallons and pounds.

4. Photograph and repeat

   Date-stamped photos after two or three storms show whether the ponds are stable or growing. Growing ponds point to a moving deck.

How much does that pond weigh?

People underestimate ponds because they look shallow. An oval pond the size of a two-car garage, just 2 inches deep at the middle, holds close to 200 gallons. Enter what you measured and pick the shape that matches best.

Already leaking under a pond? Start with our commercial roof leak repair guide, then come back to the drainage fix. The roof coating guide compares every coating family in depth.

How to fix it, cheapest first

Work up this list and stop at the first fix that actually empties the roof inside 48 hours. Skipping straight to a coating is the most common mistake we see in bids. If you are still hunting for a leak, our flat roof repair guide covers leak tracing by membrane type.

1. Clean every drain and scupper

   Add strainer baskets and put drain checks on the spring and fall maintenance list. It costs almost nothing and solves more ponds than anything else on this list.

2. Fix the drain height

   A drain sitting proud of the membrane leaves a ring of water around itself. Lowering the bowl, or tapering a sump into it, lets the last inch go.

3. Add crickets and saddles

   Small tapered pieces behind curbs, units and between drains split the water and send it where it should go.

4. Build slope across the field

   Tapered insulation during a reroof, or spray foam during a recover, builds real slope toward the drains. Foam is sprayed thicker in the low areas, so it can correct ponding without a tear-off. ArmorThane's ArmorFoam Roof 200 builds slope and adds R-value in the same pass; see spray foam roofing.

5. Add drains or overflow scuppers

   Where low spots are far from any outlet, a new drain is often cheaper than re-sloping half the roof. Parapet roofs also need secondary overflow drains or scuppers, and code sets scupper openings at no less than 4 inches.

6. Coat with a ponding-rated system

   Once drainage is right, finish with a coating rated for standing water so the leftover birdbaths do no harm.