Introduction to Plywood
Plywood is part of a category of wood-based construction materials referred to as ‘sheet goods’. Other components of this group include MDF (medium-density fibreboard), particleboard, melamine, OSB (oriented strand board) and hardboard. There are dozens of different types of plywood on the market, some designed solely for interior use and others for exterior applications. These include structural plywood (aka softwood plywood) used for sheathing, flooring, roofing and underlay applications; marine plywood used for projects that will be subject to high humidity or wet conditions (decks, boats, outdoor furniture and the like); and bendable plywood. In this article we look at the characteristics of interior plywood used primarily for cabinet and furniture construction.
Plywood is a structurally versatile product available in large dimensions and a variety of thicknesses. Compared to solid wood, it has excellent dimensional stability and a high strength to weight ratio. In many cases it can be a less expensive and more convenient alternative to use in place of solid lumber.
All plywood is comprised of a sandwich that consists of a core (made up of a single layer or multiple layers of solid or composite wood) and a veneer covering on both top and bottom. The core can be a different sheet good (such as MDF or particleboard). Various adhesives, including urea-formaldehyde, phenolic, melamine and soy-based, are used in the production of plywood, depending on its intended application.
Plywood is typically available in 4′ x 8′ sheets in 1/4″ to 1-1/4″ thicknesses. Larger sheets are available from some manufacturers.
One of the most important things to consider when purchasing plywood is the core. Here are four of the most commonly used cores for interior plywood, each with its own advantages and limitations.
Cross Grain Strength
By alternating grain direction in the laminations of plywood, the resulting panel is stronger and more stable in both directions.
Veneer Core Plywood
A very common option, veneer core plywood is available with many different face and back veneer options, as well as in many different thicknesses.
When flatness and evenness is of highest importance, using a plywood with an MDF core may be your best option.
If extreme strength isn't important, using a plywood with a particleboard core will likely be the most cost-effective approach.
The best of both worlds, the plywood provides strength, while the MDF outer layers add a level of evenness to the board.
Although it's often thought of as lesser quality, veneer is a very useful product to incorporate into furniture and woodwork. It's very thin, but if used properly, it can used with great success. Some of the best pieces of furniture in the world are built with veneer.
Veneer Core – the preferred choice
This is probably the most popular type of plywood core, and comes in a wide variety of thicknesses and face veneers. A veneer core (VC) is comprised of three or more layers of wood (called plies) sandwiched between two layers of veneer (the face and back veneer). Each ply is rotated 90° to its neighbour before the sandwich is glued together. This is what provides the integral strength of the plywood across all directions and reduces expansion and shrinkage.
The plies can be made from softwood or hardwood. The term ‘softwood plywood’ refers to panels that have a face and back veneer made of softwood, while ‘hardwood plywood’ panels (the type most widely used in furniture and cabinet construction) have a face and back veneer of hardwood, regardless of what wood is used for the core.
VC plywood is very strong, flat, reasonably light in weight and has excellent screw-holding power. However, unlike MDF or particleboard core plywood, panels can vary slightly in thickness. Its availability in a wide array of wood species makes VC plywood a favoured choice for both furniture and cabinetry.
MDF Core – super flat and consistent thickness
Medium-density fibreboard (MDF) is a sheet good made of wood fibres bound with a resin. Because it has a consistent thickness (within a single sheet and from sheet to sheet), it’s exceptionally smooth, super flat, dimensionally stable and free of knots and grain patterns, and is widely used in cabinet construction. The panels are available without a covering veneer (in which case they are typically sealed and painted before use) or they have a factory-applied, high-pressure laminate overlay.
MDF panels are also used as a core in plywood. MDF core plywood is less expensive than VC plywood. They’re also easily milled with machinery or power tools without the risk of splintering or tear-out. However, they do split and crack more easily than VC plywood, are appreciably heavier, have lower screw-holding power and can off-gas formaldehyde (see ‘Plywood Off-Gassing’).
Particleboard Core – the economical choice
Particleboard is another sheet good that’s used as a core in plywood. Where MDF is made from wood fibres, particleboard is made from sawdust and resin adhesives, which makes it less expensive to produce than MDF. It weighs about the same as MDF (on the heavy side), has a consistent thickness, is very flat and smooth, and has high tensile strength. However, because it’s made from larger dust particles that are less uniform in size, it has a weaker screw-holding ability than MDF (special screws are generally used to fasten particleboard), and is easy to crack or chip. It’s also more prone to warping if exposed to moisture, and deflects more under weight. Particleboard core plywood is not available in as wide a variety of face veneers as VC or MDF plywood.
Combination Core – the compromise plywood
Combination (combi) core plywood is typically made of an inner core of wood plies (like VC plywood), an outer layer (on both sides) of MDF or particleboard, and a face and backing veneer. The thickness of the MDF or particleboard panels can vary from manufacturer to manufacturer. The inner core of wood provides good screw-holding power, while the outer layers of MDF or particleboard ensure a super smooth and flat surface. These panels weigh about the same as VC panels. As with particleboard plywood, combi core plywood is not as widely available, nor is it available in as many veneer options as VC or MDF plywood.
Beauty is only millimeters deep
Veneer is produced by either peeling or slicing thin layers of wood from a log, and each method results in its own distinctive appearance. Rotary cut veneer is the most common method. The veneer is peeled from a log similar to paper towels coming off a roll. When applied to a plywood panel the result is typically a fairly irregular grain pattern.
Veneer can be sliced from a log in three ways. Plain-sliced veneer is usually characterized by a cathedral grain pattern; quarter-sliced veneer produces a straight grain appearance; and rift-cut veneer produces a straight, striped grain appearance without the flakes that can occur in some quarter-sliced veneer.
Once sliced veneer is cut from a log, the sections (or leafs) can be laid out (matched) in several different patterns on a plywood panel – the most common are book matched, slip matched and plank matched.
The veneer on plywood is pretty thin – about .85mm (1/30″) on premium grade plywood, much less on lower quality plywood. Regardless of the veneer thickness, you need to be careful not to sand through, especially when using power sanders.
Home improvement retailers usually carry plywood with a limited selection of face veneer – typically oak, birch, maple, spruce and pine. Specialty lumber suppliers, such as The Wood Shed, Exotic Woods and A&M Wood Specialty, carry a much wider range. They may also carry pre-sanded and pre-finished plywood.
SIDEBAR: Baltic birch
Baltic birch is a popular plywood used as a substrate for shop-made veneered panels, for jigs and templates, for drawer components, and as a bending plywood. Imported from Europe, it’s constructed from very thin plies of birch, which makes it very stable and strong – a 1/2″ thick panel has 12 plies. There are both interior and exterior versions, in thicknesses from 1/8″ to 1-3/16″, and typically in 60″ x 60″ panels (however, panels up to 72″ x 96″ can be special ordered). Also becoming more common are 4′ x 8′ panels. Baltic birch does not have face veneers, but the two outer veneers look smooth and are either without blemishes or with imperfections that are patched by veneer cutouts. Similar products include: Finnish birch (LeeValley.com), made with a waterproof adhesive for exterior use and available in thinner formats (1/64″ to 3/32″); Europly PLUS (ColumbiaForestProducts.com) in thicknesses from 1/4″ to 1″ and faced with different veneer species; and ApplyPly (StatesInd.com) in thicknesses from 1/4″ to 1-1/2″ and also faced with different veneer species.
SIDEBAR: Plywood Off-Gassing
Formaldehyde is a naturally occurring organic compound that’s emitted by all wood species. Fortunately, when the wood is processed into plywood, most of the formaldehyde is exuded at the mill. However, the adhesive commonly used to bond the alternating plies in plywood – urea-formaldehyde resin – also off-gasses formaldehyde. By the late 1980s, it was considered a probable carcinogen. Companies have looked at other adhesives to use. Columbia Forest Products, for example, uses a soy-based adhesive in its line of ‘PureBond’ hardwood plywood, which makes this product virtually formaldehyde-free. (ColumbiaForestProducts.com)