# Plys in Plywood

[Jim B. Robinson]

I'm currently studying HI - didn't know what other forum to post to.

This is in reference to plywood facts, not the inspection process. Our exterior inspection manual states "There are always an odd number of layers." And the next paragraph states: "Panels are always strongest in their long axis, though, since there are always more plies with their grain oriented with the long axis."

This didn't sound correct, so I took a jaunt down to my basement to look at some cut-offs. The 1/2" CanPly DouglasFirPlywood is 4 plys thick, each layer approx. 1/8". The two center core plys are oriented in the same direction and together oriented 90deg. between the face and back.

Another issue states: "(The tongue-and-groove approach is only common with plywood)" This is incorrect as 5/8" OSB t&g is readily available, in my locale anyway.

I've searched the net for other sources, with little luck, so here I am asking for plywood facts. I realize this doesn't involve inspection per se, I just like to know I'm reading/learning qualified material.

Comments?

Thanks,
Jim

[Jim Luttrall]

This just goes to show you, never trust facts from only one source. If you can't document from reliable sources, question the "fact" and always question books from home inspection schools.

[Jerry Peck]

I have also seen that 4-ply plywood at the Big Box stores.

As I recall, though, structurally rated panels will have an odd number of plies, typically 3, 5 or 7 plies, with the two face plies and the odd plies all oriented with the face grain in the long axis, with the even plies perpendicular to the long axis.

That information was correct in that the plywood is stronger in the direction of the face grain, even with 4 plies as the inner plies have more defects in the plies than the outer plies.

[John Kogel]

If this looks wrong, you now know why. :)

[Jim B. Robinson]

I emailed Certiwood and received this reply.

Jim,

Your are correct. This is likely an old publication. An even number of plies is allowed. And for typical Canadian softwood sheathing and select grade plywood the veneer thickness is 1/8”. Good one side, concrete formwork and other appearance/specialty plywoods may use different veneer thickness.

Feel free to call with any further questions.

Best Regards,

Paul Jaehrlich, P. Eng.
Manager, Technical Services
735 West 15th Street, North Vancouver, BC, V7M 1T2
Direct: 604 981-4182
Fax: 604 985-0342
www.certiwood.com


Testing, Certification, Inspection and Product Development for the Wood Industry

[Jim B. Robinson]

That information was correct in that the plywood is stronger in the direction of the face grain, even with 4 plies as the inner plies have more defects in the plies than the outer plies.

Agreed.

[Jack Feldmann]

Look at Baltic Birch plywood if you want to see a lot of layers/plys.