roof
I'm building a goat shed – total size is 16' x 24'. The tin roof (lean-to) will run the 16' length. Can I use a 2×6 for the rafters? Could go 16 or 24" OC. Thanks.
Update from responses: roof is 1/8 pitch 8 ‘ off the ground near Houston (snow load is not a factor). Don’t have goats yet.
What they are suggesting is done in metal buildings in the USA, the insulation is wrapped in a type of plastic sheeting on all sides, it is made that way. It is also in large rolls and very wide, maybe 3-4ft?
My biggest concern would be the distortion of the metal when it comes to removing it. If the roof is attached with screws, you have a small chance of salvaging the roofing. If the roof is fastened with nails, the metal will surely be distorted. The fasteners to reattach the roofing, which should be screws with neoprene washers, rely on a flat surface, if it is a newer system. The refastening may not take well if care is not taken on the removal. Of all the fasteners in any given roof it will only take one to create leak, with the insulation in place and it's plastic layer, it would be hard to tell where a leak may really be or if any evidence is visible until it is too late. I refer to wet, or rotted framing after a time.
If it is the older system where the nails are driven through the standing rib, and the nail has a lead coating to conform and seal the hole from the nail,, DO NOT let them remove it unless a new roof goes back down. It may be better to insulate from the underside, even if it may be finished.
This is a sample of what I was going to suggest, It may cost more overall, but it is the metal roof and insulation combined. It will not compress like the fiberglass will and give a higher R rating
We use 19.2" oc joists (5 spaces for 8' plywood) when we use Trus-Joist open web joists. They are so expensive that keeping size down and spacing maximized saves costs. (True-Joist has a software program to figure this out...)
Maybe the alternating spacing allow fasteners to secure the corrugated roofing on the "down" curve of the corrugated shape.
I think this old-time carpenter was ahead of his time.
Best Answer
If you look at the span tables here
http://www.southernpine.com/span-tables/joists-rafters/
And look at a table for roof rafters for a very light load, "RAFTERS – 20 PSF LIVE LOAD, 10 PSF DEAD LOAD, 240 DEFLECTION, CD = 1.25"
http://www.southernpine.com/app/uploads/SPtable41_060113.pdf
You'll see there's a maximum span of 16'-4" for 2x6's spaced 12" - so you might get away with it.
Even without the goat factor, I'd bump up to 2x8's for sure.