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I have a patio with a 13ft(4m) by 26ft (8m) Roof, the slope runs vertically down the short (13ft) side.. and the height difference from the high point to the low point is only about 10in (25cm).

What is considered an acceptable angle for a roof to ensure that there will be sufficient run-off during rain conditions?

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It's really going to depend on how much rain you get. Roughly speaking the more rain you get the greater the slope (up to a point of course). –  ChrisF Feb 17 '13 at 14:54
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It is not sufficient for almost any shingles, so you'll have to use an alternative covering. I like the roof paints personally, I also caulk the joints of my 4'x8' roof panels during installation to ensure full coverage. –  Jason Feb 17 '13 at 16:25
    
Yes. we get quite constant rain in the rainy season, never excessively heavy. the main problem is if there is a bolstering wind that pushes the flow up the slope. (or retards the flow) –  Hightower Feb 17 '13 at 18:12

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"What is considered an acceptable angle for a roof to ensure that there will be sufficient run-off during rain conditions?"

Generally, minimum slope for water to run off is 1% (1/8" per 1'). However, minimum slope for a flat roof by building code here is 2%. (1/4" per 1').

You have more than enough slope for your roof, however you will likely need some sort of membrane roofing since it is very low slope.

There are profiled metal roof cladding systems specifically designed for low-slope application. I don't know if any would be good for a slope as low as what you've got, so that is something you would have to look in to. Maybe call local roofing supplier and see if they have anything that would work for you.

The lowest slope you can do for a shingled roof (that I know of) is 1 in 4.

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Since water finds its own level, even a 1 degree slope will allow water to run, but it is generally better to to steep your roof 15-30 degrees for efficient running of water to avoid soaking and leaking in the roof.

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