The Perils of 3/8″ and 3-Ply Plywood
The Poor Earthquake Resistance of 3/8″ thick and 3-Ply Plywood of any thickness was proven in 1994 when shear walls made of 3-ply plywood tore exactly as shown in the photo to the right. This photo is from 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.
The California building code on the other hand rates it as high as 640 lbs. as listed under “APA RATED SHEATHING” which does not make a distinction between 3-ply ,4ply or 5 ply plywood. The APA RATED SHEATHING table is based on laboratory tests while the Los Angeles table is based on observed damage from an actual earthquake. We believe therefore that the 200 lbs per linear foot value should be used.
Look at the footnote in this table from the Los Angeles Building Code.
Footnote “n” of this table reads
After the Northridge Earthquake the City of Los Angeles immediately published emergency regulations The information below is from 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 damage inspectors 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.