Plywood Types and Application
Originally, the term “plywood” referred only to wood panels made of multi-layered veneer, laid with their grain perpendicular to adjacent layers, and bonded together with glue. While this term is still accurate, today the name plywood has expanded out, becoming more generic. It now refers to all types of engineered wood panels, regardless of their exact form or manufacturing process. The list of these can seem endless. Nevertheless, we can break the various types of plywood down into some basic categories.
Plywood is graded in a number of ways, most specifically by the type of face veneer the plywood has and the way it is manufactured. When looking at plywood for a project, one of the first things considered, after the type of plywood product is selected, is the grade or quality of face veneer the plywood is made with. There are four letter grades for these veneers:
|A||A smooth, paintable surface, containing no more than 18 neatly made repairs, which don’t affect the overall smoothness of the surface. Repairs must be made in the direction of the grain.|
|B||Solid surface, with no voids. Shims, routed repairs and knots up to 1 inch across are allowed. Repairs and patches can be made of wood or synthetic materials. Some minor splits are allowed.|
|C||Tight knots up to 1 ½ inch and knotholes up to 1 inch are allowed. Discoloration and sanding defects are allowed, as long as they do not affect the strength of the panel. Discoloration is also allowed. Limited splits are permitted.|
|D||Can contain knots and knotholes up to 2 ½ inches limited splits are allowed. Limited to use as interior panels, or the inner side of exterior panels.|
Important Plywood Terms to Know
Working with plywood, as with many other things in life, means dealing with some special vocabulary. If you’re new to woodworking or even new to working with plywood, you may not know these terms. Without them, you won’t necessarily know what you’re dealing with.
- Back – plywood side with the lower face grade, normally used as the back, when installed
- Core – inner layers of the plywood, with grain perpendicular to the face
- Crossbar – manufacturing defect where a piece of face veneer runs perpendicular to the board’s length
- Crossbanding – orientation of successive veneer layers, placing them perpendicular to the adjacent layers of the plywood
- Cut – refers to how the veneer is made; either rotary cut or sliced
- Delamination – failure of plywood panels, where the adhesive used to attach the plys fails, usually due to water
- Face – the side of the plywood with the higher grade finish
- Grade – system by which the quality of plywood’s surfaces are identified
- Grain – growth pattern of wood fibers
- Gum Spots – sap or resin leftover on wood veneer from milling
- Hardwood – any wood that comes from a deciduous tree (one whose leaves fall off) – has nothing to do with the wood’s density
- Heatwood – the center of the log, below the sapwood; also the strongest part of the log
- Knot – place on a log where a branch grew out
- Open Knot – the knot has separated from the wood around it, leaving a hole
- Pin Knot – small knots, less than ¼” in diameter
- Sound Knot – a knot that has not separated from the wood surrounding it
- Knothole – a hole or void in lumber, where a knot has come out
- Lap – place where two pieces of veneer are laid next to each other in the same ply
- Marine Plywood – high quality plywood made with waterproof glue and without any voids
- MDF (Medium Density Fiberboard) – engineered plywood product made of highly compressed, pressure cooked wood fibers, combined with synthetic resin
- Mineral Streak – discoloration of hardwood
- Oxidation – browning, graying or yellowing caused by exposure to the elements
- Particleboard – engineered wood product manufactured from small wood particles (sawdust) with a bonding resin – is not the same as MDF, which is a higher grade product
- Patches – material placed on defects in face veneers to repair voids, etc.
- Ply – single layer of wood veneer in a plywood product
- Plywood – originally referred to an engineered wood product, made by laminating several plys of wood veneer perpendicular to each other. The definition has now been expanded to include all engineered wood sheet products
- Rotary Cut – process by which veneer is peeled off a log as one continuous sheet on a lathe
- Sapwood – outer layers of living wood, just below the bark; normally not used in plywood
- Splits – cracks in the wood veneer, running parallel to the wood grain
- Veneer – sliced or peeled layer of thin wood, used to make plywood
Here are the most common types of plywood that are currently available on the market.
This is the most common plywood product, made of softwood veneer, usually fir. By “softwood” we are referring to woods that come from the various species of conifer trees, rather than broadleaf trees. Softwood plywood is commonly used for construction applications, because these trees grow throughout the year, allowing the tree to reach full stature much quicker. This also make softwoods less expensive, which translates to lower cost plywood products.
Softwood plywood was originally developed for the construction industry, although it is used in a wide variety of other applications as well. It is the prime choice where the durability of a multi-layer plywood product is needed, but a fine surface finish is not.
In manufacturing, an odd number of layers of softwood veneer are stacked at a right angle to each other and glued together with resinous glues to form softwood plywood. This plywood sold in the United States usually comes in 4’ x 8’ sheets, although 5’ x 5’ sheets are also available. A large number of grades are available, depending upon the intended application. Please see the page on CDX plywood for a more thorough explanation of the grading system.
Softwood plywood is used most commonly in the building trades for wall and roof sheathing and for installation as sub-floors. It is also commonly used in construction of crates and boxes for the shipment of expensive, delicate, or heavy machinery. Due to its ready availability, softwood plywood is an excellent choice for homebuilding and do-it-yourselfers.
This is the plywood you are most likely to encounter, when visiting the local lumberyard or home improvement center. While they will have some other types of plywood products, the majority will be softwood plywood.
This group of plywood products covers a wide range of materials, including plywood made with decorative face veneer coming from any type of hardwood tree. In practical terms, there are only a few common hardwood plywood types. All other types of wood grain are simulated by staining common hardwood plywood panels.
Hardwood plywood is manufactured the same as softwood plywood, except the exterior layers (face and reverse) are made of a thin hardwood veneer. In most cases, this veneer is thinner than the face and reverse face veneer used in the manufacture of softwood plywood. In any case, the core layers are typically softwood, to help control cost. You can also find lumber core hardwood plywood, which has a single core layer, made of strips of wood, laminated together, much like a butcher block.
This category of plywood is most commonly used for cabinet and furniture making; places where a smooth, attractive surface is required for finishing. Common hardwood plywood available includes: ash, oak, red oak, birch, maple and mahogany. It is typically AB grade plywood; meaning the face side is A grade and the reverse face, which becomes the inside of the cabinet, is a B grade.
Contrary to what I just said, the backs of hardwood plywood are not graded in the same way that the backs of other types of plywood are. A separate grading system is used, to give better understanding of how the two faces of the plywood match up, for the purpose of cabinet and furniture making:
- Sound, same species, specifically cut
- Solid, same species, specifically cut
- Rotary Grain
- Reject Back
Cabinet Grade Plywood
Essentially cabinet grade plywood the same thing as Hardwood plywood; but some people prefer this term. Generally speaking, the term cabinet grade plywood is used to refer to ash and birch hardwood plywood. The other types of hardwood plywood are not referred for by this term.
Lumber Core Plywood
This is a sub-category of hardwood plywood. However, this product is not very common and is reducing in popularity all the time. Nevertheless, you can still find it used in cases where the edges of the plywood cannot be concealed by other wood trim or need to be routed.
Typically, lumber core plywood is a three-ply plywood, the outer veneers being hardwood and the inner layer being lumber core, made of a number of strips of solid lumber, typically basswood. Those strips are bonded together, as well as being bonded to the face veneers. The end result is a plywood product that meets a particular need, but is not as strong as most other plywood.
When selecting lumber core plywood, take care to ensure that there are no voids in the core. Unlike marine grade plywood, there is no guarantee that there will not be any voids.
Marine plywood is a special softwood or hardwood plywood, designed for use in the construction of boats, but used for other applications as well, especially when some level of moisture resistance is needed. Marine plywood is specially treated to resist rotting in high-moisture environments. But the main characteristic which separates marine plywood from all other types is that marine plywood is manufactured with no “voids” – the gaps in the core layers caused by splits or knotholes. This prevents water from becoming trapped in those voids.
Most marine plywood is also graded as Water Boiled Proof (WBP), a rating that refers to the glue used in bonding the layers of veneer together. In this case, the glue is similar to what is used on exterior plywood to bond the layers together. The rating indicates that test panels of this plywood material have been subject to being boiled in water, to determine if they will delaminate. The combination of heat and moisture is hard on adhesives, tending to soften them.
The combination of features contained in marine plywood greatly affects the price. Marine plywood costs about three times the cost of standard plywood.
Aircraft plywood or “airplane plywood” was in common use in the construction of airplanes in World War II. It is a thin birch or hardwood plywood (mahogany, beech, walnut and douglas fir have all been used) and can be considered to be the highest quality plywood product made, even better than marine plywood.
Like marine plywood, aircraft plywood requires that the core veneers are void free, with no knotholes or splits. Face and reverse face veneers are allowed to contain tiny, closed knots. Due to their use, they come in much larger sheets, so as to provide a continuous surface for covering wings. Typical thicknesses range from 1.2mm to 5mm.
Overlaid plywood is a specialty plywood, used primarily in the manufacture of concrete forms, although it has found a home in other industrial applications, such as containers. It is manufactured similar to softwood plywood, with the exception of the outer face layers. Those are made to be smooth and durable, usually being resin-impregnated fibers.
Phenolic board or phenolic plywood is similar to overlaid plywood, both in its manufacture and end result. The main difference is that this plywood has been made specifically for use in the building of concrete forms. It comes in two grades: HDO (high-density overlaid) and MDO (medium-density overlaid). Each of these are available in a concrete grade and a general grade. HDO leaves a smooth “steel-form” finish on the concrete, while MDO leaves a matte finish.
Particle board, often referred to as “chipboard,” is made of sawdust, shavings and tiny pieces of wood which are mixed with glue and pressed into sheets. It is the most economical, but the weakest of all plywood products. Particle board is commonly used under laminates on countertops and for shelving. Most inexpensive furniture uses vinyl covered particle board for large surfaces, usually trimmed with solid wood.
Due to the high rosin content in this product, it tends to be heavier than other plywood products. Nevertheless, the low cost makes it extremely popular. Particle board should not be used in any application which requires structural strength or which will be subject to any sort of impact. Nor is normal particle board moisture resistant in any way. Left outdoors in the rain, this product soaks up water rapidly, expanding the thickness, while ruining what structural integrity it does have.
There is a moisture resistant particle board, often referred to as “green chipboard.” This uses a different rosin, which is much more water resistant. This allows the core of the board to retain its dimensional stability, even when the surface of it, which does not use this type of rosin, does not.
Medium Density Fiberboard (MDF)
MDF differs from Particle Board in that it is created from individual wood fibers, instead of sawdust and wood chips. This creates an extremely flat board (flatter than softwood plywood made from veneers) consistent in material thickness and density, with no voids. Due to its very smooth finish, MDF is excellent for painting or vinyl veneer coating. MDF is slightly stronger than Particle Board and is taking over the furniture market from the latter. It has the greatest weight when compared to other types.
More properly known as “high-density fiberboard” (HDF) and commonly referred to by the trade name “Masonite.” Tempered hardboard is an engineered wood product, much like MDF. The difference comes from the manufacturing process, specifically how the wood fibers are harvested from the wood. This difference in process allows the wood fibers to be pressed much tighter together, making a denser product, with a density between 65 lbs/ft3 and 90.5 lbs/ft3.
Tempered hardboard is only produced as a thin product, between 1/8” and ¼” thick. One of the most common uses is as pegboard, where perforations are made in a grid, allowing hooks to be inserted for hanging tools or merchandise on the board. The non-perforated version of this product is used extensively in skate parks, to make skateboard ramps and half pipes. Its natural flexibility and hard surface lend to this sort of application.
Oriented Strandboard (OSB)
While most people see OSB as being an inferior product to plywood, building codes recognize it as being an equivalent material, especially for roof and wall sheathing. Instead of being made of wood veneers, OSB is manufactured from wood chips or strands, creating about 50 layers in the average sheet, as compared to 5 or 7 in softwood plywood. It costs about 20 to 30 percent less than a comparable thickness sheet of softwood plywood. OSB is commonly used in construction, for sheathing, roof decking and subflooring. OSB is available in sheets up to 24 feet long.
While OSB can support as much weight as plywood, making it meet building code, it is not as resistant to impact as plywood is. In testing to break a strip of OSB by striking it with a hammer, it will break much easier than a comparable thickness of softwood plywood.
Otherwise known as “Russian Birch” is a plywood material imported from Russia, hence the name. It differs from domestic plywood in that it has a much higher number of plys (9 for 1/2” and 13 for 3/4”), making it very attractive for drawer sides and modern furniture applications. It is commonly used in applications with bullnose edges, where solid wood edging cannot be installed. Because it is manufactured to a different standard, Baltic Birch normally comes in 5’ x 5’ sheets, although 4’ x 8’ sheets are becoming more common. When looking at the price, Birch is definitely the most expensive type of plywood on the market.
This plywood product, like Baltic Birch is manufactured from a higher number of layers than standard softwood or hardwood plywood. It is made from uniform laminations of solid 1/16” Alder and Birch, both low density hardwoods. There are a minimal number of voids, and the high number of laminations provides an attractive edge for furniture and cabinetry which machines extremely well. ApplePly is available in thickness ranging from 1/4” through 1-1/4” in sheets up to 10’ long.
A recent addition to the plywood family is foam board. This is a composite material, consisting of a polyurethane foam core, with wood faces; generally a thin plywood product, which can include Lauan plywood, MDF (medium density fiberboard, and others. The core may be reinforced with fiberglass, resulting in a finished product which is as strong as softwood plywood.
The benefits of foamboard over other plywood products are the lighter weight and rot-resistance. Being made with a non-organic core eliminates the ability of bacteria or insects to damage the product. The foam core also provides some insulating value, which is not found in other plywood products. The lighter weight makes it easier to install.
Due to the foam core, this product is normally installed with adhesives, rather than fasteners. Nailing the boards, for example, would tend to crush the foam board and a miss with a hammer could put a hole in it.
Another new plywood product is bending plywood. As its name indicates, this product has been designed specifically for the purpose of making projects where curved surfaces are needed. There are more than one type of bending plywood made, so it is necessary to select the right bending plywood for the project being built.
- Bending plywood can be made to bend cross grain or long grain
- Bending plywood can be two ply, with a single veneer face or three ply with two veneer faces. The veneer faces are equal; there is no face and reverse face
- These plywood products are normally quite thin: 1/8”, 1/4”, 3/8” and their metric equivalents
These products are mostly designed for use in architectural work, making cabinets and columns. However, they can be used for cabinets and furniture making as well. They are designed for a minimum 12 inch radius. While it may be possible to bend them into a tighter radius than that, special equipment and methods may be required. As a general rule of thumb, you don’t want to curve it any tighter than you can push the wood by hand.