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Old 04-11-2013, 07:45 PM   #1
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Roof truss for low flat roof


I am getting ready to build an accessory building here in WI. Due to some stringent zoning restrictions, I am limited to 400 sq ft and 10 vertical feet. With those limitations I am planning to go with a 20x20 structure with a flat roof. To maximize head room I am going to fore go and overhead garage door and use sliding barn doors.

My question is, what should I use for the roof structure? Wood trusses? Engineered beam? Some sort of metal i-beam?

My goal is to maximize head space yes be somewhat reasonable in regards to cost. Another piece of info....I want to insulate it because I plan to heat it occasionally, and I want touse a metal roof.

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Old 04-11-2013, 08:11 PM   #2
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Roof truss for low flat roof


You need a low slope roof like EPDM. Cant use metal on low slope. One main girder and 2 x 10 joists off the girder.

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Old 04-11-2013, 08:33 PM   #3
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Roof truss for low flat roof


your going to have to talk to a local engineer, they will be able to calculate if standard lumber roof rafters will able to handle the snow load or if you need trusses
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Old 04-11-2013, 08:37 PM   #4
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Roof truss for low flat roof


why not stick build it? I do not understand the 10' hight limitation. Id that wall hight and then build a roof on top of that?
Or is it the peak of the roof hight? <<<< this makes no sense to me.

Here is a picture of how I would build the roof. Keep in mind that this picture is just to show example. Your roof would be different dimensions. I think this one is about 16"x24" There is 3 beams 16' long sitting on top of each post to connect the sides together. In the center is a king post that the ridge board sits on, consist of 2, 2x10 with a joint on the center king pin.
The joist are 2x8 and also 12' long for the over hang and the miter cuts.
If you have to keep your roof peak at 10' which is silly, you may not even have king pins and just a 2x10 sitting on top of a 8' wall. joist running from it to the walls?
But is such a old and common way to do it, and this photo shows the bones during construction and not all covered up.

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Old 04-11-2013, 08:44 PM   #5
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Roof truss for low flat roof


I hope this is not a dumb question, but why can't I use metal on a flat (or semi-flat) roof? I am thinking there will be some pitch to she'd water, just not enough to she the snow very well. My thinking was that as long as the underlying structure can support it, metal would be fine.

I would love to hear your opinions.

Thanks,
Matt
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Old 04-11-2013, 08:50 PM   #6
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Roof truss for low flat roof


Quote:
Originally Posted by funfool
why not stick build it? I do not understand the 10' hight limitation. Id that wall hight and then build a roof on top of that?
Or is it the peak of the roof hight? <<<< this makes no sense to me.

Here is a picture of how I would build the roof. Keep in mind that this picture is just to show example. Your roof would be different dimensions. I think this one is about 16"x24" There is 3 beams 16' long sitting on top of each post to connect the sides together. In the center is a king post that the ridge board sits on, consist of 2, 2x10 with a joint on the center king pin.
The joist are 2x8 and also 12' long for the over hang and the miter cuts.
If you have to keep your roof peak at 10' which is silly, you may not even have king pins and just a 2x10 sitting on top of a 8' wall. joist running from it to the walls?
But is such a old and common way to do it, and this photo shows the bones during construction and not all covered up.
The peak of the roof cannot be more than 10 vertical feet. Technically it will be a "boat house" to store my canoe and kayak, along with my table saw, lathe and drill press. :-)
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Old 04-11-2013, 09:33 PM   #7
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Roof truss for low flat roof


I have ran some pretty flat roofs with metal, you will be fine.
You do need to have some slope. The term flat roof is to broadly used. Is no such thing as a flat roof that will drain.
But are roofs with minor slope that will drain, and everyone calls them flat.

You have to do the snow load calculations for your area and find out what size of framing you need for the joist and the ledger board.
I think you can do it with standard lumber.
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Old 04-11-2013, 10:24 PM   #8
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Roof truss for low flat roof


Quote:
Originally Posted by funfool
I have ran some pretty flat roofs with metal, you will be fine.
You do need to have some slope. The term flat roof is to broadly used. Is no such thing as a flat roof that will drain.
But are roofs with minor slope that will drain, and everyone calls them flat.

You have to do the snow load calculations for your area and find out what size of framing you need for the joist and the ledger board.
I think you can do it with standard lumber.
Are there some low profile metal i-beams or engineered beams that may only require, say 4"-6"of headroom, instead of the 8"-10" that might be needed with wood framing? Just looking for ideas to explore.
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Old 04-11-2013, 11:27 PM   #9
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Roof truss for low flat roof


how much snow do you get? low sloped roofs tend to hold onto snow. the more they hold the heavy they are over time. don't want it to fall in on ya.
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Old 04-12-2013, 01:29 AM   #10
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Roof truss for low flat roof


If you could split the span equally with a central steel I-beam (or even split the span into thirds by having two equally-spaced I-beams) you could probably use 2x6s spanning onto the beams. But I doubt a steel beam 6" deep would span 20ft without excessive deflection, which could cause ponding. On that span, if incorporating steel beams, I think as a minimum you would be looking at a beam around 8" deep.
As others have said, all subject to snow loading; you would probably need to get it calculated to get the most economic roof structure.
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Old 04-12-2013, 01:44 AM   #11
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Great comments! I appreciate it.
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Old 04-12-2013, 08:32 AM   #12
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Roof truss for low flat roof


Quote:
Originally Posted by funfool View Post
why not stick build it? I do not understand the 10' hight limitation. Id that wall hight and then build a roof on top of that?
Or is it the peak of the roof hight? <<<< this makes no sense to me.

Here is a picture of how I would build the roof. Keep in mind that this picture is just to show example. Your roof would be different dimensions. I think this one is about 16"x24" There is 3 beams 16' long sitting on top of each post to connect the sides together. In the center is a king post that the ridge board sits on, consist of 2, 2x10 with a joint on the center king pin.
The joist are 2x8 and also 12' long for the over hang and the miter cuts.
If you have to keep your roof peak at 10' which is silly, you may not even have king pins and just a 2x10 sitting on top of a 8' wall. joist running from it to the walls?
But is such a old and common way to do it, and this photo shows the bones during construction and not all covered up.

Nice looking work funfool!!
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Old 04-12-2013, 09:46 AM   #13
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Roof truss for low flat roof


Well, it depends on what type of metal roof you are talking about.

Yes, you can go as low as 1/4 inch per foot with a Trapezoidal Structural Standing Seam roof with a double locked rib like the Butler MR-24 roof panel, (I wouldn't go less than 1/2 inch per foot, but you can) or you can even go dead level with a soldered flat seam copper panel such as would be used in a gusset, but generally speaking, the lower limit for an architectural panel is 3/12. There are exceptions to any rule but the break point is that above 3/12 is considered steep roofing and below 3/12 is considered low slope. It is wise to take advantage of the knowledge gained through a couple hundred years of roofing experience. NRCA and SMACNA contain that knowledge.

You can always ignore what has been learned, though, as there is always room for one more passenger on the ship of fools.

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Last edited by jagans; 04-12-2013 at 09:49 AM.
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