- The laps, where one sheet was sealed to the next
- The flashing where the roof turns up a wall
- The fall, and whether water still reaches the drain
Flat roofs
A flat roof is not flat. It is built with just enough fall to move water off it, and after that everything rests on the seams and the flashings. When one of them lets go the water does not run off an edge where you would see it. It travels along the deck and comes through a ceiling somewhere else in the building entirely, which is why the wet spot and the hole are hardly ever in the same place.