Residential plywood roof deck: does the long edge have to rest on a purlin

roof

When a 4×8 sheet of 15/32 plywood is laid lengthwise on 1×4 lath (the lath is running perpendicular to the rafters) is it permissible for the long edge of the plywood sheet not to be supported by lath directly below the edge? Is there an allowable amount of overhang?

Here is a cross-section of what I mean, where XXXXXX is the 4' edge of the plywood sheet and [p] is a purlin:

                                    |????|
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX   
[p]       [p]        [p]         [p]          [p]
                                         ^

If overhang is allowed, what is the maximum length of the overhanging plywood section marked with the question marks, where where is no purlin directly beneath the long edge?

If overhang is not allowed, additional lath would have to be added below the long edge of the sheet as a fastening surface, at the location marked by ^.

P.S. I've found an APA document (Bulletin P300C) on the subject of decking over lath; it shows the addition of lath to support the edge. "Panel edges should not be cantilevered". Here's a screen cap of the relevant drawing (the document requires a login/registration and cannot be accessed via a URL):

excerpt from APA Bulletin P300C

Best Answer

There are little metal joining clips that are used for light duty application of roof decking to rafters on 24" centers without lathe. If this is allowed when there is no lathe, then surely it would be sufficient if there is lathe.

The plywood is stiffer in one direction (I think the long direction) and it may be that the stiffness along with the joining clips (if used on the long side) are sufficient. One side of the plywood (I think the short side) is more flexible and that is what needs support by joining it on the rafters.

Editing If I could do it, I would get extra lathe and support the joints with lathe. How much better will you feel for all the time you'll be under that roof to have the joints fully supported?

More This roofer uses plywood strips to connect the long sides of the decking sheets: https://www.youtube.com/watch?v=Ij00l1PzgU4 But I don't think his strips are wide enough. I would think 2" min, 3" best.

He says in this video he uses 1 x 2" battens and supports the long edge. The batten or lathe supports the edge by tension. https://www.youtube.com/watch?v=WwsHolj_bnc

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