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Look at the edge of a piece of plywood and you'll see it's constructed with several thin layers of wood (veneers) that have been glued together. These veneers are peeled from a log in much the same manner that paper towels spin from a roll. After drying, they're spread with glue and oriented so that every ply lies cross grain to the ones on either side. The veneers are pressed tightly together under pressure until they dry before being trimmed into the familiar 4x8 panel you find in the store.
Depending on the panel's thickness, plywood is made up of 3, 5 or 7 veneer layers. Odd numbers of layers are used so there's a center core to prevent warpage.
 Buying the right plywood is critical to the success of most DIY projects. Look at a typical piece and you'll find an intimidating list of words, letters and numbers. But they tell an important story about how the plywood was made, where it was made, how strong it is and where you can use it. Once you've cracked the code, you'll be able to buy the perfect plywood for your project.
Getting a Good Grade
Generally, plywood is classified by strength, how it looks and
the type of glue used.
APA-The Engineered Wood Association sets the standard for most
of the softwood plywood you'll find at home centers.
| Panel Grades |
| Rated Sheathing |
Used for roof, wall and subflooring
applications. |
| Sturd-I-Floor |
Lightly sanded surface allows
Sturd-I-Floor to be used as a combination underlayment and subfloor.
You'll often find Sturd-I-Floor with tongue and groove edging
to form a firm interlocking subfloor. |
| Underlayment |
Use over subflooring as underlayment
for carpet and padding. |
| C-D Plugged |
Available in all exposure
ratings, the surfaces are lightly sanded to make the panel ideal
for built-ins and wall and ceiling tile applications. |
| Appearance Grades |
Consists of two letters. The
first letter indicates the surface quality of the front side
of the panel. The second letter indicates the quality of the
backside of the panel. |
Span Rating
Construction grade plywood has a span rating within the APA stamp.
It consists of two numbers with a slash in between. The first number
indicates the maximum span in inches when the plywood is used for
roofing applications. The second number is the maximum span when
the plywood is used for subflooring applications. For example, plywood
with a span rating of 24/16 would allow a span between roof rafters
of 24 inches, with a span between floor joists of 16 inches.
| Exposure Classifications |
| Exterior |
Designed for applications
where panels are subject to permanent ongoing exposure to moisture.
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| Exterior - Exposure 1 |
Fully waterproof bond, but
not intended for permanent ongoing exposure to moisture. |
| Exterior - Exposure
2 |
Interior type
with intermediate glue. Intended for protected applications
where only slight exposure to moisture is likely to occur. |
Interior Plywood is graded by its veneer (on the front and back
panels) and its exposure rating.
| Appearance
Grade |
Characteristics
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| N |
Smooth natural
finish select heartwood or sapwood veneer, free of open defects.
This grade does not allow more than six wood-only repairs per
4 ft. x 8 ft. panel. Grain and color must be well matched. |
| A |
Smooth paint-grade
veneer. May be used natural for less demanding applications.
No more than 18 repairs per 4 ft. x 8 ft. panel. |
| B |
Solid surface
veneer. This grade allows tight knots (no more than 1 inch
in diameter), round repair plugs and shims. Permits repairs
of minor splits. |
| C-Plugged |
Upgraded "C"
veneer. Splits limited to 1/8-inch max. width. No knotholes
or borer holes permitted larger than 1/4 x 1/2 inch. Synthetic
repairs permitted, as well as some limited broken grain. |
| C |
This veneer can
have tight knots up to 1 1/2 inches in diameter, and knotholes
up to 1 inch across the grain, or up to 1 1/2 inches if the
total width of knots and knotholes is within specified limits.
Wood and/or synthetic repairs allowed. Discoloration and sanding
defects which to not impair strength are allowed. |
| D |
D grade allows
knots and knotholes up to 2 1/2 inches width across the grain
as well as limited splits and stitches, and is limited to interior
or Exposure 1 panels. |
To learn more about plywood, how it's graded and its many uses,
visit the APA
website.
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