roof sheathing

[davidj]

It turns out my roof is a horrible size for 4x8 sheets - 33' x 16' 3" (30'x20' cabin, 18" overhangs, rafters 2' oc, 5/8" plywood).

I read somewhere that all sheets should cover more than one rafter bay, so I guess lengthways I can alternate 3'6" and 5'6" lengths at each end, which seems to be the only way that this is possible.  Is that the right thing to do?  It means I'll be overlapping by a 1/4 sheet, not a 1/2 like most do.

As for vertically, is it okay to have a 3" strip of plywood at the top?  (I'm gonna use open cell foam for insulation, so there won't be a ridge vent and big holes are likely to be a problem).  Do I need to use blocking for this, cf plywood clips everywhere else?

Thanks.

[MountainDon]

You should not have a narrow strip o roofing underlay anywhere pn a roof. If some day in the future a 250# roofer comes along and steps there he will go through. You might too if you forget about that narrow strip some time. I'm noit sure what the narrowest strip should be; 12", 16" ?

The sheets should run with the long direction across the rafters. You should also stagger the panel joints by at least one bay. If the cut of parts are not to large you can leave them and cut the roof end all at once with a circular saw.

[rocking23nf]

I almost made a similar mistake on my floor, i laid the first sheet sideways for 8 feet across the joists, then i laid the next sheet the other way running parralel to the joists, i put it in place to test for squareness, and my foot almost went through the floor. plywood is much stronger when placed one direction compared to the other.

[Don_P]

There is a "strength axis" on both osb and plywood so for any horizontal applications it should be laid with the length of the sheet perpendicular to the framing, that sounds like what you are proposing. You can handle the strip 2 ways. Either rip down a row and make the remaining row a good size, you know, I don't know that I've ever seen a minimum width, I try to make it a 16" (third of a sheet) when I do that. Second way is to rip down a 1x and use it at the top to make up the difference, try to pick pretty nice small knotted material, a 1x4 would work here it sounds like. There will be a small step but normally it is unnoticeable in the finished product.

[davidj]

Doh!  There was me trying to think of a structural reason why thin strips of plywood could cause problems and I missed the obvious issue with it being insufficient to hold the weight of amateur cabin builders and the like.

Thanks for all of the useful info - 16" wide strips will only use an extra two or three sheets and seem like the simplest solution.

[JRR]

Another approach is to apply purlins (2x4's, 2x6's,..) directly to the rafters.  Let the rafters extend beyond the building end to form overhang in that direction.  Then apply sheathing to purlins. 

Gives you more options, ... and strength if done correctly.  But does add to cost.  Of course, thinner sheathing may be used.