Plywood is a common sight in a DIY shop or builder’s merchants, available in sheets of different widths and finishes.  But there’s always one big question about the product that follows the obvious questions about what it can be used for and that’s is plywood waterproof?  Let’s take a look at the answer.

What is plywood made of?

Before we dive into the question of if plywood is waterproof, let’s take a moment to get to know the product better.  While it might be a common sight, not everyone knows how it is made and how this impacts what you can do with it.

Plywood, sometimes known just as ‘ply’ is an engineered sheet timber product.  It has tons of uses in construction and is made from at least three thin layers of wood veneer, known as ‘plies’ that are glued together.  They form a thicker, flat sheet and the construction makes them quite resistant to warping and cracking.

There are four main types of plywood available in different thicknesses:

• Structural plywood - this is where high strength is needed such as beams, bracing panels and formwork
• External plywood - this is used on the outside of buildings for either decorative or aesthetic purposes but not structural ones
• Internal plywood - used for non-structural purposes inside a building including ceilings and wall panels
• Marine plywood - this is used in shipbuilding because it is the waterproof option and is also used where there is a lot of moisture such in roofing or in bathrooms

What woods are used in plywood?

Different plywoods are made with different types of wood.  Softwoods such as Douglas fir, redwood and pine are used.  So too are hardwoods such as ash, maple and mahogany.  Sometimes, a mixture of hardwood and softwood is used.

Each plywood sheet has a core with the face and the back on either side.  If there are more than three layers, these will be added between the core and the outer layers.  There’s usually an odd number of layers so the sheet is balanced.

The wood is orientated with the wood grain perpendicular to the next layer, rotated 90 degrees.  Known as cross graining, this makes plywood different from things like laminated veneer lumber.

Can plywood be used outside?

From the names, you can see that generally, three of the types of plywood would be suitable to use outside - structural, external and marine plywood.  The problem comes when not everyone uses these designations on their products.  So how do you know if it is suitable for outdoor use?

There’s a couple of ways using the different European Standards that apply to plywood and how this defined their use.  The first one is EN 314-2: 1993.  This refers to the bonding quality and requirements of the wood.  It defines three classes:

• Class I - dry interior use only
• Class II - humid areas and occasionally wetting
• Class III - unprotected exterior use or frequently wet

From that, you can see that most of the time, Class III would be the one you want to use when creating something outside that could be frequently wet.  Occasionally, Class II might work if the area is only partially exposed.

The other standard used is EN 636: 2003 which is the specifications of the plywood.  Again this uses a similar three-class system to decide what the plywood is best used for:

• Class I - use in dry areas only
• Class II - use in humid areas where it might be wet occasionally
• Class II - use in wet areas including outdoors without any protection

How to waterproof plywood

While some types of plywood are waterproof as part of their construction, this isn’t the only way to get waterproof plywood for a project.  The other solution is to learn how to waterproof plywood and either add or reinforce that waterproof layer.

Step 1: Get the right waterproofing

Deck waterproofing is one product that can be used to waterproof plywood but it doesn’t require regular recoating.  A more permanent solution for somewhere harder to reach is a waterproofing material with an oil base.

Step 2: Get the plywood on a level surface

Grab a workbench, sawhorse or even an old pasting table to create a level surface to waterproof the plywood.  Try to do this outside as most waterproofing products will give off some smell and this might be unpleasant indoors.

Step 3: Waterproof one side of the plywood

Whatever product you are using to waterproof the plywood will have instructions on how to use to it and you should follow these.  If the plywood has a decorative finish on one side, wipe it down with a damp cloth and lightly sand with 100-200 grit sandpaper first to smooth the surface.  Wetting the plywood raises the grain a little and helps the waterproofing layer to be smooth.

Step 4: Repeat process on other side and edges

Repeat the waterproofing process on the other side of the plywood then finish any edges to make sure they will be waterproof too.

What happens if plywood isn’t waterproofed?

If you use types of plywood that are already treated for outdoor use, then they should be fine if they get wet consistently or to a high level.  But if you use interior plywood without adding a waterproof coating, then the water will cause the layers of the plywood to swell and break apart.

When wood gets wet, it also allows rot and mould to grow which can weaken the wood and cause it to be discoloured and smell bad.  Plus mould releases spores which can be harmful to health.

Getting the right plywood

Plywood comes in a variety of types and classes so getting the right one for the job is an important step.  Even waterproof plywoods can be easily treated to add to their protection if you plan to use them in excessively damp conditions.  Then the project you create will be solid and long lasting with little extra maintenance.

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