Eli5 flat roofs

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Flat roofs. Are they supposed to have water after rain and it evaporates? Or does the flat roof have something on it to help standing water drain?

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A flat roof should still have a gentle slope towards a drain, that is plumbed into the building’s drainage system. It shouldn’t collect standing water.

They should be slightly tilted and lead the water away (normally through a small hole in the wall at the lower end). Standing water is never a good sign and will ruin almost anything.

Flat roofs have very slight elevation shifts that help water flow to scupper boxes along the edges or drains located in the middle of the surface, which then connect to gutters to drain out water. There may still be some ponding (what it’s called when there are puddles of water left after rain, etc) but not inches of standing water.

“Flat” roofs generally are designed with a very slight slope so water drains, but in practice that slope is so small that as the building settles and shifts just a tiny amount, it becomes so flat the water is trapped. That’s why flat roofs leak a lot more than pitched roofs.

Think of them a bit like a shower base, with water running down the drain.
Also, flat roofs are usually found (traditionally, anyway) in areas where there is little or no snow. Snow can build up too much on a flat roof, causing weight problems.