Hot Topics: Pooling on a Flat Membrane Roof

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Flat roofs need to be properly build with adequate sloping and drainage, otherwise splashing water can pool around your foundation and saturate your siding, causing mold, mildew, and more! There are different types of gutters and drainage systems available, but how do you know what to choose? Will adequate drainage be a sufficient fix?

Original Post: Gutters vs. Copper Boxes for a Flat Roof?

outdoormike Member

So, as I sit and type this it's raining outside and rivers of water are splashing over the sides of my flat roof in various areas. There is no gutter system, and the pitch varies, so the water leaves in odd spots.

Had a roofing guy suggest I install copper boxes for the larger exit areas. Never heard of them until this and have since seen them on Google. I'd have to install perhaps six to handle the worst of it. They are pricey being made of copper, and installation will be a major job.

The alternative is standard gutters but only in short sections where the water cascades over the edge. Much cheaper and easier than the copper boxes, but I'm not sure they will be able to handle all the water with much it is spilling out and over just as now.

What can anyone tell me about how to decide which is best for my situation?

It's just a large flat membrane-type roof with no edging around the top, except for a small, maybe 1/2-inch, lip that is suppose to guide the water to one side. It really does not since the roof is not pitched correctly. You can see in this pic how the water is collecting near the satellite dish to go over the side.

All I can do at this point is collect what is pouring over so that it comes down a pipe and not gushing over the edge and splashing back up on to the stucco wall, removing the paint in spots.

There is a 12-inch overhang all around the building, so I do not have to go inside with the piping. Will boring holes to install a drain on the roof near the edge where the water seems to run to (within this overhang) work better than traditional gutters?


Highlights from the Thread

CarbideTipped Forum Topic Moderator

Seems to me a standard gutter would be better than trying to put a bunch of scuppers around trying to hit the right spots. The spill-over spots could change with wind and rain direction. But, the raised lip is going to impede drainage into a gutter, can it be removed?

Probably either way is going to be only a partial solution until the slope issues and low spots are corrected. But, that probably mean replacing the membrane or building a new assembly on top.

Let's see what some of the others here have to say.

XSleeper Member

I'd suggest a commercial gutter. The flashing in front shouldn't have had a lip. The lip is used on gable ends, where you want to prevent water from running off the edge and direct it down to the bottom. A gutter apron should have been used there. A good epdm roofer could replace that, if needed.

CarbideTipped Forum Topic Moderator

The installer can put a raised splash shield all along the top front edge if necessary to prevent the water from shooting past the gutter, but I doubt it would be needed with such a low slope.

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