· Drainage
Low-slope roofs are never perfectly flat, and every one holds some water after a storm. What matters is how long it stays.
The 48-hour rule of thumb
Many manufacturers treat water that is still there 48 hours after rain as ponding. Check your own warranty; the wording varies.
Why it happens
Drains set too high, deck deflection between joists, and insulation that has crushed over the years. On older buildings it is often all three.
What fixes it
Tapered insulation crickets that push water to the drain, and drain bowls reset at the right height. A coating does not fix ponding; it has to be corrected first.