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T&G Roof Decking On-centre Span

Started by Rougespear, May 22, 2016, 11:09:27 AM

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Rougespear

I am wondering on realistic rafter "on-centre" distances can be.  Obviously 24" - 32" is common I am seeing.  My calculations show I can use rafters 4' on-centre.  However, with such a long distance between each rafter, can T&G roof decking still be used without sagging between the rafters?  I am thinking 5/4" or 1 1/2" pine T&G on top of the rafters?  Or should I just reduce the on-centre distance to 32" and cut a few more 4x8 rafters?  Live+dead roof loading is 50psf, so I am wondering if that will cause the T&G to sag?  Any suggestions please?
Custom built Cook's-style hydraulic bandmill.

Jim_Rogers

You maybe able to do a "deflection" calculation with the calculators in the red tool box on this site. but you're going to have to do some of the load math to figure out what the exact load will be on one piece of the tongue and groove. But as it is tongue and groove the load will also be transferred over the adjacent pieces. So you may need to figure them into the equation.
Not something that is easily done, I would imagine.

Good luck with your research.

Jim Rogers
Whatever you do, have fun doing it!
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Roger Nair

I would ask either the lumber yard or the milling company for technical support documents concerning spans, fastening and insulation, any good commercial milling company will have docs at the ready.  Species and grade info can vary across the product line and so would span and load bearing capacity.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

timberwrestler

Pine or spruce won't make it by code.  I believe that southern yellow pine or douglas fir 2x T&G will though.  It used to be prescriptive in the building code, I'm not sure if it's in there anymore.  There's also a laminated 1.25 T&G called Lamfloor (I get it from Coastal Forest Products).
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Brad_bb

My 3/4" EWP T&G spans about 2' 3" centers (of 4" thick oak rafters) at most.  With 2x T&G you might get away with a little more span, but I doubt 4ft.
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witterbound

If you're going to put sips on top of them you can span a wider span, I think.

Rougespear

Thanks for the thoughts all.  The more I think it over, I think reducing the O'C distance is the right way to go.  I can then use a smaller 4x6 for the rafters instead of the 4x8... the 4x6 will work on both 32" and 36" centres.  What do you folks prefer: 32" or 36" spacing?  36" works out nicely for a building 36' long, but I realize it deviates from normal convention (but a lot of timber framing does it seems).
Custom built Cook's-style hydraulic bandmill.

canopy

Wide rafter spans with vaulted ceilings are a great way to show off what a timber frame can do. Fewer rafter sets is less work to make, less work to raise, and less work fastening down the boards. I went with 5' spans and made my own custom tng boards sized to suit. Some people here have gone much wider spans than that and I have seen tng boards up to 4" thick sold retail.

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