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Unread 08-27-2005, 03:18 PM   #10
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Join Date: Nov 2003
Location: LaConner, Washington
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Plywood Deflection and Grain Orientation

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.

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 tool towards the top of this screen).
  • AND make sure your plywood is laid in the correct orientation, perpendicular to the joist runs.

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.
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