Birch plywood is aesthetically pleasing and generally viewed as good-quality plywood. It is an excellent choice for underlayment, indoor furniture, kitchen cabinets, etc.
But Is birch plywood moisture resistant? Not all birch plywood types are weather-resistant, but there are ways to make them suitable for outdoor use. But, before I explain that, you should know that there are two types of birch plywood on the market.
- Standard Birch
- Baltic Birch Plywood
Birch Plywood vs Baltic Birch Plywood
What’s the difference between standard birch plywood and its Baltic counterpart? Is either of these grades of plywood water-resistant and what types of plywood should you use outdoors?
Baltic birch plywood is made exclusively from birch wood grown in the Baltic regions of Russia and Finland. Due to its manufacturing method, most regard this as higher quality than standard birch plywood.
Baltic birch plywood is made of thinner plies than standard birch plywood, making it stiffer and more stable. This manufacturing method allows for more plies to be included per sheet.
Let’s put this into context: a traditional plywood sheet contains between 5 and 7 layers of plies per sheet, while Baltic birch plywood contains 13 layers. Here, the grain direction alternates, minimizing the likelihood of shrinkage and expansion.
The number of plies included in Baltic birch plywood means that it contains fewer voids than standard birch plywood. This density makes it better suited to screw holding, which is essential for woodworking. When installing a screw in material containing a large proportion of voids, the screw doesn’t find much material to grip, making for poor screw holding. Thus, the fewer voids in the material, the better it is for woodworking.
Plywood Strength
Due to the decreased number of voids in the material, Baltic birch plywood performs better than its standard counterpart in joinery applications. Standard plywood tends to break apart when cutting finger joints and dovetails, while Baltic birch plywood doesn’t. This makes for stronger and better-looking joinery which doesn’t delaminate.
Standard birch plywood often includes softwood in the center, rendering the plywood sheet softer than standard hardwood. Since Baltic birch plywood contains only the birch hardwood, the ply is generally harder and thus stronger than the standard alternative.
Thickness
Baltic birch plywood sheets have a uniform thickness of 1.5mm, with an outer veneer that’s also 1.5mm thick. Standard birch plywood, along with other plywood made of walnut, oak, and maple, typically has an outer veneer of 0.75mm.
Adhesive Used
Various types of glue are used in manufacturing standard birch plywood. Some of these are waterproof, while others aren’t. In contrast, Baltic birch plywood is always made with waterproof glue, making it ideally suited to damp areas.
Dimensional Stability
Since Baltic birch plywood contains fewer voids, it makes for a more uniform glue distribution than standard birch plywood. This uniformity offers greater dimensional stability since all layers of the sheets are uniformly bonded.
While its physical qualities are far superior to standard birch plywood, Baltic birch plywood is expensive. It is typically only available in 5’ x 5’ sizes, the preferred measurements for European cabinetry. Then, there’s the lack of availability. You typically won’t find Baltic birch plywood in large home improvement stores but in specialty woodworking stores or from hardwood dealers.
Is Baltic Birch Plywood Waterproof?
Baltic birch plywood isn’t waterproof. However, using the correct sealants can make this type of plywood waterproof. Even though the glue used in the manufacturing process is waterproof, the wood itself isn’t waterproof.
Is Birch Plywood Suitable for the Outdoors?
As mentioned, Baltic birch plywood isn’t waterproof as-is. If you use it outside without the proper sealants, the wood will absorb water, leading to rot and delamination. However, this type of plywood is better suited than its standard counterpart to handling low-level moisture. The many layers of plies present in Baltic birch plywood render it more dimensionally stable. This allows for more shrinkage and expansion as the wood gets wet and dries again. This is only valid up to a point, though. Suppose unsealed Baltic birch plywood is exposed to moisture for too long. In that case, it will cup and bow and eventually delaminate, rendering it structurally unstable.
How Do You Seal Birch Plywood Outdoors?
The finishing you apply to Baltic birch plywood depends on the grade and thickness of the material you’re using.
If you’re using lower-grade Baltic birch plywood, there will be some surface defects, like knots and voids. These must be filled and sanded before sealing the plywood.
After finishing the surface, there are three options to seal the plywood: painting, epoxy sealing, and applying oils.
1. Painting
Painting is the cheapest and most common method of waterproofing Baltic birch plywood. Typically, you would use water-based acrylic latex paints or epoxy paints. While not essential, it is best to use a primer to seal the wood before applying paint. This ensures a higher quality finish and reduces the overcoat needed to complete the job.
Plywood tends to absorb less of the primer than it would of the overcoat, providing a better bond to the overcoat.
Some paints combine the primer and overcoat into one product. This saves time and effort since you don’t have to apply two layers of paint, waiting for the layers to dry between applications. These are typically more expensive, so you won’t save much on cost when using these.
2. Epoxy Sealing
Applying a layer of epoxy resin to your Baltic birch plywood renders it weather resistant. These products are typically clear, thus maintaining the plywood’s natural appearance.
Epoxy resins are hard and thus increase the wood’s hardness. This further improves the material’s durability.
3. Applying Penetrating Oils
Penetrating oil improves the plywood’s moisture resistance and often acts as a light stain, slightly altering the wood’s appearance. Linseed oil is an example of an oil acting as a light stain. Tung oil, by contrast, is clear, leaving the wood’s appearance unaffected.
Oil works by penetrating the wood’s surface layers, thus providing protection from the weather. Note that you’ll need multiple layers of oil, allowing some time between layers for the oil to penetrate. Exposure to moisture and other elements will cause the oil to dry out and thus require regular reapplication for it to be effective.
What Plywood is Waterproof?
Plywood meant for outdoor use is typically made with waterproof glue and manufactured from Douglas fir. This type of plywood generally has visible knots and other blemishes, so it’s not very pretty. It’s meant to withstand the weather, not please the eye.
Suppose you’re working on a project where aesthetics are essential. In that case, there are types of plywood with fewer blemishes that are still waterproof. These tend to be more expensive, though.
ACX and CDX Plywood
Typically, type CDX and ACX plywood would be used for outdoor applications. Here, the X denotes that the material is meant for exterior use. The first two letters indicate that it’s construction grade. The first letter refers to the front of the plywood sheet, while the second refers to the back.
Premium grade plywood is marked with an A and is suitable for higher-end uses, such as cabinetry. This type of plywood tends to be more expensive and aesthetically pleasing. It is typically used in applications where appearances are important.
CDX plywood is the more affordable option. This type of plywood isn’t aesthetically pleasing but is weather resistant. Here, the layers can be made of veneers, light hardwood, or high-density hardwood.
Overlaid plywood is CDX or ACX plywood with a cleaner-looking overlay. This improves the aesthetic appeal while improving the wear resistance.
Marine Plywood: Best for Water Resistance
Marine plywood is the highest quality waterproof plywood. Denoted by AA, AB, and BB, it’s made from wood without knots, improving its durability. Knots in the surface provided areas where pockets of water can collect. Eliminating these minimizes the possibility of water collecting in the wood and thus affecting the wood’s durability.
Due to its structural integrity, marine plywood is used in manufacturing boats and in other marine applications. Unlike other types of plywood, it’s not prone to cracking while being shaped. This type of plywood is typically easier to sand and cut than other types of plywood, thus leaving cleaner edges.
Pressure-treated plywood is another type of plywood well suited to outdoor use. Regular plywood is infused with chemicals, increasing its resistance to mold and mildew.
Dan Miller is a professional toolmaker with extensive experience in the metalworking and tooling industry.
An avid DIY enthusiast and passionate hobbyist woodworker, Dan shares his expert knowledge here.