Flattest Roof Slope

[hocho]

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.

 

[KootK]

1/8" per foot with the following caveats:

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.

 

[hocho]

1/8" per foot with the following caveats:

1) Depends of your roofing assembly to some degree and;
2) Your local codes might have limits that need to be adhered to.

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.

 

[thaidavid40]

Check your local "big box" stores - most of them are 1% roof slopes. This works well, if designed properly.

Thaidavid

 

[hocho]

Can you make trusses with 1% slope? What is the minimum slope for trusses roof?

 

[JAE]

Actually most roofs are 1/4" in 1 foot today - the 1/8" slope went away some years ago (about early 1980's I think).

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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[cvg]

as someone who lives in a house with a "flat roof", I can attest that water does indeed "stand" on a "flat roof" not withstanding any deflection caused by weight of water. It only takes a pile of leaves or other debris to block a scupper and render a "flat roof" a non-draining bathtub. I highly advise using the steepest slope you can get away with and neither 1% or 2% is recommended in my book

 

[Ron]

International Building Code and other "replicate" codes require 1/4" per foot. As JAE noted, 1/8" per foot was the previous minimum.

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!

 

[hocho]

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?

 

[Ron]

You can make trusses that are any slope....including flat; however, if the roof membrane selected to go over the trusses is not correct, then you have accomplished nothing. For instance, you can make trusses with a 1" in 12" slope; however, you would not use shingled roofing on such a slope because it would be prone to failure.

 

[Triangled]

curious what your annual precipitation statistics look like...

 

[msquared48]

You can also keep the trusses the same depth but vary the height of the supporting framing to get the slope.

Mike McCann, PE, SE (WA)

 

[KootK]

Just google "high heel truss" for images showing how structural depth can be achieved in low pitch roof trusses.

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.