drainageroofroofing
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?
From the 2012 International Plumbing Code Section 1106, I get a pipe diameter of 3" based on table 1106.3 and this map:
Your pictures appear to show a steeply sloped section with erosion, a flat section, and another sloped section (without erosion.) If the flat section resulted from poor compaction of an original sloped profile that did not have the flat part, you may be able to simply fill that back in so the slope is not as steep. If you want to keep it in mown grass, that might be the best bet.
Otherwise, "rip-rap" or fairly large rocks are arguably the standard method of holding a steep drainage slope.
Best Answer
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.