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.
Exposure ratings
Softwood plywood is also rated according to one of four exposure ratings, which indicates its durability when exposed to moisture. Those ratings are:
Exterior: Has a fully waterproof glue joint, and is designed for use in areas that are permanently exposed to moisture or weather.
Exposure 1: Also has a fully waterproof bond, and is designed for use where the construction process will subject them to long periods of weather exposure before they are fully protected.
Exposure 2: Intended for use in protected construction areas where they will be exposed only to moderate moisture or weather conditions.
Interior: Limited strictly to use in protected interior environments.
Hardwood plywood grades
Plywood is also available with hardwood veneer faces, for use in cabinets, furniture and other projects where appearance is important. Hardwood plywood usually has softwood veneer inner plies, but may also have an inner core of lumber, particleboard or other material. There are dozens of different types of hardwood plywood, from more common grades such as birch and oak, to exotic species from all over the world.
As with softwood plywood, hardwood veneers may be rotary cut or sliced. If they’re sliced, the veneers may be slip-matched, meaning they’re butted side by side as they’re cut off the log, or book-matched, meaning that every other one is flipped over, creating a mirror image of the one before it. The two different types of matching can greatly affect the appearance of the sheet, so it’s worth looking at.
Sheet sizes are similar to softwood plywood, although a multi-ply Baltic birch plywood in a 5-foot-by-5-foot sheet is also common among cabinetmakers. Thicknesses are less uniform than softwood plywood, due to thinner veneers and the number of sheets that are imported from countries that manufacture to different standards.
Hardwood plywood has a slightly different grading system, and it can be a little less uniform than that of softwood plywood. Basically, A-D grades (best to worst) apply to the face of the sheet, and 1-4 applies to the back of the sheet. So an A-1 sheet is the best, with a good veneer on both sides.
An A-4 would have a good grade on the front, with a back that has knots, blemishes and repairs; it would be used where only one side of the sheet will be seen.