One of the things that I look for when inspecting flat roofs is evidence of ponding.
"Ponding" is a word we use to describe the phenomena of water that remains on the roof 48 hours after it has rained.
Roofs should shed water quickly. The longer water stays up there, the greater a chance it has of finding a way into the house.
It's possible to see signs of ponding on a completely dry roof. Keep an eye out for areas of dry silt - like little dry lakes. In extreme cases, ponding can support the growth of small plants and even saplings. The presence of vegetation growing into the roof sheathing is a sign of a troubled roof.
According to Carson Dunlop's Home Reference Book, ponding water can decrease the lifespan of some flat roofs by as much as fifty percent.
Correcting the drainage slope of the roof often means a roof replacement. If the roof's in good condition at the time of inspection, free of leaks, with good flashing and good seams, no further action is warranted. When replacement becomes necessary, the slope issues should be addressed to prevent the recurrence of ponding.
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