Beam span length for a tin roof

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Old 08-31-13, 10:44 AM
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Beam span length for a tin roof

I've been contemplating installing a roof over my patio. My patio sits in the middle of my house, carport, and breezeway. My idea was to attach ledgers to the eve of each structure, which is not an issue. The longest beam span is approx 20ft and will be supported by two 4x4 post's. The next two beams will be 17ft and 16ft, and will not have any support underneath. I wanted to use 2x4/plywood sandwiched beams so that at no point do I have any beams hanging lower than my existing eve. This roof will have galvalume panels screwed directly to the beams, so weight isn't an issue. I figured I could construct the beams in two pieces and then butt them end to end and bolt an additional 2x to each side of the splice. I'm trying to avoid sag in the future. Am I fighting a losing battle?
 
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Old 08-31-13, 11:22 AM
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The word "beams" and 2x4's shouldn't even be used in the same sentence. Your proposed method and your desired results (avoid sag) just aren't possible.

We would probably need a picture to understand the layout better, and then could make some better suggestions. You can upload pictures directly to this website. Just click the "go advanced" button when you go to reply and you can attach the pictures to your reply.

I would suggest that you instead consider making a gable roof over the patio, and tie the new roof line into your existing roof, which would probably create 2 new valleys on each side of the new roof.

To do what you want, a framework of welded 4" iron beams supported by iron columns is likely the only way that you will be able to have a framework thin enough to lay flat 2x4's perlins onto so that you can attach your panels. So that's a minumum of 5 1/2" of thickness just for the "framing" that would attach to your fascia.

Other options might be SIP's and/or carport style roofs, but they are usually aluminum and would need metal beams that are wider than what you originally were thinking.

Another idea might be to use steel flitch plates rather than plywood, bolting the steel on each side of the joist. But that also won't work for the spans you are wanting, and it would also add to the weight of something you are trying to keep light.

SIP's are the lightest, strongest, thinnest roof out there.
 
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Old 08-31-13, 11:40 AM
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Yeah I figured it wouldn't be feasible. Thanks for the advice.
 
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