lower cost - low pitch gable or flat(shed)roof?

[1201]

Ive been looking at the 20x34 lately. I think it could work great for what Im trying to do if doubled up to 20x68

my question is this. say I want to do a 2/12 pitch gable - we have no snow load that I know of here in north texas.

would a 0.25/12 or 0.5/12 pitch flat roof be cheaper?

thanks

[NathanS]

The potential issue I can think of is at .25 or .5 / 12 you can't use shingles or metal, you will have to use a flat roof material like EPDM or hot mopped tar. I think that would end up costing more in the end.

The manufacturer will usually tell you the minimum roof slope.

[1201]

The potential issue I can think of is at .25 or .5 / 12 you can't use shingles or metal, you will have to use a flat roof material like EPDM or hot mopped tar. I think that would end up costing more in the end.

The manufacturer will usually tell you the minimum roof slope.

thanks for that, and thats the kind of info I need.

I thought the irc allowed metal roofs at 0.5/12? maybe I was wrong , and even if Im correct it may not be that good an idea. on a 20ft span it would only rise approx 10 inches  from low to high.

I havent looked at epdm costs but that is something to take I hadnt taken into consideration. I guess I should look into it.

thanks man

[NathanS]

They might, just make sure the manufacturer also allows a .5/12.

[1201]

They might, just make sure the manufacturer also allows a .5/12.

copy that.

[Don_P]

Technically the rafter tables in the IRC do not apply below 3/12 and all members get designed as beams, read the footnotes in and around the tables in chapter 8, that's the only language that springs to mind. You really don't want to go below that anyway. I'm working on a very low slope commercial roof, you do not want to go with epdm or tpo, I'm sitting on a 60k bid right now on ~4000 sf.
At 20' some form of truss is going to be cheaper unless there is an interior bearing around midspan, if so, you would need to compare prices.

Just eng info, ponding on low slopes is a common way they fail. Deflection causes water to pond, the weight causes more deflection, the pond grows, the weight causes more deflection, the pond grows, etc until it collapses. I'm the proud owner of 3 huge billboard tarps at the moment and its still raining inside, flat roofs suck.

[1201]

Technically the rafter tables in the IRC do not apply below 3/12 and all members get designed as beams, read the footnotes in and around the tables in chapter 8, that's the only language that springs to mind. You really don't want to go below that anyway. I'm working on a very low slope commercial roof, you do not want to go with epdm or tpo, I'm sitting on a 60k bid right now on ~4000 sf.
At 20' some form of truss is going to be cheaper unless there is an interior bearing around midspan, if so, you would need to compare prices.

Just eng info, ponding on low slopes is a common way they fail. Deflection causes water to pond, the weight causes more deflection, the pond grows, the weight causes more deflection, the pond grows, etc until it collapses. I'm the proud owner of 3 huge billboard tarps at the moment and its still raining inside, flat roofs suck.

Wow.ok, that's a nice scared straight story. You've got me definitely rethinking the shed roof.


You said not to go below 3 12 porch on the gable? Can you expound? I know you would have to use a ridge Beam if going rafters but what other issues spring to mind?
Thanks

[Don_P]

There's nothing wrong with a shed roof, just keep the slope up. On a 20' shed roof at 3/12 pitch the high side would be 60", 5', higher than the low side. Check your roofing material specs for minimum slope, or the slope where they start requiring detail changes. When you see them calling for enhanced detailing you are in the frequent failure zone, stay out of that if possible.

On a gabled roof, there are a number of threads on here explaining the difference between a ridge board and a ridge beam. Basically if you will have a level ceiling, a ridge board is used, if a sloped ceiling then a ridge beam, likely with one or more interior supports or a wall under the ridge. If you can maneuver trusses they typically go up quicker, have engineering, less skill required. The roof and stairs are where geometry class comes into play, not hard but you are thinking in more planes.

[1201]

Thanks Don.

That is great info. I know I don't want to do a 3 12 pitch shed. Max is 1 12 so it may be out of the question