Plywood consists of resin-coated veneer pieces pressed together under heat. That sounds straightforward, but things become a little more involved when it comes to choosing the right type and thickness of plywood for a job. Knowing the safety and strength requirements of any finished product makes it possible to choose the right type of plywood.
Plywood Grading and Rating
Aside from thickness, plywood classifications run according to appearance and condition. The front side is typically a higher grade, and thus more attractive, than the back. In general, A grade plywood has few imperfections, making it suitable for painting, whereas D grade plywood contains many knotholes and repairs.
Plywood for Furniture
Many furniture fronts, such as drawer fronts and cabinet tops, feature high-grade plywood with attractive veneer. Most cabinet construction uses ¾” plywood, or else ½” plywood, which is usually less expensive, and makes the finished piece somewhat lighter. Lower grade veneers are fine for interior areas, where appearance is not so important. Drawer sides only require ½” thick plywood, and drawer bottoms need only ¼” thick plywood.
Thicker plywood is heavier, but it is also stronger and more durable. For instance, building a large tabletop from ½” plywood, supported only by legs at the corner, will result in a dinnertime disaster. Many table-building plans specify the thicknesses to use according to tabletop dimensions:
- ¾” plywood for smaller table tops, up to about 18” x 24”
- 1” plywood for tops up to 24” x 36
- At least 1”, and, even better, 1 ½” for larger tables
Plywood is a popular choice for building entertainment units and shelving. However, while ¾” plywood may work fine for shelves less than 30” or so in length, the likelihood of mid-shelf sagging increases with shelf length. Using a thicker plywood helps reduce sag, especially over wider spans, thus avoiding potentially dangerous occurrences involving falling objects and collapsing furniture. Another way to avoid such a problem is by using wood edging to strengthen the shelves, or by installing mid-span supports.
Proper Plywood Choices for Safe Flooring
Plywood plays an important part in ensuring safe and durable flooring. Floors are composed of many different layers. The finished flooring sits on top. Next down is the underlayment, which provides the smooth surface needed for installing the top. Different surfaces require different underlayment, but all underlayment must be a thin but strong layer upon which the finished flooring surface rests. Floor underlayment typically uses 5/8” plywood. Beneath the underlayment is the subfloor, which requires the use of a thicker product, at least ¾” thick, perhaps more if the distance between floor joists is large.
Plywood for Walls
Only use exterior grade plywood anywhere where weather exposure is a concern. Interior plywood is not waterproof and its use in outdoor applications may result in serious building complications. Furthermore, exposure to weather compromises the integrity of the material, leading to collapses and weak spots that can cause serious injury.
The plywood used to sheath house frames does more than provide a surface on which to install the siding on the house. It also makes the structure stronger by connecting all the studs together. On top of that, structures with siding only connected to the plywood sheathing require a thicker plywood. If the siding is attached to the studs, thickness is not as much of an issue. Using ½” thick plywood for walls is standard, although the International Residential Code specifies only 3/8” wall sheathing when studs are spaced 16” apart and the siding is attached to the studs and not just the plywood. For buildings with siding attached only to the sheathing, the IRC requires ½” plywood.
The Right Plywood for Roofing
Many local building codes only require 3/8” plywood for roofing. Spending a little extra money, and using ½” or even 5/8” plywood results in a longer lasting and safer roof. Plywood that is only 3/8” thick often will not support someone working on a roof, especially if they are big and/or there is a lot of space between trusses and rafters. Handling heavier weights is also important in high snow areas. Additionally, using thinner plywood for roofing provides less material for shingles to attach to, which makes it easier for the shingles to fly off in high wind situations.
Consider the requirements of the finished product when choosing plywood thicknesses. It is not worth spending the time to craft a piece of furniture that will begin to sag in a relatively short time. The same goes for construction. Consider the weather conditions of the building site, especially high winds. It takes only one severe storm to prove the worth of spending a bit of extra money on thicker plywood.