I read that an edpm rubber roof requires a minimum fall of 1 in 80 in order to be able to shed water. I calculate that to be 0.7 degrees. My flat roof is 1.7 degrees. I was thinking to also install a sheet of polycarbonate on the roof (or even better perspex – glass like window onto the sky). But it is often stated that polycarbonate requires a minimum 5 degrees fall! Am I missing something – polycarbonate requires more than 5 times the fall than edpm rubber to shed water?
Would using perspex make any difference?
Water – What’s the minimum fall for a flat roof with edpm perpex polycarbonate
roofroofingwaterproofing
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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.
I'm a huge fan of EPDM rubber roofs. They are even starting to be feasible for DIY installation.
The most important thing is to glue them down correctly to the roof deck. Otherwise weather forces will make them shift around, fold, stretch, tear seams, or pull away from corners and edges.
The roof decking must be in good condition - so it can glue down properly, and not be stressed or punctured by the decking coming up.
I would not put it over an old roof either, same reasons.
Also, it does not like tar. Tar causes it to melt. If you have simpleminded buffoons who work on your roof and think tar is for all roof repairs, that will be a problem.
Fortunately rubber roofs aren't terribly hard to patch provided you use proper materials.
Cost of the rubber material is quite low, often under $1/foot, but the correct glues and supplies will cost more than the rubber! The stuff comes in widths up to 20 feet.
Almost all rubber material is black. That means your roof will get quite warm. "The thing to do" nowadays especially in CA is to paint roofs white, if you aren't sticking solar panels up there. You can get white rubber, or white paint, but you must be selective as to type of paint. Painting the roof will make it less repairable.
Edit: Apropos a comment, one idea I've thought of to make rubber roofing easier to work with, is to create some vertical ribs on the roof, perpendicular obviously to the water flow. Something like this, assuming a parapet on three sides.
Those vertical ribs could become supports for the solar panels. It's really desirable to NOT have the solar panels introduce roof penetrations, because those leak.
The ribs would be carefully positioned so they can be covered by a single sheet of rubber roof material. A cap over the ribs and you're all set.
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Best Answer
No, you have to follow manufacturer’s recommendations for fall. And by the way, fall is discussed as a ratio of rise/run. Nobody uses degrees for that; it’s an unwieldy unit that you have to keep converting in/out of. Build some roofs; you’ll get it.
The reason for the fall is simply that an EPDM roof is a membrane. It is glued and sealed, and it could nearly be used as pool liner.
Whereas, “tin roof”, perspex etc. rely entirely on lap seams to keep rain out. If the roof is too shallow, water will do an “S-curve” and run backward up the lap seam and get into the roof.