The human body becomes acclimatized to the environment in which we spend most of our time. We often hear about blood “thickening” in colder climates, and we get used to the colder temperatures; we hear, too, about those in the US south and their “thinner” blood that has become accustomed to the heat.
In the northern US, 50 degrees might be tee shirt weather or at least not winter coat weather. But, in the south, like my friend in Sarasota the other day who was complaining about 50-degree weather as being cold, it’s at least sweater or coat weather.
That kind of acclimation is easy to understand. We experience it, feel it, and adjust our clothing accordingly to it. We’re not expanding or contracting, though, as the climate around us changes. But wood does, and as woodworkers, we must be aware of this.
When our reaction is to put more clothes on or take some off, wood will tend to crack, warp, cup, or split as the environment around it changes. Moisture content can lead to any of those as the wood dries, or if it does not can lead to decay and rot. Temperature changes cause expansion and contraction, and if the wood has already been installed/nailed/screwed/glued, a large swing in temperature can cause the wood to shift, crack, or pop a nail.
Plywood is affected by all of this just as solid wood is affected. Does this mean plywood must first be allowed to settle into its environment first before using it, just as we humans settle into ours? Let’s take a look.
In This Article
What Is Plywood?
We know plywood, and we work with plywood often. It has many applications, is easy to work with, and is less expensive than solid wood, making it a suitable substitute to keep the cost of wood projects down.
Plywood is a manufactured wood product made by gluing veneers, or plies, of real wood in layers to create sheets. Each ply is rotated 90 degrees as it is glued to the previous ply in the manufacturing process, and this is done for three reasons:
- It enhances the strength of the sheet of plywood consistently in all directions.
- It helps avoid the expansion and contraction that is inevitable with a changing temperature in its environment.
- It helps prevent splitting when nailed or screwed.
In essence, the rotation of the plies during manufacture makes the plywood sheet stronger than if the plies were not turned.
Plywood sheets come in 3-ply, 5-ply, and multi-ply types, with 3-ply being the most common. The 3-ply sheets are most often chosen for interior uses as their appearance is more appealing than thicker sheets.
Is Plywood Real Wood?
Yes, the plies are veneer cut from real wood. It’s a manufactured product, but the wood is real. The plies are glued together with a resin, and different glues are used depending on the rating of the sheets being manufactured.
Virtually all structural plywood today is manufactured using waterproof phenolic resins that will maintain their bond during expansion caused by exposure to moisture. Phenolic resins, invented in 1907, were originally called Bakelite, effectively the first commercially sold form of plastic. As a synthetic thermosetting resin, it was used to make the old rotary dial telephones.
A form of phenolic resin with the properties of a bonding agent, a glue, is used to hold one ply of veneer to the next, etc., as the sheets of plywood are manufactured. Once dried, it has waterproof qualities and forms a tight bond to the plies, helping to keep the plywood cores dry and resistant to expansion and contraction.
The American Plywood Association, the APA, rates and offers trademark designation to plywood. APA-trademarked plywoods are suitable for a variety of uses in construction, including:
- wall and roof sheathing
- ceiling and deck sheathing
- and many others.
The APA is a third-party quality control association that rates plywood designated for particular uses, especially when the plywood is to be used in the structural construction of buildings. In our research for this article, we consulted the APA website and, in particular, to its recommendations for using plywood for particular purposes.
Common Plywood Uses in Woodworking Shops and Construction Sites
As we said, plywood is a versatile wood product that is very cost-effective and easy to work with on home shop projects and in-home construction. It works well in each for purposes that include:
- Exterior wall framing when building a new home or an addition to an existing home
- As floorboarding over joists
- Furniture-building, cabinetry, shelving, and more, usually with a higher grade of plywood than the first two
The advantages of using plywood rather than solid wood include:
Stability. The rotating plies add strength to the sheets, as well as resistance to changes in its surrounding environment.
Strength-To-Weight ratio. Plywood shear strength is twice as strong as solid wood and at a lighter weight and has good impact resistance.
Stands up to chemicals. It doesn’t corrode, so it is useful in chemical works and as material for concrete forms work.
If you work in construction, you already know this. You also know the relatively lower cost of plywood for these purposes and for these advantages in plywood’s use.
Like all woods and wood products, there are different grades of plywood, each with different features and advantages and at different price points.
Grade A is the highest quality and the highest price point. It is sanded smooth and will have no apparent flaws or defects in appearance.
Grade B is the next highest quality and a lower price than Grade A. It, too, is sanded smooth but will have some defects in appearance. Those defects will have been repaired during manufacture. You might see knots, but there will be no wood missing among the plies.
Grade C plywood will not have been sanded and will readily display defects and blemishes. It will be used where it will not be seen, such as flooring material, wall and ceiling sheathing, and such. It is a third-tier price point below the higher grades.
Grade D is neither sanded nor blemish-free. It might be discolored, also. You will use this grade where it will not be seen, again as a flooring material, sheathing, and such.
Beyond grades, plywood will also have different ratings that have to do with where and how the sheets will be used. They range from:
Exterior – waterproofed, suitable for outdoor projects, made with a different glue than plywood manufactured for interior use.
Exposure 1 – waterproofed, too, suitable for outdoor use temporarily during construction, but eventually should be covered and protected from the elements.
Exposure 2 – only partially waterproof and should be limited to interior use.
Interior – as the rating implies, for interior uses only.
There are also features and categories of plywood beyond grades and ratings, but for our purposes today, we’ll skip them except to say they have to do with the type of wood veneers used to manufacture the sheets – softwoods and hardwoods – and the uses to which the plywood sheets might be put.
You’ll find more detail on these grades and ratings in a previous piece we wrote about sealing plywood for exterior use.
We will be mostly concerned with plywood used as a subfloor and underlayment during construction.
As you can no doubt see, we will not be using Grade A or Grade B plywood-rated exterior for these purposes. They are more expensive than the purpose can justify, and the added measures of protection from a wet environment or high humidity level are not essential.
A well-constructed floor in your house will have a proper mixture of plywood subfloor, plywood underlayment, and finish flooring (which may be carpet or hardwood).
The plywood subflooring will sit on the floor joists, the plywood underlayment will sit on the subflooring, and the finished flooring choice will sit on the plywood underlayment. This is the order of construction in the framing and preparation of the flooring for the proper finish.
Depending on that finish, there may be a particular choice of plywood underlayment that will serve best, something that will offer the required strength and stiffness needed for the finish (wood floors, for instance, rather than a pad and carpet).
We know that water is the enemy of wood. It can lead to decay and rot, and a high moisture content makes the wood susceptible to this damage.
We also know that changes in the surrounding environment will cause some expansion and contraction in the wood. The greater the variance in the expansion and contraction level can cause cracks, warping, cupping, and splitting.
We rely on the strength and stiffness of flooring to provide a stable and level performance of the materials used to construct the floor.
Plywood underlayment, usually a Grade C or D, must always be protected from moisture and damage before being installed. Moisture and/or temperature variance will cause wood expansion, and installing wet underlayment will present a problem as the plywood dries and contracts. We don’t want wood movement in the floor after installation.
This is where some form of plywood acclimation comes into play. The APA recommends the underlayment plywood sheets be allowed to acclimatize to the environment in which they will be used before installation.
The plywood sheets should be stood on their edges and separated from each other to allow plenty of air circulation for a day or two in the room where they will be installed. This latter point is not written in stone, though. An adjacent room, assuming the humidity level and temperature are the same, is certainly fine.
The point is to bring the moisture content of the plywood underlayment into equilibrium with the surrounding environment. You can use a moisture meter, of course, to measure the moisture content. Moisture meters are not expensive, and you can find them at large DIY stores, your local hardware store, and at online retail stores. You simply want to bring the moisture level of the plywood down to match the moisture level of the room.
We aren’t necessarily talking about a couple of weeks for that acclimatization to occur. A few days may very well be enough, and a moisture meter can help you verify that fact. Besides, just as we don’t want to watch grass grow, we also don’t want to watch plywood dry for a couple of weeks., especially if it is not necessary.
Once the plywood has become acclimatized to the surrounding environment, it is ready to use. During installation, and again knowing that a changing environment in both temperature and humidity level will inevitably occur, you will want to allow a 1/8 ” or so gap between the sheets as you install it. This will permit some expansion and contraction that will likely occur during the course of a year.
The subfloor should be dry before installation, too. Be sure to inspect the evenness and flatness between the joists to which the underlayment will be installed. Also, remember to install the underlayment smooth side up. Finally, install the underlayment with its face grain perpendicular to the joists to maximize stiffness, and leave a bit of a gap at the ends of the sheets to allow for a little bit of expansion.
Other Plywood Installation During Construction
For ceiling sheathing or for framing walls during construction, it probably isn’t necessary to consider plywood acclimation. These parts of construction come at the early framing stages of a house or addition and will lead to making the house or room weather-tight.
Once a house has been made weather-tight, the portable heaters will be brought in if it’s winter or the warm weather is present, each of which will tend to dry out the moisture content of the plywood. There will still be the rough plumbing and wiring, and those couple of days or even a couple of weeks will be plenty of time to allow the plywood to become acclimatized to its surroundings.
That’s really all you want, too – an equilibrium to be established.
Video On Buying Plywood
Although not on point about acclimatizing plywood, this video will give you some very helpful guidance on using the right plywood for your project. It’s worth a look, and since we went into some detail about plywood grades and ratings, we thought it would be a worthwhile addition to this article.
The answer to the question, then, is yes, there are some instances where your plywood should be acclimatized to its permanent environment before being installed. It’s no big deal, though – good air circulation, shelter from the elements, and a little bit of time. The effort and the time are well worth any minor inconvenience, and the experts at APA recommend it.