What Is Plywood?
Plywood is an engineered wood product made from thin wood veneers or plies. The grain of each ply is set at a 90-degree angle to that of adjacent plies, reducing expansion and contraction. The plies are glued together, pressed, heated and cut into sheets.
The Plies of Plywood
In general, more plies create more stability and better resistance to breaking and warping:
- 3-ply plywood is durable and offers edges that are more attractive than those of sheets with more plies.
- 5-ply plywood is suitable for a wide range of project and construction uses where improved rigidity and strength are needed.
- Multi-ply plywood has seven or more layers and is common for applications such as subflooring and roofing, where greater strength and stiffness are necessary.
Plywood Veneer Grades
Plywood veneer grades indicate the number and size of defects. The first letter applies to the front and the second letter applies to the back:
- Grade A: The veneer is smooth, sanded and nearly free of defects. The surface is ready for finishing and is ideal for cabinet doors and furniture.
- Grade B: Also sanded, this grade has small knots and a few defects that might be filled.
- Grade C: The surface is unsanded, has larger knots, defects and patches and is good for subflooring or other applications where appearance isn’t important.
- Grade D: Also unsanded, the surface can have large knots, lots of patching and unrepaired defects.
The most common plywood grade is B/C.
Plywood Exposure Ratings
Exposure ratings relate to the adhesive that bonds the plies together:
- Exterior: Waterproof adhesive secures the plies, making these sheets suitable for outdoor buildings that have continuous exposure to the elements.
- Exposure 1: These sheets have waterproof adhesive that’s only designed for temporary exposure, such as during construction.
- Interior: Designed for interior use only, the ply adhesive isn’t waterproof, and the sheets aren’t suitable for moisture exposure.
What Is CDX Plywood?
CDX plywood is suitable for outdoor use. The CDX designation tells you grades and the location rating:
- C indicates a Grade-C top surface.
- D indicates a Grade-D bottom surface.
- X indicates an Exposure 1 rating and temporary adhesive waterproofing.
Common CDX plywood uses include roof decking, sheathing for exterior walls and subflooring.
Plywood Sizes
Plywood thickness is a nominal measurement, meaning the actual thickness might be slightly less due to drying during the manufacturing process.
Exterior Plywood Types
Exterior-rated plywood is necessary for projects like exterior walls, roof decking and outdoor structures. While the adhesive that binds the plies is waterproof, the plywood itself needs sealing if it’s exposed to moisture.
Pressure Treated Plywood
Pressure treated plywood is a type of exterior-rated plywood. In addition to a waterproof adhesive, it has preservatives that make it resistant to insects, rot and decay. Pressure treated plywood uses include shed floors, storage buildings, fencing and accessibility ramps.
Pressure treated plywood requires corrosion-resistant fasteners and hardware, such as stainless steel material or a hot-dipped galvanized finish.
Plywood Sheathing
Exterior plywood sheathing is common in roofs and walls underneath shingles or siding. Plywood sheathing is often CDX plywood, suitable for only temporary exposure to moisture.
Building codes specify materials for some construction and building projects. Always check local codes before choosing plywood for your application.
Interior Plywood Types
Interior plywood isn’t resistant to moisture, rot or insects but works well for indoor furniture, interior cladding, flooring underlayment and craft projects. Check out our article on simple plywood projects for ideas.
Plywood Underlayment
Plywood underlayment installs over subflooring. It creates a smooth, defect-free surface for installation of thin flooring, such as sheet vinyl, that can show imperfections in the subfloor. Tongue and groove or T&G subflooring fits together tightly to reduce squeaking.
Plywood Project Panels
Plywood project panels are pre-cut plywood pieces available in a variety of plies and wood types. They’re easy to transport, easy to work with and make an excellent beginner-friendly choice for anyone starting out in DIY.
You can also find project panels for exterior use.
Sanded Plywood
Sanded plywood can be either exterior or interior plywood. A sanded face provides a smooth surface that’s ready for finishing with minimal or no additional sanding. Typical uses include stair treads and risers, as well as projects where appearance is important, such as building cabinets, shelving, paneling, accent panels and furniture.
Hardwood and Softwood Plywood
Hardwood plywood is made from oak, maple, poplar, birch or walnut and offers excellent strength, stiffness and durability. The density of the hardwood gives the panels resistance to damage and wear and makes them a good choice for cabinets, shelving and furniture.
Softwood plywood is usually made of Douglas fir, pine or Southern yellow pine and is typically used for exterior frame sheathing, roof sheathing and subflooring. Softwood plywood is also ideal for sheds, plant boxes, doghouses and shelving.
Plywood Sheathing vs. OSB
Like plywood sheathing, OSB (oriented strand board) is an engineered wood product common in construction projects. It’s suitable for walls, roof decking and subflooring. Here’s a breakdown of key differences between plywood sheathing and OSB:
Material Composition
- Plywood sheathing is thin layers or plies of wood veneer glued together with the grain of each ply at a 90-degree angle to the adjacent plies.
- OSB is wood chips or strands compressed and bonded together by wax and resin, with the strands of each layer at a 90-degree angle to those of the adjacent layers.
Strength
- Plywood is impact resistant and better able to hold nails and screws.
- OSB offers comparable strength for exterior wall sheathing, roof decking and subflooring.
Moisture Tolerance
- Plywood can withstand water better and is less likely to swell and warp.
- OSB edges are especially vulnerable to moisture if they‘re not sealed.
Appearance
- Plywood offers a smooth and uniform surface.
- OSB has a rougher appearance and is typically used where it won’t be visible.
Cost
- Plywood is usually more expensive due to use of better materials.
- OSB is more budget-friendly and a popular choice where costs are a concern.