Where is the house? If it's going to hold snow, you need to consider the
weight of the snow and the plywood vs. the strength. You'll get a hundred
opinions on this, but I live in New England, and I have 3/4 4-ply.
I just reroofed after 19 years and found that my roof plywood is not
quite 1/2 in. I believe it was 15/32. Not quite thick enough as
it would really sag when I was standing between the rafters and I only
weigh 165. Of course after the roof and underlayment was on, I
didn't notice any sagging.
I would guess that my builder, General Homes, used the cheapest
plywood they could find, so I certaintly wouldn't recommend doing what
I recently completed the roof of our new house which has rafters and
trusses spaced at 24" OC. I looked at two different options, 1/2" OSB and
1/2" CDX plywood. The span ratings are stamped right on the plywood sheets.
Either would span 24" fine, but the CDX had slightly higher span ratings
and more important to me the sheets were lighter. Much nicer when you have
to get those big sheets up on the roof in the first place.
Anyway, my final choice was 5-ply CDX plywood. I also used metal "H-clips"
midspan between each rafter. I weigh over 200 pounds and the roof feels
plenty strong to me with virtually no deflection between rafters.
Of course, if you regularly get heavy snow, have a low slope roof, or just
want to upgrade the roof ply, then by all means go with 5/8" or 3/4". It'll
just cost you more money, and you'll have a slightly harder time lifting
the sheets up to the roof.
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