Plywood Floors -- CDX vs. Flooring Grades

[John Bridge]

This is a must read for anyone considering the use of CDX sheathing for subflooring material.

http://johnbridge.com/vbulletin/showthread.php?t=38913

[Sponsored Links]

[Mike2]

Plywood Deflection and Grain Orientation

Here is a visual intended to demonstrate the importance of laying plywood sub-flooring and underlayment perpendicular to the joist runs.

Relevant materials used in this demonstration include:


Two strips of 3/8” (actually 11/32”) ACX Douglas Fir plywood cut into 5 ¼” strips. One was cut/ripped with the face grain. The other was cross-cut against the grain. Both strips were cut from the same panel which has 3 plys (face, core, and back).
Two 2"X6" 'joists' spaced 16” on center.
Six bricks (the load) weighing in at 29.9 pds. total.
One dial indicator for accurate measurement of deflection.


Results are shown in picture below:

Case #1 - Plywood laid perpendicular to joists
Depicted on the left, this plywood strip is laid in the right direction with the face grain running perpendicular to the joists. Actual deflection under load was .069” or slightly more the 1/16”

Case #2 - Plywood laid parallel to joists
On the right, this strip is laid in the wrong direction with the face grain running parallel to the joist. Actual deflection was .380” or 6/16” (3/8”), six times greater than Case #1.

http://www.johnbridge.com/images/mike2/For%20Liberry%20Stuff/Plywood%20deflection.jpg


In summary: Tile needs a stiff floor, disliking movement of any kind.

Pay attention to your joist structure - i.e., joist size, spacing, and length of unsupported span (See DEFLECTO (http://www.johnbridge.com/vbulletin/deflecto.pl) tool towards the top of this screen).
AND make sure your plywood is laid in the correct orientation, perpendicular to the joist runs.

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Note: Actual results (deflection amounts under a given load) will vary of course depending upon plywood thickness, number of layers, grade, species of wood, etc. However having said all that, orientating the face grain perpendicular to the joist runs will always yield the least deflection given the common grades of building plywood readily available.

[Mike2]

Here is another article on plywood co-authored by Frank Woeste, P.E., and Peter Nielsen, Schluter Systems L.P. : Position of Underlayment to Prevent Cracked Tile and Grout (http://www.johnbridge.com/images/mike2/For%20Liberry%20Stuff/Underlayment-Nielsen-Woeste-0604.pdf..pdf)

[Mike2]

JerseyReef (Mike) a member here recently posted purchasing some plywood Sheathing panels stamped PRP-108 (see picture below). With no veneer grades on this stamp such as C-D or C-C, he contacted the APA (American Plywood Association) for additional information on this panel.

APA response was that Exposure 1 Sheathing manufactured under either PRP-108 or PS 2-92 standards is equivalent to a DD grade. The full text of that response is in this post here: http://johnbridge.com/vbulletin/showpost.php?p=522771&postcount=56

Clearly then, plywood stamped APA Rated Sheathing, Exposure 1, PRP-108 or PS 2-92 is not suitable as sub-flooring or underlayment under a tiled surface.

[Mike2]

Plywood veneers are graded into five levels, as follows:

A – Highest grade level. No knots or knotholes. Some patches allowed

B – Small round knots. Patches and round plugs allowed.

C Plugged - An improved C grade, found in Sturd-I-Floor and some of the better underlayments.


C – Small knots and knotholes allowed. Lowest grade allowed in Exterior rated type plywoods.

D - Larger knots and knotholes. Limited voids/pockets allowed in wall and roof sheathing panels. This grade is not allowed in Exterior rated panels. Furthermore, it should not be used under a tiled surface

Example: When two grades are used together as in AC , the panel has grade A veneers on the face, C grade on the backside.

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If you are interested in more technical information, the APA (Americal Plywood Association) publishes many free documents on plywood. This one here provides a thorough discussions of the various grades. APA Panel Handbook & Grade Glossary (http://www.johnbridge.com/images/mike2/For%20Liberry%20Stuff/Panel%20Handook%20&%20Grade%20Glossary.pdf)