Plywood span questions

[Arky217]

For subfloors, would 5/8" plywood spanning 12" joists be stronger and less deflection than 3/4" plywood spanning 19.2" joists ?

And for roof sheathing, would 1/2" plywood spanning 19.2" rafters be stronger and less deflection than 5/8" plywood spanning 24" rafters ?

Thanks,
Arky

[MountainDon]

I don't think that can be asnwered without knowing anything about the joist and rafter materials/sizes.

I've always believed that the deflection / stiffness for a floor is calculated on the joists, depending upon the species, grade, size and spacing. The subfloor sheathing can add to that stiffness, but I've not seen it involved directly in those calculations. Ditto on the roof rafters.

[Arky217]

I'm talking about the strength and deflection of the plywood only, not what it rests on.

In other words, pretend that the members the plywood rested on were solid steel, which thickness and span combination named above would be the strongest and have the least deflection ?

[PEG688]

The issue with the 5/8" is no T&G edges , at least I've never seen 5/8 CDX or OSB that was T&G,  so I'd throw that idea out.

The 3/4" with T&G laid  on " I " joist with wide 1 3/4" or 2"  flanges would be OK on 19.2 layout , BUT skimpy IF laid on dimensional lumber so that a trick question and your steel analogy is,  in ways,  wrong. As the spacing left between the joist is what matters here and 19.2 layout that spacing was part of the figuring  / factor of the layout and resulting space , it was designed with  "I" joist in mind.

  On the roof I've  never seen a 19.2 layout used , not that you couldn't do it that way , it just would be very uncommon.

Either way I'd guess about a wash as with either of the two detractors you have given us I'd use H clips / plywood clips.

  The best option for roof sheathing on dimensional lumber is a 16" OC layout with 5/8 CDX plywood. IMO.