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Does Marine Grade Plywood Warp and Rot? Is it Waterproof Plywood?

Warped Plywood

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Updated April 27, 2021.

Working on a boat build is a great and time-consuming endeavor. The last thing a boat builder wants is to use materials that will not be conducive to a wet environment. Moreover, one is likely asking the question, “Does marine grade plywood warp and rot?”.

Although marine plywood avoids warping and rot, it is not impervious to such detrimental effects. However, the marine-grade board will last considerably longer than standard exterior ply products. Marine plywood, using Exterior and Boil Proof glue, also known as WBP, is one of the most water-resistant fabricated wood materials. The wood is also typically pressure treated and fabricated from tropical hardwoods with a natural resistance to moisture and humid conditions1.

To best understand this specialty wood product’s abilities for use in the construction of seafaring vessels, it is advisable to understand what the differences are between marine-grade plywood and treated or other types of boards.

What is the difference between marine grade plywood and treated plywood?

Most plywood uses multiple layers of wood known as veneers in the fabrication. These are shaved layers of wood that are of a particular thickness and glued together. These veneers oriented with each grain’s layers at ninety degrees to the orientation of grains in the layer above or below 2 are fabricated for maximum strength.

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The performance of the plywood is dependent upon several factors. First, the quality of the veneers plays a major role in the overall quality of the finished product. Some types of tropical hardwoods, such as Teak or Mahogany, make excellent marine plywoods. Other common types of marine boards include fir, Sapele, meranti, and Okoume.

Not all types of wood used for marine plywood are equal in their ability to perform. For instance, Okoume is known to be able to rot. Moreover, Teak is extremely rot-resistant but is also extremely expensive.

Due to the variations in different woods, performance abilities, and costs, it is not uncommon to find a boat made with multiple types of marine plywood.

What to look for in a marine plywood

When purchasing marine plywood, there are several things to watch for in the quality of the wood. First, the board should be solid, with no layer delamination of any kind. Also, watch for any holes or loose knots; there should be none. If there are loose knots or holes, the quality is poor even in a single outer veneer. Remember that any hole or loose knot could allow water to seep in between the layers.

If fir is the plywood of choice, check for cracks or ‘checking’ as they are also known. It can mean the plywood has been outside, exposed to the elements for some time.  

If one uses fir plywood, it will require a layer of fiberglass and resin to prevent leaking.

A wooden boat sits half sunken in reeds at the waters edge.
Proper sealing of wooden boats is essential to the success of the boat.

Does one need to seal marine plywood?

Marine-grade plywood veneers are bound using waterproof glue. Nevertheless, that does not make the panel waterproof. The board still requires treatment and sealing.  

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Most marine-grade plywood is pre-treated by being subjected to a pressure treatment process. This process forces chemicals into the wood grains, which prevent the wood from rot and decay.

The pressure-treating process is essential to the prevention of wood warp and rot. However, the wood still requires sealing.

Sealing most marine-grade plywood with epoxy is essential. Moreover, the epoxy, applied in multiple coats, forms a fantastic seal. Although this can add significant weight to the boat, it is an essential process in the panels’ waterproofing to fabricate the boat. The sealing process is even more important for the board’s edges as this area can allow water to seep between veneers and cause delamination.

What is the best marine grade plywood?

Note from the author: If you’re looking for a source for plywood near London, England, check out our friends at Build4Less.co.uk (London) for all your plywood needs.

Although one can buy marine grade plywood from a hardware store like Home Depot, the quality, according to reviewers, is medium grade. However, the product states its construction using the wood of fir trees3.

Many of the reviewers have complained about the wood. However, a few of the seemingly more experienced reviewers commented that it was indeed medium-quality marine grade plywood. People make a common error with this type of marine-grade plywood that they still need to coat the plywood.

As mentioned previously, fir boards will crack and weather, so they must be coated with (ideally) fiberglass skin and resin epoxy. Even the best of woods will disintegrate over time unless properly treated and maintained.

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One of the most common types of marine plywood in North America is the Douglas-Fir marine plywood. It is a hefty wood best used for boats meant to be strong and rot-resistant. The downside to using fir is the ‘checking’ (cracking) issue when exposed to the elements. For best results, this wood should be used as a structural hull skin and then coated with fiberglass and epoxy. 

One of the best types of marine-grade plywood is Teak. However, it is also one of the most expensive. The general rule is that one gets what one pays for, so the saying goes. Mahogany is another popular choice as well, although more so for its decorative characteristics. However, it is also suitable for marine plywood.

Best Types of Marine Wood Board?

For the more discerning hobbyist, a good type of marine plywood used typically in making canoes and other small vessels is meranti. This wood originates in Southeast Asia and is relatively inexpensive. It can, however, be difficult to find in good quality, especially in the thinner board sizes.

When it is a flexible and beautiful wood one wants, one might want to choose Okoume. This type of wood from Africa is the most flexible of marine plywoods. However, it is not nearly as durable as most other types.  

However, it is so beautiful and easy to work with that it typically can be found in the construction of many small dinghies, canoes, and even kayaks.  

How does one waterproof marine plywood?

The type of wood that the marine plywood consists of determines the best way to waterproof the wood. For example, one might be using Douglas-Fir to build the main hull due to its lower cost than woods like teak.  

However, as was just discussed, fir tends to crack when exposed to the sun and weather. This type of wood is an excellent ‘builder grade’ marine plywood. Moreover, it requires being coated in a substance such as fiberglass epoxy to achieve a waterproof seal.

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Moreover, if exposed to water, fir plywood, even marine-grade, will break down.

More expensive and higher quality marine plywoods may not require something as drastic as fiberglass skin. However, sealing all marine woods is a necessity.  

For the higher quality marine plywoods, the sealing process typically involves multiple layers of epoxy applied to the wood to achieve waterproofing qualities.

More Than Boats For Marine Plywood?

Is marine plywood only for boat building?

Marine plywoods are employed for more applications than just boat manufacturing. One such application is building covered docks.  

Any time a dock requires some side or roof panels, using plywood, such as marine-grade plywood, is smart.

Marine-grade plywood, often used in wet conditions, not just on boats. Boathouses, covered docks, even in washrooms where steam and humidity are prevalent are all applications that have seen the use of marine plywoods4.

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Does one have to use marine-grade plywood?

The question frequently arises as to whether or not marine grade plywood is essentially necessary for boat building. Boat enthusiasts often ask this question when they see the price tag of marine plywood. Marine plywood is not inexpensive.

So, does one have to use marine-grade plywood? That depends on a few things.

First, the exact application of the board is a necessary piece of information in determining whether or not it is appropriate for a specific purpose.

For example, let us assume that Jim, a hobby boat builder, wants to use exterior plywood instead of marine grade plywood in his boat build. He chooses exterior plywood due to the cost.  

If Jim decides to coat the exterior plywood with epoxy simply, it may not be adequate to preserve the condition of the plywood for long. However, if treated like fir marine plywood, exterior plywood can be acceptable.

The common domestic North American type of marine-grade plywood used as a ‘go-to’ for many shipbuilders is Douglas-Fir Marine Plywood5. It is excellent marine plywood for use when the plywood has a coating of fiberglass instead of varnish. It can be cumbersome. This type of wood for boat fabrication has not changed much over the last five decades.

Sources

  1. Marine Plywood, Wikipedia, https://en.wikipedia.org/wiki/Plywood#Marine_plywood, Accessed June 18, 2020.
  2. Esterle, Paul, “Underway: The lowdown on marine plywood” LakeExpo, https://www.lakeexpo.com/boating/boat_projects/underway-the-lowdown-on-marine-plywood/article_f5d1bbfa-c4fd-11e0-9cbc-001cc4c03286.html, Accessed June 18, 2020.
  3. “3/4 in. X 4 ft. X 8 ft. AB Marine Grade Pressure-Treated Fir Plywood“ Home Depot, https://www.homedepot.com/p/3-4-in-x-4-ft-x-8-ft-AB-Marine-Grade-Pressure-Treated-Fir-Plywood-154459/203527614, Accessed June 18, 2020.
  4. “When Should You Use Marine Grade Plywood?” OHC, https://www.ohc.net/outdoor-living-today/, Accessed June 18, 2020.
  5. Harris, John C., “Marine Plywood” Wooden Boat, https://www.woodenboat.com/marine-plywood, Accessed June 18, 2020.

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