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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