With the price of lumber skyrocketing right now, homeowners and contractors want to eliminate waste as much as possible. If you are planning a project where your plywood might get exposed to moisture, or looking for the best storage options in wet conditions, you may wonder whether water might harm your plywood.
Long-term exposure to wet conditions can ruin plywood, especially if it has not been treated for waterproofing. Excessive moisture penetrates between the layers and causes the sheet of plywood to swell and warp. The longer the plywood stays wet, the more likely the damage will be permanent.
Read on to learn more about plywood and why it is so susceptible to moisture damage. You will also discover better ways to store it to prevent water damage and what to do if your plywood gets wet.
Plywood: The Basics You Need To Know
Plywood is a piece of engineered composite lumber made up of thin layers of wood veneer that have been glued and pressed together. Usually sold in 4-feet x 8-feet (1.21 x 2.44 meters) sheets of various thicknesses, plywood is versatile, relatively inexpensive, and has been a popular and practical building material for centuries.
The wood grains of adjacent layers are rotated up to 90° from one another. This process of cross-graining provides several advantages over natural wood of the same dimensions:
- Reduces splitting when the wood is nailed at the edges
- Limits shrinkage and expansion
- Greater dimensional stability
- Less warping
- Resistance to bending
When Plywood Gets Wet
Among the disadvantages of plywood is a greater vulnerability to water damage. If sheets of plywood are allowed to stay wet for too long, water can seep in between the layers. Because the veneer layers are so thin, any degree of swelling can cause considerable damage such as:
- Warping – Excessive moisture makes the plywood expand and contract, and when it finally dries, it may be bent out of shape.
- Separation – If water seeps in, it can cause the individual veneers to swell and move apart from each other. Invasive moisture can also dissolve the resin that holds the layers together. This weakens the board’s structural integrity.
- Splitting – This damage occurs when the stress of swelling with water and shrinking as it dries causes the plywood to split or crack.
- Rot– If the sheet of plywood is not entirely dried out, it can be irreparably damaged when attacked by mold, mildew, bacteria, or fungi.
In most cases, if the plywood stays wet for an extended period, to the point that any of these forms of damage turn severe, the wood will be completely ruined and unusable.
How Different Grades of Plywood Resist Moisture
Depending on your project, you may be storing different grades of plywood. Although no type should be left submerged in water, some grades are more resistant to moisture than others.
- CDX (3 layers) – “Class C to D exposed” plywood is the lowest grade and is the worst at resisting water damage.
- T1-11 – “Textured” plywood is raw and untreated with any protectants, making it very vulnerable to moisture damage.
- ACX (2 layers) – “Class A to C exposed” plywood has an exterior layer of higher-grade wood that is far more resistant to warping.
- Overlaid (3 to 7 layers) – Superior to CDX and ACX at resisting moisture due to an overlay that is more waterproof and scratch-resistant.
- Marine-Grade (5 to 9 layers) – Despite the name, marine-grade plywood is not wholly impervious to moisture damage. While the layers are held together with waterproof glue, the wood itself is not treated, leaving it susceptible to rot.
- Pressure-Treated (5 to 15 layers) – This type of plywood undergoes 160 pounds of pressure to treat it with protective chemicals. The process makes it more resistant to rot, insects, and moisture, but it is still not totally waterproof.
The bottom line is that while marine-grade plywood is the most water-resistant, thanks to the type of resin used, no grade of plywood can be considered fully waterproof.
The Type of Wood Matters
Hardwood plywoods are made with formaldehyde-free glue, which means they are less resistant to moisture. This is why it is typically used for interior projects such as making cabinets or furniture.
Despite the name, softwood plywood is superior in terms of standing up to the elements. This is why it is commonly used for more demanding projects such as roof and exterior frame sheathing, sub-flooring, and in erecting outside buildings such as sheds and dog houses.
Below are examples of each wood type:
Softwoods
- Redwood
- Pine
- Fir
- Cedar
Hardwoods
- Poplar
- Oak
- Maple
- Cherry
- Birch
- Beech
- Alder
Check the Exposure Rating
When buying plywood, always check the exposure rating.This will tell you how water-resistant the specific type is. Here are the different ratings available.
- Exterior – Suitable for applications where the plywood will be subject to ongoing and permanent exposure to moisture. The bond between the layers is fully waterproof.
- Exterior Exposure 1 – While the bond is fully waterproof, this type of plywood is not designed for ongoing and permanent exposure to moisture.
- Exterior Exposure 2 – Designed for applications where the exposure to moisture is expected to be light. Manufactured with intermediate glue.
- Interior 2 – Only suitable for interior applications.
It is important to note that the exposure ratings focus primarily on the quality of the glue binding the layers together. It does not rate the wood itself.
How To Waterproof Plywood
The good news is there are things you can do to protect your plywood from water damage. Moreover, you can save a lot of repair and replacement money by proactively taking precautions.
Below are some helpful suggestions:
Apply Waterproof Paint
With plenty of texture and color options, waterproof paint is easy to apply – simply brush or spray the paint directly onto the plywood. For best results, use at least two coats, letting the paint fully dry before applying the next coat.
Remember that you will have to repaint periodically for continued protection.
Use Epoxy To Seal Or Varnish Your Plywood
Epoxies are an excellent way to give plywood superior waterproofing protection. Properly applied, epoxy lasts longer than paint.
If the epoxy is merely for use as a varnish, apply no more than two layers. If using it as a sealant, four layers are recommended. Let the coat dry thoroughly, lightly sand, and then wipe the wood down with a clean and dry cloth before applying the next layer.
Caution: When working with epoxy, always use gloves and make sure that there is adequate ventilation.
Use Polyurethane
Although it takes longer to dry than either paint or epoxy, polyurethane is a fantastic waterproofing choice. Take the following steps:
- After cleaning the plywood with a dry cloth, brush the polyurethane onto the plywood, remembering to go with the grain of the wood.
- Let the coat dry for at least 24 hours.
- Lightly sand to create texture and remove blemishes, wipe it off again, and then apply the next coat. You will need at least three coats.
Caution: Again, to be safe, you must use gloves and only use polyurethane in well-ventilated areas.
Apply Protective Oils
Oils are a good way to make plywood water-resistant while beautifying it at the same time.
Here’s what to do:
- Using a brush or roller, liberally apply the oil.
- Let it soak in and penetrate the plywood for at least 10 to 15 minutes, then wipe off any excess.
- For best results, apply at least three coats of oil and allow up to 24 hours between each coat.
What To Do if Plywood Gets Wet
If your unprotected plywood has already gotten wet, the actions you take and how quickly you take them will largely determine whether you can prevent significant damage and save the wood.
The first step is always to separate the plywood from the wetness and, if possible, deal with the source. For example, if it is raining, get the plywood off the ground and cover it tightly with a waterproof tarp. Or, if it is exposed to a leak, move the plywood, dry it off with rags or towels, and take whatever measures are necessary to stop the leak.
If you are trying to dry stacked plywood, separate the sheets for better airflow. Lay them out in the sun or use fans or a dehumidifier to speed up the drying process.
Always take special care to ensure that the edges of the plywood are completely dry. These parts of the sheet are the most vulnerable to water damage.
Conclusion
Because no grade of plywood is utterly impervious to water:
- Only use plywood with the appropriate grade and the proper exposure rating for your particular project.
- Protect your plywood by waterproofing it with paint, varnish, polyurethane, or oil. Take special care to include the vulnerable edges.
- Keep plywood away from unnecessary exposure to moisture. Store it correctly, and if it does get wet, dry it off as soon as possible.
If plywood gets oversaturated to the point of damage, it may no longer be structurally sound. Have it inspected by a professional to see if repair or replacement is necessary.
