What ponding water is
Ponding is water that remains on the roof surface well after the rain has stopped. A common working guideline is 48 hours. Water still standing after that point is a sign that the drainage isn't moving it off the roof. A flat roof has a slight slope, and small low spots are normal. Ponding is the problem when the water sits in the same spots after every storm.
What causes it
- Settling or sagging decks. Over time, the structure can dip, creating low areas that collect water.
- Blocked or undersized drains. Debris, leaves and roofing grit clog drains. Drains that were sized for an older building or an earlier code may not handle heavy Atlanta downpours.
- Clogged scuppers and overflows. Scuppers at the roof edge can block just like drains, and the water then backs up over the membrane.
- Failed or uneven insulation. Insulation that has compressed, shifted or been installed without the right slope holds water in place.
- Rooftop equipment. HVAC units, curbs and added equipment can change how water flows, often creating a new low spot.
Why it matters
- Faster membrane aging. Standing water speeds up wear on TPO, PVC, EPDM and modified bitumen. Ponds are often where a membrane fails first.
- Seam and flashing stress. Water pressure and repeated wet and dry cycles strain seams and the flashings at the edges of a pond.
- Hidden leaks. Water can travel through the insulation and deck before it reaches a ceiling, so the leak shows up far from its source.
- Winter ice load. Standing water that freezes adds weight and can damage the membrane and the structure.
- Debris buildup. A pond collects leaves, grit and dirt, which clog drains and feed the cycle.
How to tell if you have ponding
- Water sitting on the roof two or more days after a rain
- Drains that back up or overflow during storms
- Dried outlines or dirt rings where water stood and evaporated
- Soft spots, bubbles or discoloration in the membrane under the pond
- Stains or moisture on ceilings below the low areas
Fixes
1. Clear drains and scuppers on a schedule. Most ponding problems start here, and this is the cheapest fix. Add it to a maintenance plan. 2. Check drain size and location. A drain that can't keep up with the roof's area may need to be upsized or joined by another drain. 3. Correct the slope with tapered insulation. Tapered insulation directs water toward the drains. It is the most effective fix for a low spot that is caused by the roof's structure or design. 4. Repair the membrane under old ponds. Areas that have stood in water for years may have blisters, splits or failed seams. Repair them before any coating or overlay. 5. Add drains where water collects. If a pond sits far from any drain, a new drain at that spot is often the right answer.
Fixing the drainage is usually the real repair. A new membrane over the same low spot will pond again.
Questions for a roofer
- Where does the water pond, and how deep is it?
- Are the drains and scuppers sized for this roof and our rainfall?
- Is the ponding caused by the structure, the insulation, the drains, or the equipment?
- Will you correct the slope, and what does that add to the scope?
- What repairs does the membrane need under the old pond areas?
- Can drain clearing be included in our maintenance plan?
Next steps
If your flat roof holds water after rain, call (404) 260-3168 or request a commercial roof assessment. We map where the water sits, check the drains, scuppers and insulation, and give you a written plan with costs and priorities.
If the roof is leaking now, see our emergency commercial roof repair page. To keep drains clear and catch ponding early, read about a commercial roof maintenance plan.