| In the United States, standards for most softwood plywood are established
by the American Plywood Association in cooperation with the National Institute of
Standards and Technology. A Voluntary Product Standard for plywood became effective November 1, 1966, and was revised in 1974
and again in 1983. The mark ¡°PS 1-83¡± indicates that the
panel conforms to the Product Standard published in 1983, which
was still in force in 1992. [Commercial Standards (such as
CS45-60, ¡®Douglas Fir Plywood.¡¯] Panels are stamped with a grade mark, unless such a
stamp would deface a high quality surface that might be given a natural finish. Such
grades are marked on the edges.
Lumber yards usually carry 4 foot by 8 foot and 4 foot by 10 foot panels, but will cut
half and quarter sheets. Other sizes are manufactured, almost always in increments of 12
inches, for example, in widths of 36¡å, 48¡å and 60¡å, and in 9 foot lengths.
The exception to the whole feet rule are panels marked ¡°sized for spacing,¡± which
are slightly shorter than normal panels (e.g., 48 inches by 95½ inches instead of
48 inches by 96 inches) in order to leave space between panels in, for example,
sheathing a roof with rafters on 24" centers. The space is necessary to allow for the
panels' expansion.
Thicknesses of sanded panels range from ¼ inch to 1¼ inch or more, in
steps of 1/8 inch. The thickness tolerance is ¡À 1?64 inch for
panels ¾ inch or less and ¡À 3% of specified thickness for thicker panels.
Nominal thicknesses of unsanded panels range from 5?16 inch to 1¼
inch or more, ¡À 1?32 inch for panels with a specified thickness of 13?16
inch or less, and ¡À 5% for thicker panels.
| Grade |
Requirements |
| N |
- A premium grade (for natural finish), available on special order from some
manufacturers.
- Either 100% heartwood or 100% sapwood.
- Repairs must be made with well-matched wood parallel to the grain, and only 6 are
permitted in a 4 foot by 8 foot panel.
|
| A |
- Smooth, paintable surface.
- Not more than 18 neatly made repairs, which must be parallel to the grain, and of the
boat, sled, or router type. Repairs with synthetic patching compound are permitted in this
as well as all lesser grades.
|
| B |
Shims, circular repair plugs, and tight knots as large as 1 inch measured across the
grain are permitted, as well as minor splits. |
| C plugged |
Splits may be no wider than 1/8 inch, and knotholes and borer holes no larger than
¼ inch by ½ inch. Some broken grain is allowed. |
| C |
Tight knots as large as 1½ inch, discoloration and sanding defects that do not
affect strength, and stitching are permitted. |
| D |
Knots and knotholes as wide as 2½ inch measured across the grain and even
larger (within limits) in other directions. Splits and stitching permitted. |
A number from 1 to 5, with wood from the species of trees in group 1 being the
strongest and stiffest (such as beech, Douglas Fir from certain states) and 5 the least
stiff and strong (basswood, poplar).
Exposure durability classifications
| Exterior |
Panels intended to exposure to the weather. D grade veneer may not be used in Exterior
panels.
Contrary to a common misconception, CDX is not an exterior panel.
Exterior plywood is not necessarily the best choice where plywood will be constantly
exposed to the weather; plywood pressure-treated with preservative would probably be
better. |
| Exposure 1 |
The glue is the same as that used for Exterior plywood, but other characteristics that
affect bonding are not. These panels are for use in high moisture condition or where,
during construction, long delays may be expected before the panel is protected from the
elements. |
| Exposure 2 |
Same glue. The wood itself is a bit worse and the panel less rugged. |
| Interior |
A different glue is used. Plywood in the other exposure classifications can act as a
vapor barrier if glued and nailed; interior plywood cannot. For use only in protected
areas indoors. |
Panel grade is either
- a pair of veneer grades, such as ¡°A-B¡± (one side is rated A and the other B), ¡°C-D¡±,
and so on
or
- a name identifying the principal end use, such as: APA Rated Sheathing, APA 303 Siding,
or APA Underlayment.
Certain designations are trademarked by the APA, such as ¡°APA Rated Sheathing,¡± ¡°APA
Rated Sturd-I-Floor,¡± and ¡°APA Rated Siding.¡± These panels are marked with span ratings, which indicate the maximum center-to-center distance,
in inches, between the joists or studs to which the panel is fastened during construction.
In the case of sheathing, the span rating is two numbers separated by a slash:
- the first number is the span rating if the panel is used for roofing
- the second number is the span rating if the panel is used for subflooring
In both cases the rating assumes that the long dimension of the plywood crosses at
least three supports. |
