Flat roof - do they have to be flat?
February 1, 2016 10:25 PM   Subscribe

I have the opportunity to rebuilt an area of flat roof on my house, with a slight slope. My question is, do flat roofs have to be flat? If not, why aren't they more often built with a little bit of a slope (intuitively that would seem to be helpful to get water off it). I live in a wet state (ie rain is the issue not baking sun). Thanks
posted by Xhris to Home & Garden (11 answers total)
There's no reason a flat roof has to be flat (all other things being equal). It's usually an aesthetic choice. I'm always baffled that people choose flat roofs here in the PNW.

If they were more often built with a little bit of slope, then they wouldn't be flat, would they? "Flat" is simply a description of a particular roof.
posted by humboldt32 at 11:55 PM on February 1, 2016

Best answer: I believe that most correctly installed flat roofs are built with a slope - either to a peripheral gutter or to a central sump. Here are some design guidelines.
posted by rongorongo at 11:56 PM on February 1, 2016 [8 favorites]

A sloping roof yields a volume inside that isn't as useful as if the roof were level.

Getting water off is important, but getting snow off is a lot more important. A flat roof can accumulate many, many tons of snow and the building could collapse. In areas where there's a lot of snow they build the buildings with very steeply sloped roofs so that the snow will spontaneously slide off.

It's all a tradeoff.
posted by Chocolate Pickle at 12:38 AM on February 2, 2016 [1 favorite]

Go for the slope!
posted by amanda at 2:29 AM on February 2, 2016

In the UK a flat roof is defined as any roof with an incline under 10 degrees.
posted by biffa at 3:28 AM on February 2, 2016 [1 favorite]

Standard slope for flat roofs are 1/4" per foot. That is enought to move water. You could go to 1/2" per foot, but that is overkill. "Flat" roofs are fine in rainy climates- just not so great in extreme snowy climates. That's when you want enough slope to keep snow from accumulating.

If you're rebuilding an existing portion of your roof- you might have to make major interior and structural modifications to change it to a much steeper slope. I'd say change it if you want, but don't do it because of water drainage.
posted by KMoney at 3:45 AM on February 2, 2016 [3 favorites]

If you're replacing an existing low slope (membrane) roof, install sloped (ridid) insulation to increase your slope if it will not impact the roof edge where you are adding the increased height. Get some well- respected local installers to look at your situation and explain everything until you have a complete understanding of what's involved.
posted by mightshould at 5:18 AM on February 2, 2016

Why are you considering rebuilding the slope?
Is the roof not performing correctly? Is water piling up and not draining off correctly?
If your roof is performing correctly, there's no reason to spend extra money rebuilding the slope. If it's got low spots and has ponds that don't go away, of course, fix it.
If you want to put a rooftop deck in, you might need to make other considerations.

I can't imagine any reason to mess with something that's working correctly.
I live in the Upper Midwest under a flat roof designed well over 100 years ago. My house was built correctly to handle the rain and (lots and lots of) snow, and yours probably was too.
There's lots of fearmongering over flat roofs and I have no idea why this is perpetuated. Keep your property in good condition and you have no worries. When you see "building collapsed due to snow" it's always in a place that doesn't get snow, and I bet $200 that the roof's supports had been weakened through years of neglect of a leaky roof. Correctly built and maintained buildings don't collapse.
A flat roof will also last much longer than a shingle roof.

Don't buy into anyone selling you anything based on flat roof fear. Find a contractor that specializes in flat roofing and get the straight story.
posted by littlewater at 6:30 AM on February 2, 2016

Just chiming in to say that a flat roof shouldn't be perfectly flat and should have a slope to allow water to drain and not pool on the roof.

Flat roofs are perfectly fine in snowy areas as well. When the roof is first designed it will take into account a snow load specified by the local building department and/or code. Just think, most commercial buildings in snowy areas have flat roofs and if collapsing due to snow was such a big problem then they would have sloped ones instead.
posted by any portmanteau in a storm at 8:16 AM on February 2, 2016

On roads you only need, typically, a slope of 0.5%, so you don't need a lot of grade in order to move water. What's more important is having a constant slope so that it drains into your gutters and doesn't pond somewhere in the middle.
posted by hwyengr at 8:24 AM on February 2, 2016

You can use your existing flat roof as a guide to see where additional slope or cants may be needed to avoid ponding - you'll be able to see discoloration or sediment where water sits and evaporates instead of running off.

If this is a membrane system with a parapet similar to this detail it's easier to adjust the slopes than if you have a low-slope shingled roof running to a gutter - but the latter type should have enough pitch that it wouldn't look flat.
posted by a halcyon day at 10:42 AM on February 2, 2016

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