Flattest Roof Slope
Flattest Roof Slope
(OP)
For typical gable roof.. what is the formula for flattest slopes possible where the rain can still flow to the gutter. The area is typical 12 meters width by 12 meters length with gable roof from middle extended to the sides. I need the flattest slope. Any experiences in those actually having built flattest roofing?
Any online calculator ok too. Thanks.






RE: Flattest Roof Slope
1) Depends of your roofing assembly to some degree and;
2) Your local codes might have limits that need to be adhered to.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Flattest Roof Slope
1/8" is 0.125" and divided it by 12, you get 0.0104 or about 1%. Did you type it right? 1/8" per foot? It's almost flat.. has anyone actually done this slope? Won't the roof be flooded first before
water slowly flows down the gutter. I'm asking about the more practical flattest slop where rain won't puddle. Most installers reason for using steeper slope is so there is no leak because flashing may not be perfect.
RE: Flattest Roof Slope
Thaidavid
RE: Flattest Roof Slope
Can you make trusses with 1% slope? What is the minimum slope for trusses roof?
RE: Flattest Roof Slope
The slope you should use should also consider the type of roofing. Single ply roofs and built-up bituminous roofs can go much flatter than asphalt shingles.
In fact, with asphalt shingles I believe 3:12 is a good limit with an ice shield below but 4:12 is preferred.
Also - "water doesn't stand on a flat surface" so if you check for ponding and the roofing works, you can go flat if you like. Might not meet code but it will theoretically work.
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RE: Flattest Roof Slope
RE: Flattest Roof Slope
KootK is correct in that it depends a lot on the roof membrane system selected. As an example, a coal tar bitumen system typically cannot tolerate a 1/4" per foot slope because of the rheology of the coal tar bitumen (it will slide downhill!); however, coal tar is rarely used these days (even though it is perhaps one of the better roof membrane systems ever devised!). Most current membranes are stated to require a 1/4" per foot slope and no ponding on the roof surface that remains at 72 hours. This is common and a consideration for design. Don't design something structural that is incompatible or inconsistent with performance/serviceability requirements!
RE: Flattest Roof Slope
So can you really makes trusses that are 3% slope? Then the members have to be very short.. how would it even be strong? Maybe there is a minimum slope on trusses framing? What is that?
RE: Flattest Roof Slope
RE: Flattest Roof Slope
RE: Flattest Roof Slope
Mike McCann, PE, SE (WA)
RE: Flattest Roof Slope
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.