Is Plywood Waterproof? [Complete Plywood Waterproofing Guide]

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When building exterior projects or projects in wet areas of your home, you want to choose a material that will withstand moisture. Not all wood products are made equal, and it’s important to know which wood building materials will perform best, whether in the outdoor elements or the bathroom. Plywood is a popular choice for many projects due to its large size and incredible strength, but is it waterproof? For your convenience, we’ve found the answer!

Yes, some types of plywood can be purchased waterproofed at the building supply store. It generally costs more than untreated plywood, but it saves you from the time and hassle of waterproofing the untreated plywood yourself.

So, how do you know which plywoods are waterproof and which ones aren’t? How do you waterproof untreated plywood? Keep reading to learn about the different kinds of plywood and how to prepare it for use in your project.

Stack of plywood sheets, Is Plywood Waterproof? [Complete Plywood Waterproofing Guide]

Pressure Treated Plywood

Pressure-treating wood is a process that makes it resistant to moisture; almost any type of lumber can be pressure treated. The process involves sealing the wood in an airtight tank, extracting the air, then introducing a chemical solution made of copper, chromium, and arsenic which soaks deep into the wood and works to repel water, rot, and insects.

Pressure-treated plywood is a great option for outdoor projects that are heavily exposed to the elements. It also makes a perfect choice for interior projects in rooms that see a lot of moisture, like the bathroom or the kitchen. When using pressure-treated plywood outdoors, it is recommended to begin using a sealant on it after the first year of use, then once per year after that.

Set of plywood

Untreated Plywood

Untreated plywood is commonly used for interior projects, but it will work pretty well outdoors if you treat the lumber. While you can’t pressure treat your own wood without special equipment and chemicals, there are plenty of great products on the market to help you quickly and easily protect your wood surfaces.

Untreated plywood will not last long in wet conditions without a protective coating, so for bathrooms and kitchens, it is recommended to use pressure-treated plywood if you don’t want to go through the process of treating the wood yourself. Untreated plywood will need to be re-finished whenever signs of wear begin to show through the original coat to ensure longevity.

How Can I Tell The Difference Between Treated and Untreated Plywood?

Treated plywood is sold with a tag on the end that will list the chemicals used to treat it. If you can’t find a tag or previously removed it, it becomes a little harder to tell. Pressure-treated wood tends to have a darker color and can appear to have a green or brown tint left behind by the chemical curing process.

Another way to identify pressure-treated wood by appearance is a moist look left behind by the chemical treatment process; this moist appearance can stick around for a long time after the wood has been treated. Treated wood will also give off a chemical smell, but avoid getting your face too close, as the chemicals used to treat wood are very dangerous and should not be inhaled.

How Important Is The Wood Species?

Both treated and untreated plywood is available in a variety of different wood species. Some wood is more weather resistant than other species, so this can greatly impact how long the material will last. Whether or not the wood is naturally rot-resistant or pressure-treated, your plywood should always receive an application of sealant when exposed to moisture.

Some naturally rot-resistant wood species include Mahogany, Fir, and Hemlock. These rot-resistant varieties can be harder to find and will cost more, but they will significantly increase your project’s lifespan. Less resistant species include Pine, Poplar, and Red Oak.

Can You Seal Plywood For The Outdoors?

There are a ton of waterproofing products available for wood surfaces, but make sure you choose a sealant designed for outdoor use if this is an exterior project. If you are waterproofing your plywood for an indoor project, find a sealant recommended for interior purposes. Exterior sealants can contain harsher chemicals that you may not want inside your home every day.

Klean Strip Boiled Linseed Oil

Boiled Linseed Oil is a great, more natural alternative to chemical sealants like polyurethane. Click here to view it on Amazon.

What Do You Put On Plywood To Make It Waterproof?

Plywood can be treated in many ways. There are waterproofing products designed for use on wood, like sealants. You can use polyurethane varnish for an effective seal as well, but it can obtain a slightly yellow tint when exposed to sunlight. Marine varnish and aquarium sealants are popular choices for exposed projects.

Certain oils like linseed oil are popular for waterproofing wood as well. Plywood should always be waterproofed before you paint it as well, so if you plan to paint afterward, make sure to choose a sealant that will accept a coat of paint on top.

Thompson’s Waterseal Wood Protector

Thompson’s WaterSeal is a trusted and dependable product that contractors have put their trust in for years. Click here to check it out on Amazon.

How Long Will Plywood Last Outside?

Untreated plywood without a waterproof finish would last about two years outside before it begins to rot away, depending on the exposure it receives. Untreated plywood with a waterproof finish can easily last more than five years if you re-apply the finish regularly.

Pressure-treated plywood will last around nine years, even without a waterproof finish. Regularly using a waterproof finish on pressure-treated plywood will give you a project that could last between 30 and 50 years depending on exposure and how regularly you re-finish it.

Red Oak Plywood 1/4″ x 24″ x 48″

This Red Oak plywood has a beautiful natural grain and tone. It’s untreated, meaning it’s best for interior use and will need to be sealed for outdoor use. Click here to look at it on Amazon.

Will Rain Hurt Treated Plywood?

Yes. Rain, moisture, and other elements will cause damage to any type of wood. Pressure-treated plywood can withstand rain better than untreated plywood, but waterproofing will greatly increase its life expectancy. Given enough time and exposure, all wood will rot eventually, but properly sealing can extend its life exponentially.

What Is The Strongest Plywood?

Marine plywood is the strongest plywood available on the market today. It is made to be used in the water for things like boats and docks. During manufacturing, they use special water-resistant glues to adhere the ply’s of the wood together, and the wood is also treated with stronger chemicals to withstand being submerged. Even marine plywood should be sealed. If you want strong plywood that is not weather-treated, consider purchasing the thickest plywood available at your supply store.

3/4″ Marine Grade Plywood 24″ x 48″

This marine-grade plywood is pressure-treated for extreme moisture. Click here to see it on Amazon.

Only Build With the Best

Now that you have learned all the differences in the types of plywood available and how to maintain them, you are ready to start your next building supply list. When working with tools, always use care and make sure to wear gloves, masks, and protective eyewear when working with pressure-treated lumber.

If you are planning on building a new porch, our article What Is The Best Material For A Porch Floor [A Complete Guide] is an excellent guide.

For more help on building a porch, or finishing outdoor wood projects, check out our article Does Porch Paint Need Primer?

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