Do this first
What to do right now
- Photograph the ponded areas with a tape measure or a ruler in frame so the depth is documented.
- Clear the drains, scuppers and gutters you can reach safely, leaves, gravel and a stray plastic bag cause a surprising share of ponding.
- Check whether the water sits over a low spot in the deck or simply upslope of a blocked outlet. These need completely different repairs.
- Press the ponded area with a foot or a broom handle: if it feels spongy, the insulation underneath is already saturated and the deck may be involved.
- Note how long the water takes to disappear after rain stops in San Bernardino, anything past 48 hours is the industry definition of ponding.
- Get a low-slope specialist to survey it before the next storm season, not during it.
What counts as ponding
- Industry definition of ponding
- Water still on the roof more than 48 hours after rainfall stops, in conditions where evaporation should have cleared it.
- Water gone in under 48 hours
- Normal drainage lag on a low-slope roof.
- Water still there after 48 hours
- A defect.
- Weight of standing water
- Roughly five pounds per square foot for every inch of depth.
Why it destroys a membrane
Standing water attacks a roof in several ways at once:
- It magnifies UV as it sits, accelerating the breakdown of the sheet.
- It keeps seams and flashing details permanently wet, so any small weakness becomes a path.
- It grows algae and vegetation whose roots find their way into laps.
- It is heavy, so a broad pond deflects the deck slightly, which makes the low spot deeper, which collects more water.
- On many roofs in San Bernardino it freezes and thaws just often enough overnight to work seams loose.
It also has a contractual consequence people discover far too late: most single-ply manufacturer warranties explicitly exclude damage in areas of ponding water. A perfectly installed TPO roof with a ponding defect can be outside its warranty on day one.
How ponding actually gets fixed
Cleaning the drains is step one and sometimes the whole answer. Beyond that, the fix depends on why the water sits there. If the deck has deflected or the roof was framed dead-flat, the usual solution is tapered insulation, polyiso boards cut to a slope, typically a quarter-inch per foot, built into crickets and saddles that route water toward the outlets. If the drains are simply too few or badly placed, adding a drain or a through-wall scupper is more effective than any amount of tapered board. Sump pans around existing drains help where the deck dips slightly the wrong way.
Watch out for the shortcut: pouring a levelling compound or piling extra coating into the low spot. It adds weight where the deck is already deflecting and rarely lasts.
Rebuild or restore
If the membrane is otherwise sound and the ponding is mild, a reinforced coating system over corrected slope can extend the roof many years and, in California, is often a chance to bring the surface into Title 24 cool-roof compliance at the same time. If the insulation under the pond is saturated, that section has to come off, a coating over wet insulation traps the moisture and shortens the life of everything above it. A moisture survey or a few core cuts will tell you which situation you are in.
