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When a 4x8 sheet of 15/32 plywood is laid lengthwise on 1x4 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

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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

  • Since the sheets will be laid perpendicular to the rafters, the short side will always be supported. The long side edge might have a 2 or 3 inch overhang, so that the fasteners would be 3 or 4 inches in from the edge. – TRomano Feb 7 '17 at 12:26
  • The lathe has gaps and the short side will be unsupported over those gaps. All the time in Dallas I see roofs with lathe (originally shingled with wood) have OSB decking placed on them. I have not asked the roofers if they use clips or other support along the joints on the 8' sides of the decking sheets or the 4' sides. – Jim Stewart Feb 7 '17 at 12:36
  • I've found a document on the APA website. It shows on page 3 the addition of lath to support the long edge of the sheet when it falls between existing lath. I will attach it to my question. – TRomano Feb 7 '17 at 12:49
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To me, the wood members running up and down the roof are "joists" (not purlins.) The wood running perpendicular to the joists are "stripping" (not lathe).

The plywood can be installed parallel or perpendicular to the "joists". So, if you install the plywood perpendicular to the joists, then stripping is required at the LONG EDGE of the plywood. If the plywood is installed parallel to the joists, then stripping is required at the SHORT EDGE of the plywood.

Remember, the important thing is that you can nail into the stripping at the perimeter of each sheet of plywood but you MUST nail into the joists (through the stripping) in the middle of each sheet of plywood.

By the way, ply-clips (aka H-clips) are not a substitute for edge support (stripping) in this situation.

  • The purlins are the smaller 1" x 4" strips running perpendicular to the large framing members. The drawing on the right shows that the APA does not sanction even a small cantilever of only a few inches; a strip has been added to support the lower edge. – TRomano Mar 13 '17 at 11:46

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