Plywood is a great building material, but not every sheet is made equal. If you’ve got standard plywood sheets exposed to water, you need to learn how to waterproof plywood. This will help you avoid costly water damage and repairs down the road.
Waterproofing plywood is a simple process that can save you hundreds in water damage repair costs. You can waterproof plywood using different materials, ranging from waterproof paint to oil and varnish. Doing this extends the life of plywood considerably, preventing rot, warping, and splitting.
In this article, you’ll learn about plywood’s vulnerability to water, the different ways to waterproof plywood, and what types of plywood are available.
Is Plywood Waterproof?
Plywood is best considered water-resistant, not waterproof. And, even then, “water-resistant” may be too strong a term! Small amounts of water won’t affect it, but prolonged exposure to moisture can be disastrous. Plywood is made of thin wood veneers layered and glued together with pressure and heat. It is a very strong material, but lots of moisture can seep into the layers and cause them to separate or split.
When plywood is being made, the wood is treated with water, which is soaked up and makes the wood expand. Then, when the layers are pressed together and heated, the water is squeezed out, making the wood layers bond together even tighter. For this reason, water and plywood don’t exactly mix.
Plywood is vulnerable to water because it will make the individual wood layers expand again. Over time, this will separate the layers from each other, peeling apart since the glue that holds them together can’t withstand the slow expansion. This can take many years, or just a few months, depending on the amount of water the plywood is exposed to.
What happens to plywood when it gets wet is different than what happens to solid wood. When a wooden beam gets wet, it may soak up water and eventually deteriorate, but it doesn’t lose its integrity easily. Because water separates the layers in plywood, the effects of water damage on plywood are much greater.
Two other things can happen when plywood gets wet, other than splitting: rot and warping. Rot occurs when the moisture so thoroughly corrupts the wood that it cannot hold weight any longer.
In plywood, this can happen in spots where screws/nails are used, and the sheets can come completely loose since the wood no longer holds tight to the nail. Warping results in bent, curved plywood that looks horrible and is useless in any building application.
Can You Waterproof Plywood?
Plywood is very receptive to waterproofing material. The smooth finish is still porous enough on a micro level that the plywood will easily bond with sealers and paints. This means plywood can still be used outdoors and in marine environments. You can waterproof plywood to use on your outdoor deck, boat dock, or on the boat itself, without worrying!
Waterproofing your plywood is very important because it saves you the headache of completely replacing it every year or two. Since plywood is usually used in entire sheets rather than thin boards, one damaged sheet of plywood affects a large surface area. If parts of your floors and walls are made from plywood, you could end up replacing large parts of your home!
Benefits of sealing Plywood
So, how does waterproofing help? Here are some of the ways that doing a waterproof treatment on your plywood can benefit you:
Untreated wood boards, like 2X4’s, last about two years outside. Since it is less moisture-resistant, plywood will likely fail even quicker if left exposed to the elements. The same is true indoors- a water leak is just as deadly to untreated plywood as rain and snow. With properly treated plywood, however, you can expect a lifetime from 20-40 years!
Even if the plywood doesn’t fully split, prolonged water exposure can cause rot just like it would with normal wooden boards. Wood rot doesn’t just look bad- it smells bad, and it drastically weakens the material. Wood rot can spell disaster for your deck, home, or boat. Waterproofing your plywood can save you the headache of dealing with a slowly crumbling plywood panel!
Prevents Dry Rot
Wait, isn’t dry rot the same as normal rotting? While it’s true that the effects of dry and wet rot are similar, they have different causes. Dry rot occurs when microscopic fungi grow on the surface of the wood and is almost impossible to see until it’s too late. It doesn’t require much moisture- the dagame occurs after the wood has dried out and the fungi are already growing!
Protects from Moisture and UV Rays
Repeated exposure to moisture and heat warp plywood quickly. After one rainy summer, a completely flat plywood sheet turns into a modern art sculpture. Those bends and curves might be interesting, but they make for a pretty terrible deck, floor, or siding. Waterproofing plywood sheets is a great way to make sure they last for decades, not months!
Splitting is one of the most damaging results of water exposure in plywood. It causes the wooden layers to separate, completely ruining the sheet. The worst part about splitting is that it can sometimes be hard to see; once you finally notice it, the water is likely damaging other parts of the deck/home/boat already. Make sure to waterproof before installing plywood to avoid splitting!
How to Seal Plywood for Outdoor Use
There are a few common techniques for outdoor plywood sealing. The good news is, most of them are very affordable and relatively simple to put into use. And, with proper application, they’re more or less equally effective. That can make it hard to find the absolute best way to seal plywood because each method does the job rather well.
Let’s take a look at the top 6 methods of waterproofing plywood:
Using an epoxy sealer is probably the most popular way to seal plywood against the elements. The epoxy is usually found in paint or spray forms. The advantage that epoxy provides is that it makes the plywood stronger, in addition to waterproof. The epoxy forms a tough, hard outer layer that is resistant to scratches and dents as well as water!
The epoxy soaks into the surface of the plywood, and it goes on clear, so wood/plywood retains its natural color. Plus, if you run a bit of sandpaper over the dried epoxy, you can still paint over it, any color you like!
Drying OilDrying oil is great for outdoor and marine use because it allows the plywood to keep some of its natural moisture and flexibility. Whereas epoxy forms what is essentially a shell around the plywood, drying oil soaks in and provides an effective yet thin coat.
If you’re planning to use plywood in a high-moisture area, like a dock, boat, or you live in a wetter environment, drying oil is going to be the best waterproofing method for you. It works a bit differently from epoxy, paint, and other sealers because it strengthens the wood against water deterioration rather than completely blocking water from the wood.
This means it will dramatically extend the life of the wood without making it 100% “waterproof” in the traditional sense.
Water-Based PaintWaterproofing with paint isn’t quite as long-lasting as epoxy or drying oil, but it is effective nonetheless. Water-based paints work outdoors, but they will probably need to be touched up or repainted every few years. Water-based paint won’t crack or peel easily, but you will notice slight issues like these over time.
However, these problems are easy to fix, and your plywood will be safe from moisture damage with waterproof paint. Plus, most other methods of sealing plywood don’t give you the advantage that paint does: color! Choosing the exact color you need is more than worth the “hassle” of touching up waterproof paint every few years.
VarnishUsing a varnish is a great way to seal plywood without adding a noticeable thick coating. The term “Varnish” refers to any mixture or resin that is brushed on thin and cures to form a hard coating.
It’s not as thick as epoxy or noticeable as paint but has many of the same qualities. Most varnishes will leave an appealing glossy look once dried.
The varnish is ideal for above-water line marine uses; for instance, it’s great for the upper parts of wooden boats or the above-water parts of your dock. As long as it isn’t submerged, it will resist salt, mildew, and moisture for many years. It’s great for outdoor (non-marine use) for that reason as well!
Liquid LatexUsed in spray or brush-on form, liquid latex might be the easiest method of waterproofing plywood: spray it on and forget about it! The only downside to using liquid latex is that it can be hard to tell whether or not you’ve got enough on the plywood to seal it adequately.
On top of that, it isn’t very cost-effective on a large scale. However, you may want to pay the extra price (compared to brushed paint) to have the easiness of spraying liquid latex generously.
If you use it appropriately, as in getting adequate coverage, liquid latex is one method that works just about everywhere. It bonds well to plywood and will last a couple of decades, worry-free.
If you don’t get adequate coverage, water will be able to get into the plywood through the gaps left by poorly sprayed latex, and all your work will be in vain! So, if you choose liquid latex, make sure to completely cover the plywood with a generous amount of spray!
PVAThis is a type of glue that is traditionally used for things like bookbinding. PVA has every quality you need in a waterproofing material: it dries thin and clear, bonds well to plywood, and is completely waterproof.
If you want to use PVA to seal plywood, it’s a good idea to dilute it with water so that it will go on thinner and soak into the wood a bit before drying.
How to Waterproof Plywood
Now that we’ve discussed the different available methods of plywood waterproofing, it’s time to do the work of sealing it.
Step 1: Prepare the Plywood
Before the liquid latex, paint, or PVA is applied, the plywood needs to be readied. Clean the surface of the plywood sheet with a brush or dry cloth, removing any dirty spots you can find.
Then, use wood filler to fill in any rough spots on the surface of the sheet. This will give it a uniform smoothness and help prevent any gaps in the waterproofing. Give the wood filler time to dry and harden before proceeding to the next step.
Once the wood filler dries, wipe down the sheet with a wet cloth. That small amount of moisture will raise the wood’s grain a bit, helping the sealer bond to the plywood sheet.
The final step in preparation is to lightly sand the plywood with medium-grit sandpaper (60-100 grit). This will help smooth the sheet and remove any rough spots that could “gunk” up the painting/sealing.
Step 2: Apply the Sealer
If you’re using a brushed-on sealing method, paint it on with a brush or roller, just as you would normally. Make sure you get adequate coverage and let the first coat dry before applying the second. Some materials and waterproofing needs might lead you to add a third coat. While this might not be necessary, but it can never hurt!
If you’re using a spray method, carefully read the manufacturer’s instructions. Improper use of spray-on sealer results in one of two things:
- not using enough sealer to cover the surface adequately, and
- having a spotty concentration, with lots of sealer in some places and very little in others.
The reason these risks exist is that spray-on sealers are harder to see and control than brushed-on sealers. So, when you spray the sealer, be very intentional with where you are pointing the can!
Step 3: Let it Dry, and Keep an Eye
Give the plywood at least a few hours to dry, up to 24 hours if you have the time to spare. Then, once it’s completely dry, you’re free to install it! After plywood is installed, add a bit of sealer over the screws/nails to ensure it stays watertight!
Finally, although your plywood sheets are now waterproofed, you should still check them out every few months to make sure water isn’t wreaking havoc without your knowledge. Water damage will be easy to spot, so this shouldn’t feel like a comprehensive inspection- just an “eyeball test” to ensure all is well!
How to Seal Plywood Edges
We’ve gone over how to seal the surface of a plywood sheet, but what about the edges? Should they be waterproofed, and if so, how do you seal plywood edges? Because plywood is made of several layers, the edges are particularly vulnerable to water.
When water is introduced to plywood edges, it can split much faster than it would on the sheet’s surface. So, it’s very important to waterproof the edges- perhaps more important than surfaces!
Sealing plywood edges is relatively simple; apply several coats of varnish, allowing each coat to dry for about an hour. The edges will soak up the varnish quickly, so you need more coats to make sure it dries to form that protective layer.
Which Plywood to Choose For Outdoor Use?
It would be a mistake to assume that all plywood is the same and equally vulnerable to water damage. The reality is that plywood is a diverse category of wood products, and each type of plywood is better for a specific application. Some plywood is pre-treated for waterproofing before you buy it, so you don’t even have to worry about sealing it yourself!
Two types of plywood are the best for outdoor use: CDX Plywood and ABX or “marine-grade” plywood. CDX is made of rough, lower-quality wood that is heavily treated for water resistance and is ideal for things like roof sheathing.
Marine-grade plywood is much smoother and higher quality and has the best waterproofing of any plywood. As the name implies, marine-grade plywood is designed for decks, boardwalks, and other waterfront construction.
Types of Waterproof Plywood
Plywood is graded on an A-D system; A is the highest and D is the lowest. The first letter represents the front face, and the second letter represents the back face and is usually more coarse.
The third letter is less important but signifies the type of glue used to bond the layers. These letters represent the appearance of the wood, not the strength it has. So, ABX looks much nicer than CDX, but they both have similar strength and water resistance. Let’s take a look at the different types of waterproof plywood and their recommended uses!
ABX, also known as marine-grade plywood, is a smooth, strong plywood. It’s intended for marine use, but it can also make great exterior plywood anywhere. It is specially treated to be resistant to salt and mildew along with water, and has the smoothest, most appealing surface of any plywood, hence the “AB” grade.
ACX is waterproof plywood that has much in common with marine-grade sheets but doesn’t have the marine-specific treatment. It is waterproof, tough, and perfect for use in decks, exterior walls, and renovations.
This type of plywood is pretty much the same as ACX, but the back face of the sheet is of the rougher, knottier variety. It can be a cheaper option because only the front face is the pricier “A” graded veneer. It’s great to use in exterior construction or as a subfloor in a home.
The letters in this plywood stand for “Boiling Water Proof”- and it’s exactly what it sounds like. This is extremely durable plywood and will keep its integrity even in the face of boiling water! It’s ideal to use when you are constructing a kitchen or bathroom where hot water and steam will be ever-present.
MR Grade Plywood
The “MR” stands for “moisture resistant” and is good indoor-use plywood. It isn’t completely waterproof- it resists moisture and water droplets but will falter against standing water. In humid climates, MR plywood is ideal for interiors because it won’t warp from humidity.
BWR Grade Plywood
“Boiling Water Resistant” plywood is like the slightly weaker cousin of BWR plywood. It will hold up well against steam and boiling water, although it is not technically watertight. It’s best used in places where water and steam might be present but not all the time- cabinets, sinks, etc. are all great places to use BWR grade plywood.
Finding a way to seal and waterproof plywood is very important to the longevity of your home, deck, or shed. It’s a relatively simple process, and with a bit of care, anyone can make plywood completely waterproof with a few easy steps! And, if you choose your plywood correctly, you may never even need to waterproof it yourself because several types of plywood are waterproof or water-resistant.