Choose the right plywood for your project

A primer on grades, composition and exposure ratings

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A primer on grades, composition and exposure ratings

Paul Bianchina
Inman News®

If you've tackled any kind of home improvement project, you've almost certainly had occasion to pick up a piece of plywood. It's one of the most common and versatile construction materials anywhere, with hundreds of uses in the home, shop and job site. But how do you choose the right one for your next project? Here are some basic guidelines to get you going in the right direction.

What is plywood?

Plywood is actually a composite of several thin sheets of wood veneer. Each sheet is manufactured to have a specific set of strength, span and appearance characteristics.

Most plywood veneers are rotary cut by peeling thin, continuous strips of wood off a horizontally rotating log. The veneer is then cut into pieces, graded and dried prior to assembly. Veneers are also made by slicing a log along its length into long thin strips, which are then matched for best appearance, a more labor-intensive process typically reserved for more expensive hardwoods.

After the veneers are cut, they're coated with resin and pressed together under heat and pressure to form a solid sheet. For exterior- and marine-grade plywood, the resin is typically phenol formaldehyde; for interior plywood, urea formaldehyde is often used due to its lower cost.

The face and back veneers are oriented parallel with the long dimension of the sheet, and each alternating, inner veneer layer runs perpendicular to the ones above and below it. To balance the sheet, there are typically an odd number of veneer layers, usually three, five or seven.

The more layers a sheet has, the more each veneer layer overlaps potential defects in the other layers. So, in two sheets of equal overall thickness, the one with more layers will have a higher strength rating.

The sheets are pressed and trimmed into uniform sizes. The most common sheet size is 4 feet by 8 feet, and common thicknesses range from 1/8 to 1 1/4 inches. To confuse things a bit further, the actual thickness of the sheet will be about 1/32 of an inch less, due to final sanding.

Softwood plywood grades

Most of the plywood for home improvement and construction projects is softwood plywood, meaning that the face and back veneers are made from softwoods such as Douglas fir, cedar, pine, and what's known collectively as SPF (spruce-pine-fir).

The sheets are designated by a pair of letters, which are the grades of the exposed-face veneers. The grades range from A, the highest, to D, the lowest:

A: Smooth and paintable. Not more than 18 neatly made repairs are permitted, parallel to the grain. Wood or synthetic repairs are permitted. It may be used for natural finishes in less-demanding applications.

B: Solid surface. Repairs and tight knots up to 1 inch across the grain are permitted. Wood or synthetic repairs are permitted. Some minor splits are permitted.

C: Plugged. An improved C veneer, with splits limited to 1/8 inch in width and knotholes or other open defects limited to 1/4 by 1/2 inch. Wood or synthetic repairs are permitted. Some broken grain is allowed.

C: Tight knots up to 1 1/2 inches. Knotholes up to 1 inch across the grain and some to 1 1/2 inches. Synthetic or wood repairs allowed. Discoloration and sanding defects that do not impair strength are permitted. Limited splits are allowed. Stitching (a type of repair) is permitted.

D: Knots and knotholes to 2 1/2 inches in width across grain and 1/2 inch larger within specified limits. Limited splits are permitted. Stitching is permitted. Use is limited to Exposure 1 (see below).

Some of the more common grade combinations include AC, CC Plugged, and CD. If the plywood is intended for exterior use, it will carry a third letter designation: X. Therefore, a sheet rated CDX would have one C face, one D face, and exterior-rated glue.

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