Water still sitting on a flat roof **48 hours after rain is called ponding, and it’s a defect** — not a normal characteristic of flat roofs.
Flat roofs aren’t actually flat. Code requires a minimum slope of one quarter inch per foot toward the drains. When water lingers, it’s usually one of these:
– **Blocked or undersized drains.** The most common cause and the cheapest fix. Houston rainfall intensity regularly overwhelms drains sized to generic national standards.
– **Deck deflection.** Structural decking sags between joists, creating low spots. Ponding causes deflection, which causes more ponding.
– **Settled or saturated insulation,** which compresses and leaves a depression.
– **HVAC curbs blocking flow,** where a unit was installed without a cricket to divert water around it.
– **Tapered insulation installed incorrectly** or omitted on a re-roof to save cost.
Ponding matters for three concrete reasons. **Weight:** water is about 5.2 pounds per square foot per inch — a 20 by 20 area holding two inches adds over 4,000 pounds. **Degradation:** constant saturation breaks down TPO and EPDM seams. **Warranty:** most manufacturers explicitly exclude ponding, so an unresolved pond can void coverage on an otherwise healthy roof.
Coating over a ponding roof without correcting slope traps the problem underneath. It’s the most common bad advice we see on Houston commercial buildings.