A low slope roof needs a quarter inch of fall per foot and a clear path to a drain.
At a glance
- Water still sitting 48 hours after rain counts as ponding.
- Ponded water adds weight and breaks down most membranes.
- Every roof needs an overflow path when the main drain clogs.
- Tapered insulation rebuilds slope without touching framing.
Slope and why it disappears
Framing deflects over the years. Added rooftop equipment presses down. A remodel raises a parapet or blocks an old scupper. Any of those turns a roof that drained fine into one that holds water in a corner.
Ponding is not just cosmetic. Water weighs about five pounds per square foot per inch of depth, and a large pond loads the frame it already sagged.
Drains, scuppers and overflows
Interior drains pull water through the building in a pipe. Scuppers push it out through the parapet wall. Most Los Angeles low slope roofs use one or the other and need an overflow set about two inches above the primary.
The overflow is the part people forget. When leaves pack the main drain in the first storm, the overflow is what keeps the roof from becoming a pool.
Fixing a flat spot
Tapered insulation is cut in a slope and laid under the membrane to rebuild fall toward a drain. Crickets do the same job in a small area behind a curb or a chimney.
Adding a new drain at the low point is sometimes cheaper than retapering a whole roof. A roofer should price both.
Common questions
How much ponding is acceptable
Most manufacturers allow standing water for 48 hours after rain and no longer.
Can I add a drain to an existing roof
Yes, with plumbing work below and proper flashing at the new opening.
Do flat roofs need gutters
Usually not. They use scuppers and downspouts at the wall instead.