Plywood sheets have a grade stamp that contains approximately 15 pieces of information. Within the grade stamp you will find the sheet span rating, which tells you the maximum distance the sheet can span between supports.
For APA-rated sheathing, the rating is expressed as two numbers, such as “48/24″ — the first number is the distance between supports when the sheet is used for roof sheathing and the second is the maximum span when used as subfloor.
Plywood is made up of several thin sheets of wood veneer, coated with resin (glue) and
pressed together under heat and pressure to form a solid sheet.
The face and back veneers are rotated parallel with the long dimension of the sheet, and each alternating veneer layer runs perpendicular to the ones above and below it, giving the sheet its extraordinary strength and span characteristics.
Plywood is graded by its face and back veneers, the weather exposure it’s approved for, the maximum distance it can span, the species of wood used for the face veneer, and any special characteristics that the sheet has. The most common sheet size is four-feet by eight-feet, although four-feet by nine-feet, four-feet by 10-feet and other sizes are also available. Common thicknesses include 1/8-inch, 1/4-inch, 3/8-inch, 1/2-inch, 5/8-inch, 3/4-inch, and 1 1/8-inch, with square, tongue-and-groove and ship lap edges.
The veneers are given a grade based on the allowable number of defects and repairs and range from A (best) to D (worst). Under the rules of the American Plywood Association (APA), here is an abridged look at what is permitted under each grade:
A: No more than 18 neatly made repairs are allowed, matched with the graining.
B: Repairs and tight knots up to 1-inch in size are permitted across the grain. Some minor splits are permitted.
C: Tight knots up to 1 _-inch in size and knotholes up to 1-inch across the grain are permitted. Limited splits are permitted, as well as sanding defects that do not impair strength. Stitching (a type of rough veneer repair) is also permitted.
C-Plugged: This is an improved version of the C veneer. Splits are limited to 1/8-inch. Knotholes and other defects are limited to 1/8-inch by 3/8-inch.
D: Allows knots and knotholes up to 2 1/2-inch wide across the grain. Limited splits are allowed, as is stitching.
Construction plywood is also rated by the APA according to one of four weather durability classifications. These include:
â€¢ Exterior: These sheets have a fully waterproof glue joint and are designed for use in
areas that are permanently exposed to moisture or weather. LI> Exposure 1: These also
have a fully waterproof bond and are designed for use where the construction process
will subject them to long periods of weather exposure before they are fully protected.
â€¢ Exposure 2: This grade is intended for use in protected construction areas where they
will only be exposed to moderate moisture or weather conditions.
â€¢ Interior: These sheets are limited strictly to use in protected interior environments.
A finished plywood sheet is designated by the grades of its face and back veneers respectively. For construction projects, some of the typical grade combinations include:
â€¢ A-C: With an A face and a C back, this is a common plywood for use where one smooth,
solid face is required, such as cabinet making, signs and some types of painted paneling
â€¢ C-C Plugged: This is a good plywood for use where one smooth, paintable surface is
needed. Perhaps the most common construction application is roof overhangs on porches
and exposed soffits.
â€¢ C-D-X: A common panel for use on roof and wall sheathing where strength and moisture
durability are important, but because the panel will be covered, outer veneer is not a big