Spacing of roof rafters

I am building an enclosed porch using native lumber. The underside of the roof will be visible from inside so instead of using 2x6 and plywood, I am using using rough sawn 3x6 rafters spaced at 30", and 1x12 pine smooth on one side for the roof decking. My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient at 30" spacing? The roof will be covered with asphalt shingle. Thanks,
-- Jeff
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The 30" may be too far. If you do a google search you can find web sites that can help with load calculations. My guess is that there is less of a requirement in Florida than say, Ontario Canada with a snow load to figure in.
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set 2 2x4s 30" apart and span w/1x12
step on it
do you think that's sufficient for your roof?
I imagine what you could do is lay down the 1x12 underneath of a layer of plywood (osb, whatever) sheathing but make damn sure you're nailing the ply into rafters and not the 1x12. You'll need to add 3/4 in to the length of the roofing nail you'd usually use as well.
Also there's a reason that rafters are 24 inch on center vice 30. 4x8 ft plywood sheets can be easily made to butt halfway into the thickness of a rafter rather than hanging between them.
These are questions for your building inspector or at least hitting the library and looking at the code in your area.
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On 6-May-2005, snipped-for-privacy@yahoo.com wrote:

didn't mean roofing nail.. meant the nails you're using to attach the sheathing with. my bad
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On 6 May 2005 08:36:30 -0700, snipped-for-privacy@hotmail.com wrote:

Can you T&G the boards. that would help. This applies to laws in my area yours may vary. It would be enough but probably not legal. Your rough saw rafters might not be legal either unless they have been graded. Of course if you have an engineer friend who will sign off on the job for you then you're OK.
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I realize that 30 is pushing it even with 1" roof decking. 24" is more common in these parts (Vermont). I am using 3x6 rafters which are beefier than normal and putting in some 30" wide skylights. That's why I wanted to go with the rafters on 30" to keep the spacing even because it will be visible from inside. But I guess I should go with 24-26" rafter spacing and just frame in around the wide skylights with extra rafters to the headers as is usually done. I am not using plywood anywhere. Thanks,
-- Jeff
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If you're using real 3xN rafters, you shouldn't need to double up on the rafters flanking the skylights. The problem isn't the ability of the rafters to hold up the weight of the roof, it's the ability of the roof-decking to span 30" with the weight of one man plus tools without flexing so much that it damages things.
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As other posters mentioned, there are two question: are the rafters strong enough for the roof load, and is the decking strong enough for the span between rafters?
For the rafter strength question, it is impossible to say without knowing the rafter span. However, it is worth noting that the load per rafter is proportional to the rafter spacing, and the rafter "strength" (shear, bending and deflection) is proportional to its width. So if your rafters will be a full 3" thick at 30" spacing, their strength is equivalent to 1.5" wide rafters at 15" spacing. Just check a standard rafter sizing table for your load and span to verify that nominal 2x6s at 16" o.c. will suffice.
[BTW, if the skylights are a full 30" wide, then you'd need your 3" wide rafters 33" o.c. to have a 30" wide space between them.]
As to the strength of the roof decking, my quick calculations suggested that a full 1" thick material would actually be OK at 30" o.c. However, I'm not an engineer, so I'm not confident enough in my calculations to post them, and you shouldn't believe me.
Cheers, Wayne
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On Sat, 7 May 2005, Wayne Whitney wrote:

I didn't see the original post, but I'd probably be talking with the local planning & zoning people. If its going to be in a house or commerical facility, they will want to see plans. Ask them what is OK.
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