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Plywood is an extremely strong product composed of thin sheets of veneers glued to each other at right angles. Plywood thickness varies, but the most common are ¼”, ½”, and ¾”. Panels with tongue and groove edges are available for use in sheathing floors. Minimum thicknesses for various applications: Roofing: ½”; Floor sheathing: ¾”; Floor underlayment installed atop the floor sheathing: ¼”; Wall sheathing: ½”.
Types: Plywood comes in a dizzying variety of types, ranging from simple interior grade plywood, acceptable for normal inside woodworking projects to marine grade plywood, made with waterproof glues to resist moisture. Special plywood products made with smooth surfaces and no voids are called underlayments, and are appropriate for use on top of the plywood subfloor as a base for vinyl or ceramic tile and carpeting. If you’re building furniture or cabinetry of any kind, you’ll want to use hardwood veneer plywoods, made with a face veneer of oak, birch, maple, or walnut. Softwood veneers (pine or fir) are used for many siding, roofing, and general construction activities for their ease of nailing and general economy.
Grading: Plywood quality is determined by a grading system, with each face receiving its own letter grade: A is the highest quality (no major blemishes, sanded smooth); B grade may contain minor blemishes; C grade has noticeable patches and is not appropriate for finished products; D grade can contain holes, knots, and splits, and is used as a purely economy product where it will be hidden or appearances do not matter. In the case of hardwood veneers, a combination letter and number system is used, with the letter referring to the face grade and the number referring to the back grade (the opposite side of the board). These rankings range from AA for the best face grade to E for the lowest utility grade (pallets); and 1 for the best back grade to 4 for the worst.
Appearance: Plywood veneers are cut and arranged in different ways to achieve different looks. Most plywood is cut with a rotary blade, but flat-sliced veneers (produced by moving the log back and forth over a knife) result in the most attractive veneers. Here are the most common ways in which in which veneers are matched on a piece of plywood:
Random Matched: Veneers are not matched with any regard to grain or color;
Slip Matched: Veneers are matched with the same face always upward;
Book Matched: Veneers are joined as if they are two pages of the same book. This is one of the most common types of veneers in finished furniture; and also the most expensive type.
Other Board Products
Particleboard (also known as flakeboard) is made of sawdust and wood shavings bonded with resin under high pressure. It’s a dense, heavy board ideal for economy cabinetry or shelving, as a base for plastic laminate countertops, and for flooring underlayment. Particle board breaks down quickly when exposed to moisture, so it should never be used in exterior or wet conditions. Also, never use it as structural flooring - only as an underlayment atop the structural plywood floor. In addition, particleboard does not hold nails well at all, and the cheaper grades (coarser) do not even hold screws that well. It does glue well, and although it needs to be primed with a sealer first, accepts paint very well. Particleboard comes in many thicknesses, the most common of which are ½”, 5/8″, and ¾”.
Medium Density Fiberboard (MDF) / Hardboard are two products that are similar to particleboard, but have the advantage of being made of smaller fibers. They are therefore denser, smoother, and stronger than particleboard (and more expensive, as well). MDF is used mostly by cabinetmakers and countertop manufacturers, and is prized for its ability to hold screws and nails well, and its easy workability. You may often see MDF used as a base for wood or laminate wall panels you can buy for your bath, kitchen, or den. Hardboard is an even denser version of MDF, and is made with waterproof glues for use in home exterior siding products. You may also find it in your home as garage perforated pegboard, drawer bottoms, and the backs of cabinets or other furniture. Occasionally it is used as a floor underlayment.
Oriented Strand Board (OSB) is an economy product touted as a replacement for plywood in many situations. This product is made of strands of wood bonded in various directions on each face, with the various layers bonded at right angles to each other. Although it is rated nearly as strong as plywood, OSB is very susceptible to moisture and, for that reason, is not the preferred product for roof sheathing or floor decking, unless you are confident the board will be protected from moisture shortly after installation. OSB is ideal as wall sheathing and attic flooring.
Medium Density Overlay (MDO) is a very expensive, high-quality exterior grade of plywood with a smooth surface. It is widely used by sign painters and furniture makers for its denseness, weatherability, and ease of use. MDO can be purchased with paper overlay on one or both of its finishes for applications requiring an exceptionally smooth finish.
No product is as useful as plywood for making home projects with a durable and paint-ready finish. Pick your product and and enjoy making a useful addition to your home!