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Can Plywood Get Wet?


Plywood is an engineered or manufactured type of wood used for everything from walls to furniture.

From its long-term use in an outdoor project to the risk of it getting wet during the trip home from the lumberyard, a common question people often have is whether or not plywood can get wet, and that’s what we’ll be discussing in this article.

Plywood can get wet, and short, isolated periods of moisture should not cause any long-term damage. Standard plywood is not waterproof, though, and the use of marine plywood or a waterproofing sealant is recommended to prevent rot in especially wet conditions, for example, from outdoor use.

Read on to find out more about what happens when plywood gets wet, what you can do to both dry out and flatten warped plywood, and three ways you can waterproof your plywood for outdoor use.

Is Plywood Waterproof?

Plywood is not waterproof as standard.

If you’re planning on using plywood outdoors where it’s likely to encounter moisture, either from the odd bit of rain or where it’ll be left sitting in standing water, there are a few different choices available:

  • Applying a protective layer in the form of paint, epoxy sealant, or oil to protect the wood from the elements.
  • Alternatively, you could use an exterior grade sheet, for example, marine plywood or pressure-treated plywood, for protection in particularly wet conditions (although we’d still recommend applying a sealant for extra moisture resistance).

If you opt to forego sealing your plywood or choose not to use exterior grade plywood, it’s likely to rot and decay when exposed to moisture for extended durations.

The most common form of deterioration in plywood exposed to water is delamination, where the glue between the various layers fails, causing portions of the plywood sheet to peel away.

Another thing to watch out for with plywood is the glue used between its plies or layers, which can be either interior or exterior grade glue.

A common misconception is that if plywood uses exterior grade glue, the wood is waterproof and suitable for outdoor use, but this isn’t always the case.

For example, various types of birch plywood and baltic birch plywood sheets use exterior grade glue, and these aren’t waterproof as standard.

Without some form of sealing, the wood layers will still absorb water as with regular plywood, leading to rot and delamination (in other words, when the layers start to separate).


Can Plywood Be Used Outdoors?

You can use plywood outdoors, but you’ll need to match the extent of the treatment you apply or the specific type of plywood you opt to use to the conditions that it’s likely to encounter.

Plywood That Will Be Exposed to Moisture

Will the plywood be subjected to the odd bit of rain and standing water, or will it be fully submerged in water, for example?

In other words, how wet is the plywood likely to get?

The exact conditions will dictate whether a finish such as painting or epoxy sealing will be appropriate or whether you’ll need to use marine or pressure-treated plywood to ensure it can withstand the elements.

For less extreme conditions, such as occasional exposure to rain that won’t have the chance to puddle or pool on the plywood, we’d recommend waterproofing the plywood with paint, epoxy sealant, or oils to prevent water-related damage.

You probably don’t need to go to the extreme of using marine or pressure-treated timber for plywood exposed to occasional rainfall, as in many cases, the sealant layer should be sufficient to prevent moisture ingress and damage to the plywood. You will likely need to reapply the sealant layer every few years or so, however.

For the wettest applications, marine or pressure-treated plywood is probably the best option. To provide maximum protection, we’d recommend applying an additional layer of sealant, whether in the form of paint, epoxy sealant, or oil that blocks the pores in the plywood and prevents moisture ingress.

Plywood That Will Be Exposed to Sunlight

Exposure to sunlight, and the subsequent damage this can lead to, is another important factor that you need to consider when using plywood outside.

In his article, Paint, Wood and Weather, expert Colin Mitchell-Rose explains that untreated wood, including plywood, will gradually deteriorate when exposed to sunlight for long periods.

At the risk of getting overly technical, this is caused by cellulose (one of the primary components in the various layers that make up a plywood sheet) undergoing a process known as auto-oxidation due to the UV radiation that comes from sunlight.

The typical result of this process is the graying of the plywood’s surface, with the color fading due to surface bleaching.

Thankfully, in most cases, the damage from sunlight is purely cosmetic, as the fading only occurs in the layers close to the surface, meaning that it shouldn’t affect the strength or structural integrity of the plywood.

One of the easiest ways to prevent sunlight-related damage in plywood is by applying a sealant layer to the surface.


What Happens if Plywood Gets Wet?

Plywood is renowned for its dimensional stability, and this means that it is much less susceptible to shrinking, swelling, or warping versus other types of timber.

The main source of plywood’s dimensional stability is its alternating layers or plies, which are glued together, and this also results in alternating grain patterns throughout each sheet.

You can see the alternating grain patterns in the image below:

an image showing the various plies or layers in plywood

©bumi100/123RF.COM

By contrast, non-engineered, conventional types of lumber typically feature a grain that runs in one direction only throughout their entire depth, and this is less dimensional stable than plywood as a result.

Let’s look at a piece of 2×4 that is left sitting on wet ground as an example.

Provided it isn’t raining, the side facing upwards will inevitably dry faster than the side left in contact with the wet ground.

As the top dries, it shrinks faster than the bottom face, and this process can cause stresses throughout the wood leading to cupping or bowing.

Plywood can resist these stresses better due to the movement across the various alternating grain patterns, effectively cancelling each other out.

Because of this, plywood is good at coping with low amounts of moisture, and it takes a lot more moisture and internal stresses to cause warping versus conventional lumber.

This only occurs up to a certain point, though, assuming that you’re using conventional plywood that hasn’t been sealed versus a pressure-treated or marine plywood.

High levels of exposure to moisture in unsealed plywood, such as leaving sheets on wet ground for long periods, can be a massive issue leading to cupping and bowing problems in the short term and rot and delamination in the longer term.

An example of delaminated plywood is shown below:

an example image showing what happen to plywood when it gets wet and rots

©michaelviphoto/123RF.COM

It’s also important to remember that wood is biodegradable, and plywood is no exception.

Above around a 20% moisture level, plywood can experience damage from various types of mould, fungi, and even certain species of beetle that can significantly damage the wood.


How Long Will Untreated Plywood Last Outside?

We’d estimate that untreated plywood would last anywhere from between two to five years outside.

The exact length of time will be influenced by a variety of factors, including:

  • The thickness of the plywood sheet, with thicker sheets likely lasting longer.
  • The condition of the plywood prior to being left outside, with any areas of damage caused during initial transportation or storage potentially accelerating deterioration.
  • The presence of any defects like knots or voids that could assist with moisture ingress and further speed up deterioration.
  • The amount of ultraviolet rays from direct sunlight that the plywood is exposed to, with a quicker fading or ‘graying’ of the plywood’s surface in more extreme conditions.
  • The plywood’s exposure to moisture, ranging from occasional contact with rain to sitting in standing water, with more exposure leading to faster deterioration, especially in untreated plywood.

In terms of what kinds of damage will occur and when, the first signs of deterioration would probably be the fading or ‘graying’ of the plywood caused by exposure to ultraviolet rays in sunlight.

Warping would then occur in unsupported timber sections over time, followed by splitting and delamination that could cause significant reductions in the plywood’s strength.


How Do You Treat Wet Plywood?

We’ll discuss how to treat or waterproof your plywood in the next section, but it’s worth covering the steps you need to take beforehand if you want to work with plywood that’s already wet.

There are two steps that we recommend:

  1. First, you need to let the plywood dry out.
  2. Second, you need to flatten the plywood if it is warped.

Let’s look at each step in more detail:

How to Dry Plywood

If your sheets are both warped and wet, it’s probably best to sort the moisture issue first.

A common question is how long plywood takes to dry, and depending on how wet your plywood is to start with, it can take anywhere from a few days to weeks to dry out.

Here’s how we typically dry out wet plywood sheets:

If you’re working with single sheets, we’d recommend placing each sheet on at least three supports (ensure these are equally sized and level) on the floor.

Raising your plywood off the ground in this way will allow air to circulate under the sheet, ensuring that both the top and bottom faces have a good chance of drying out.

If you want to dry out more than one sheet, you can place another three supports on top of the first sheet (make sure these line up with the supports on the floor propping up the first sheet), and then go ahead and stack your second sheet on top.

It’s possible to keep stacking more sheets in this manner, but be careful and make sure there’s no chance of the sheets toppling over.

It’s usually a good idea to move the position of the supports every so often, as this allows the area that was in contact with the sheet also to dry out.

We’d recommend running a cloth or sponge over the sheets before stacking to remove most of the moisture, and if you’re able to, placing the sheets somewhere with a heat source or dehumidifier will speed up the drying process.

Leave your sheets stacked like this and check them periodically to see how the drying process is going. Remember, they might appear dry on the surface, but they could still be wet internally, so allow extra time for the sheets to dry fully.

How to Flatten Warped Plywood

Now that your plywood sheet is fully dry, the next step is to flatten the sheet and remove any warping.

Here’s what we recommend depending on the degree of warping:

For Minor Warping

When it comes to flattening out minor warping, our go-to method is sandwiching the bowed plywood between two flat sheets of wood (this could be plywood, MDF, or whatever else you can find).

You can also sandwich the board between flat ground and a second board if you don’t have enough material to hand. In this case, we’d recommend placing a polythene sheet between the plywood and the ground to prevent any moisture ingress if the ground is wet.

Regardless of the option you choose, make sure your plywood sheet is placed bow side up, as shown below.

an example image showing how to flatten warped plywood

You might also need to add some weight to the top sheet to provide enough downward pressure to flatten the warped plywood.

We usually leave our plywood in this ‘sandwiched’ state for a couple of days, and we come back every so often to check on progress.

For More Severe Warping

For plywood with more severe warping, we adopt a slightly different process.

The first step is to place your plywood sheet on two supports, with the warp going upwards as shown here:

an example image showing how to flatten warped plywood

Next, to account for the difference in moisture between the two faces of the plywood, spray a light mist of water across the concave face of the sheet (the bottom face in the example above).

You’ll then need to apply weight to the crest of the warping (the point where the warping is at its worst), which is close to the center of the plywood sheet in the example above.

The weight shouldn’t be so heavy that you risk breaking the plywood, but it needs to be heavy enough to start to flatten the sheet out noticeably.

From nipped fingers to dropping weights on your feet, there are plenty of risks here, so be sure to pay attention to your safety.

When working with thicker sheets or more severe warping, the weight you’re using will need to be increased to make a difference.

We’d recommend checking on your plywood every hour or so to gauge progress. You’ll know that it has been successful if the sheet no longer springs upwards when you remove the weight.


How to Make Plywood Waterproof

As discussed, untreated plywood will rot and deteriorate if left outside for extended durations, and this is the case for all types of plywood, from the cheapest varieties to the most expensive baltic birch.

So, whatever you’re using plywood for outdoors, from panelling to outdoor furniture, you’ll need to take a few steps to waterproof it before use to prevent any water-related damage.

It’s also worth bearing in mind that different levels of finishing may be required before waterproofing your plywood, and this is due to the fact that there are multiple ‘grades’ available.

Lower-grade, cheaper plywoods, for example, might have more surface-level defects like knots and voids, which need to be filled and sanded before applying a sealant or waterproofing coat.

Once finished to an adequate standard, there are three main options available for waterproofing plywood:

Painting

Painting is one of the most common ways of waterproofing plywood, and it’s also one of the cheapest.

Water-based acrylic latex paints or epoxy paints are the most common.

Although not 100% essential, we’d recommend using a primer first to seal the wood, as this provides a better opportunity for a high-quality finish.

Using a primer can also reduce the amount of overcoat needed as the plywood tends to absorb less of the primer than the overcoat. It can provide a better bond for the overcoat versus painting directly onto the plywood, too.

Some modern paints consist of a combined primer and overcoat, which saves time and effort applying a separate primer layer. These are usually more expensive, so you probably won’t save as much as you’d think versus buying separate paints.

Epoxy Sealing

Another option for keeping plywood dry and in good condition outside is by applying a layer of epoxy resin that will create an impenetrable barrier between the wood and the elements.

Unlike epoxy paint which applies a color, epoxy sealing products are typically clear, allowing the plywood to keep its natural appearance.

An added advantage of applying epoxy resin is that it drastically increases the hardness of the wood, improving overall durability.

Applying Oils

In addition to providing a certain level of moisture resistance, some oils, for example, linseed oil, can change the appearance of plywood through light staining.

However, some oils like tung oil dry clear, which will leave the plywood’s original appearance unaffected.

These oils work by penetrating the plywood’s surface to provide a layer of protection from the elements.

It’s important to understand that multiple layers of oil may be required. Exposure to the elements can cause the oil to dry out, meaning it will need to be reapplied periodically to continue to provide optimal waterproofing.


Final Remarks

Standard plywood is not waterproof, and it’ll need a layer of protection in the form of paint or an epoxy or oil sealant to prevent moisture-related damage when used outdoors.

We’d recommend using marine or pressure-treated plywood where high moisture levels are possible, for example, in particularly humid conditions or applications where standing water is likely. In these cases, we’d still recommend applying a sealant for additional waterproofing.

It’s important to remember that untreated plywood can typically last for between two to five years outside, and this is due to its dimensional stability that resists a lot of the shrinking, swelling and warping common in natural timbers.

Because of this, you don’t need to worry about isolated or one-off situations where your plywood gets wet briefly, for example, when transporting it home from the lumberyard or home improvement store. As we’ve shown in this article, provided you deal with it as quickly as possible and let it dry out as a priority, there shouldn’t be any lasting issues.

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