The Perils of 3-Ply Plywood
The Poor Earthquake Resistance of 3-Ply Plywood was proven in 1994 when shear walls made of 3-ply plywood tore exactly as shown in the photo to the right in the Northridge Earthquake. For this reason, the City of Los Angeles downgraded the acceptable limits for 3-ply plywood to a maximum of 200 Lbs. per linear foot.
Footnote “n” of this table reads
This information comes out of the Los Angeles Building Code.
On page 10 of the Wood-Frame Subcommittee Findings Report published immediately after the Northridge Earthquake it says: “The performance of 3-ply construction has raised questions of its ultimate capacity. Horizontal tearing has occurred on some outer face plies above the inner ply seam. Values for all 3-ply panel construction were therefore reduced to 200lbs/ft maximum.” In other words, the earthquake resistance of 3-ply plywood makes it too weak to consider.
This is what one of the field damage engineers with the Los Angeles Department of Building and Safety had to say.
“So, a little more history.
Part of the history includes that 3/8” plywood was typically made of 3 plies, and ½” structural 1 plywood was traditionally 5 plies. Over time, to decrease the cost of making ½” plywood the number of ply’s was reduced to 4 ply’s and then 3 plies’ (now have less glue), but individual plies were now thicker to maintain the overall ½” thickness requirement. Yes, you could still get 5 ply struct 1 plywood, but it was more expensive than ordering ½” plywood with 4 or 3 ply’s and there were long lead times to get it as the mills had moved away from making the 5 ply sheets in ½” thickness.
After Northridge, the engineering community discovered that much of the ½” plywood being installed was just 3 plies. Engineers had called for struct 1 plywood, but didn’t always say the number of ply’s required, as they had typically understood that ½” structural 1 plywood with exterior glue was typically made of 5 ply’s and didn’t realize the mills had moved away from manufacturing 5 ply ½” thick sheets. After Northridge, 4 ply ½” structural 1 became the minimum requirement if you wanted to use the diaphragm and shear wall table design values. If you had ½” 3 ply sheets, they became limited to 200 lbs. / ft. You would also need the 4-ply minimum as well for ½” thick exposure 1 sheathing.
So, 3/8” sheets and ½” sheets made from 3 ply’s were given the same design value = 200 lbs./ft as their construction was basically the same- just 3 plies’. Some of the issue was overdriven nails and gaps in the individual ply’s, such that once you drive the nail head through the first ply, you only have 2 ply’s left to resist the seismic forces. If you over drive just one ply in 5 ply ½” thick sheets, you still have 4 ply’s left. Having more glue in the 5 ply sheets makes it harder to overdrive the nails as well. There was also the discussion of what is an overdriven nail versus properly installed nail: Properly installed discussion depended on who you spoke with:
- Bottom surface of nail head rests on plywood surface or
- Top of nail head is flush with surface of plywood, and no deeper.
Each ply for 3 ply ½” thick sheets is approximately 3/16” thick. 2x 3/16” = 6/16” = 3/8” plywood. So, the ½” plywood with 3 ply’s was treated as having similar behavior to 3/8” plywood, which may have overdriven nails as well.”
Thickness of 3-plywood makes no difference
This report applies to ALL thicknesses of plywood. The poor resistance of 3-ply plywood does not change if it is 3/8, 1/2, or 5/8″ plywood.