I'm building a shed/barn and apparently had a bit of a brain fart when nailing down the plywood floor sheathing. I nailed it down parallel to the joists (like t1-11 sheathing on an exterior wall), with a few 4x4 squares that I ended up rotating correctly. I've already nailed down a few exterior walls so it would be a big pain to pull everything up to rotate them.

How big of a deal is this in terms of structural integrity? The floor joists are 16in o.c. Plywood is 19/32. This is a 16x16 shed to store tools, lawn mower, atv, etc. The floor feels perfectly stable, but I'm not sure if there are any long term effects I need to be worried about.

  • 2
    I probably would not use use a pogo stick in it, but beside that it should be okay.
    – crip659
    Oct 5 at 12:04
  • Do you know how many layers the plywood has? Since plywood has alternating grain directions, it should have more structural integrity in the direction it has more grain in, but if it's 3 layers out of 5 then it would matter more than if it's 5 layers out of 9, for example.
    – kaya3
    Oct 5 at 13:30
  • 1
    @crip659 dammit... Oct 5 at 13:38
  • @kaya3 My best guess is that 5/8 plywood has 9-11 layers. So that would mean 4-5 of them are in the "correct" direction. Oct 5 at 13:41
  • Are all the long plywood joins on the joists? if so then there won't be much difference. However, if there are long joins in "free space" that will be the issue - perhaps extra support...
    – Solar Mike
    Oct 5 at 13:42

Assuming your Plywood is Grade 1 (Birch/Pine), at 19/32 you can run the joists at 20in perpendicular (floor) or 24in parallel (roof) for 50psf, so at 16 o.c. you're well within both limits.

I am mixing roof and floor rating, but comparing at 50psf total, since there is no floor rating for parallel installations. This matters for how the load is distributed along the sheet: for a roof it is assumed that the loading is more even. Also note that with floors all sheet edges must be supported, as you have done correctly. Unsupported edges are allowed for roofs.

Where you store the heavier equipment you could always add another layer of plywood or some other lumber to spread the load across at least two joists and achieve an even higher load rating, plus prevent long term sagging of the subfloor. This would also be effective along the perimeter of the shed where you'd have any storage racks or tables which have legs that form concentrated point loads.

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Ref: https://codes.iccsafe.org/content/IBC2012/chapter-23-wood

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