Plywood Grades

[awcivil]

Can anyone explain the difference between CDX (and OSB) panels and Structural 1 panels. Is there a difference in allowable shear values in the NDS Table 4.3A? If so, which values should be used for CDX and OSB? Also, why is it that contractor want to use these types of materials? I assume cost but is there another?

 

[msquared48]

Very few structural situations require Structural I panels when thought has been put into the design to consider the structure.

For residences and commercial applications CDX and OSB are more available and $$$$$$$$$$$ cheaper. This gets contractors' attention.

Mike McCann
MMC Engineering

 

[Ron]

CDX is a "non-designation". It is not structurally rated and is considered a utility grade. Generally, the letter designations refer to face and backing grades of the exposed surfaces. A "C" face indicates that the facing surface is rated as a "C" material, meaning that it has a certain allowable number of knots and blemishes. If a plywood is rated C-D, that means it has a face rating of C and a backing rating of D. CDX was originally intended to mean C-face, D-backing,Exterior rated, but that doesn't properly describe CDX as we currently buyit. OSB can be structurally rated or not. The APA-Engineered Wood Association outlines the criteria for structural grades of plywood/sheathing. In general, they must meet the criteria of Public Standard 1 (PS-1) and PS-2 for plywood and for OSB.

Check the APA websit for additional info.

 

[awcivil]

So there is a difference between Structural 1 and OSB & CDX. Would the design values in the NDS be for the plywood panels? It seems the allowable shear is about 2/3rds less.

 

[Ron]

Structural 1 is a structural performance designation. OSB or plywood can be rated as such, but not all OSB or plywood will meet the requirements. If it is Structurally rated, it will be stamped with the rating on the sheet. If it isn't stamped, assume no structural rating.

 

[JAE]

I agree with Ron. Don't confuse OSB/Plywood (sheathing types) with Structural I or non-structural sheathing (sheathing "grades" so to speak).

Check out the APA website here: APA website

They have a lot of free publications (after registering) that will answer all your questions.

Attached is one document that I found helpful.