What is CAT Plywood

The plywood we use can be unofficially categorized in a large number of different ways. We who use this engineered wood material categorize it by the type of wood it is made from, what manufacturing technique is used, whether there are any voids in the material or how the surface is finished. But if we want to get technical about categorizing plywood, we need to go to the specifications. That means understanding just what CAT plywood is and what ps1-09 deals with.

The term β€œCAT” in this regard doesn’t refer to a fuzzy creature which purrs sometimes and scratches at others. Rather, it is a shortening of the word β€œcategory” and according to ps1-09, β€œthe product standard for the production of plywood intended for structural (load-bearing) use” it mostly deals with plywood thickness. The abbreviation β€œPERF-CAT” or simply β€œCategory” can be used in place of β€œCAT.”

Other product standards exist, all of which provide some sort of security to the purchaser. For example:

  • PS 1 deals with structural plywood
  • PS 2 deals with performance standards for wood-based structural-use panels, in other words, OSB and MDF 

While CAT plywood is mostly concerned with plywood thickness, this isn’t the only plywood standard that exists. There are other plywood standards, dealing with the construction of all the different engineered wood products in the plywood family.

These standards are voluntary product standards, not regulatory ones. That means that it is up to the mill making the plywood to decide whether or not they are going to follow them. Most mills do, as making products to a standard helps the customer know what it is they are buying. Considering that a large percentage of plywood manufactured goes into the construction of buildings, where the building code imposes strict requirements and that another large user is the furniture and cabinet industry, which needs consistent products, it’s actually no wonder that mills would volunteer to meet those specifications.

This is not to say that all plywood manufactured meets the requirements laid out in ps1-09 or the other product standards. Blondwood, a relatively new designation, is being sold by some of the larger home improvement chains. This is not manufactured to meet the standards, allowing it to be produced at a lower cost, which is then passed on to the consumer. But it can be inconsistent.

Nevertheless, in meeting those specifications, mills are able to stamp their products, letting the consumer know what it is that they are getting. While it is rare to find plywood that meets or exceeds those specifications, there is considerable confidence that it at least meets the minimum requirements.

When looking at purchasing plywood, all the various factors need to be taken into consideration; plywood construction, materials used, voids, special ratings and recommended purposes. Each product has been developed to meet specific needs which either aren’t met by another product or can be better met by the new one. In some cases, meetings that need better consists of nothing more than a lower price to the consumer. But we need to be careful that the lower priced product will still meet our needs.

For what is known as CAT plywood, the main thing to look for is the panel thickness. This is based upon ps1-09, the plywood specification that deals with the finished thickness of plywood products. Different thicknesses are provided for both sanded and unsanded panels, so caution must be taken to avoid confusing the two, using one chart, when needing the other. Sanded panels are expected to be slightly thinner, as the process of sanding removes some of the material.

Unsanded  and Overlaid Plywood

(nominal thickness)
Minimum Allowable ThicknessMaximum Allowable ThicknessRecommended Labeling by Manufacturer
1/4”0.219” (5.56mm)0.281” (7.14mm)0.225”
11/32”0.313” (9.94mm)0.375” (9.53mm)0.322”
3/8”0.344” (8.73mm)0.406” (10.32mm)0.354”
7/16”0.406” (10.32mm)0.469” (11.91mm)0.418”
15/32”0.438” (11.11 mm)0.500” (12.70mm)0.451”
1/2”0.469” (11.91mm)0.531” (13.49mm)0.473”
19/32”0.563” (14.29mm)0.625” (15.88mm)0.578”
5/8”0.594” (15.08mm)0.656” (16.67mm)0.609”
23/32”0.688” (17.46mm)0.750” (19.05mm)0.703”
3/4”0.719” (18.26mm)0.781” (19.84mm)0.734”
13/16”0.781” (19.84mm)0.844” (21.43mm)0.788”
7/8”0.831” (21.11mm)0.919” (23.34mm)0.849”
1”0.950” (24.13mm)1.050” (26.67mm)0.970”
1-1/8”1.069” (27.15mm)1.181” (30.00mm)1.091”
1-1/4”1.188” (30.16mm)1.313” (33.34mm)1.213”

The thicknesses shown in this table are based on a tolerance of 1/32” for panels with Performance Category up to 13/16” and a tolerance of 5% for those which are thicker. According to the specification, this measurement is to be done with a panel moisture content of 9%.

sanded plywood panels, plywood thickness
Sanded plywood panels, Greg Guarino

Sanded Plywood panels 

(nominal thickness)
Minimum Allowable ThicknessMaximum Allowable ThicknessRecommended Labeling by Manufacturer
1/4”0.234” (5.95mm)0.266” (6.75mm)0.234”
11/32”0.328” (8.33mm)0.359” (9.13mm)0.328”
3/8”0.359” (9.13mm)0.391” (9.92mm)0.359”
7/16”0.422” (10.72mm)0.453” (11.51mm)0.422”
15/32”0.453” (11.51mm)0.484” (12.30mm)0.453”
1/2”0.484” (12.30mm)0.516” (13.10mm)0.484”
19/32”0.578” (14.68mm)0.609” (15.48mm)0.578”
5/8”0.609” (15.48mm)0.641” (16.27mm)0.609”
23/32”0.703” (17.86mm)0.734” (18.65mm)0.703”
3/4”0.734” (18.65mm)0.766” (19.45mm)0.734”
13/16”0.788” (20.02mm)0.837” (21.26mm)0.788”
7/8”0.849” (21.56mm)0.901” (22.89mm)0.849”
1”0.970” (24.64mm)1.030” (29.34mm)0.970”
1-1/8”1.091” (27.72mm)1.159” (29.43mm)1.091”
1-1/4”1.213” (30.80mm)1.288” (32.70mm)1.213”

Thicknesses in this table are based on a tolerance of 1/64”, rather than the 1/32” allowed for non-sanded panels. As the panels get thicker, panels over ¾” thick are expected to maintain a tolerance of 3%. As mentioned for unsanded panels, the moist content of panels, when measured, should be 9%.

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