Marine plywood is a great sheet material that can be easily cut using both hand saws and basic power tools. Marine plywood is often used to build boat interiors and to replace flooring during boat renovations. But why do we have to use plywood specifically made for the marine environment and are we wasting our money on expensive ‘marine’ plywood when outdoor grade ply may work just as well?

I’m currently working on an 18ft fibreglass fishing boat that has been totally gutted by the previous owner. The boat is currently a ghostly empty shell with nothing but a loose BMC 1.5 engine. With help, the engine has now been moved to the back of the boat in preparation for the rebuild of the boat’s interior. Although it’s a fibreglass boat, it will still require a plywood floor that will need to be reinforced with fibreglass matting. This is to protect it against the elements and to also form a waterproof structural bond to the hull of the boat. It would be a pretty awful to be in a capsized boat but imagine the floor conking you on the head as well if it wasn’t attached. With this in mind, I took to searching the internet for the solution.





What makes marine plywood so special?

Whilst planning the renovation of my fishing boat my first thought was; why would I need a marine grade plywood when it is going to be coated in resin+ and fibreglassed when it’s finished? Surely if the wood is not exposed then the material lying underneath doesn’t need to be water resistant?

The main issue with using a non-marine plywood is the eternal problem of time. Given sufficient time and use it may be that the protective fibreglass layer becomes damaged allowing water to permeate the surface and begin to rot the underlying plywood. If the plywood is required structurally then this becomes a messy problem indeed. Your floor will begin to sag and rot through. You lose the strength of the plywood backing over time meaning that the fibreglass is then vulnerable to cracks and structural damage. Unless a large amount of fibreglass matting has been used with a good amount of strength then your floor could potentially fail. In theory and like most things in life the more you spend the better your product will be and the longer it will last.

So why not skip the plywood and just fibreglass?

Fibreglass can easily provide enough strength to support the weight of a person without the use of ply, but the amount of matting needed to create a floor would cost a considerable amount more than simply fibreglassing over plywood.

Without the use of moulds, you will always need a backing or core material to laminate over. This is why marine plywood has been the boat builders choice for many years.

So what are we paying for?

Marine Plywood offers high resistance to water, fungus and damp. It is composed of durable face and core veneers and should be free of voids. Essentially you are paying for the time it will last when exposed to harsh environments. Some ply is guaranteed for 15 and 25 years depending on where and what you purchase. Marine ply uses a waterproof higher grade of glue in between the layers of veneer which also has less imperfections than standard plywood, and is marked with ‘BS 1088’ meaning:

“BS1088 is the British Standard specification for marine plywood that applies to plywood produced with untreated tropical hardwood veneers that have a set level of resistance to fungal attack. The plies are bonded with Weather Boil-Proof (WBP) glue.”

Although this stamp is said to be the seal of approval, there have been various arguments over the years regarding the quality variance. A reader submitted a photo of a piece of so-called ‘marine plywood’ that he recently purchased. It can be seen above the Illustration 1 caption. Don’t worry there are some ways to test the quality of your plywood (at your own risk of course!):

Cut some small samples and leave them in the dishwasher for a few cycles.

Simmer samples in hot water for a few days to see if they delaminate.

With these factors now in mind, it is possible to find cheaper alternatives and more importantly available alternatives. It’s all good and well only wanting to use marine plywood but if none of your local building supply store’s sell it, then you won’t get very far!

Exterior grade plywood

Exterior grade plywood is a good alternative to marine ply. Whilst exterior ply may not be pre-sanded and visually free of imperfections, it should still use 100% waterproof glue and should be suitable for permanent exposure to outdoor environments. Exterior ply may also have voids in between the layers that aren’t immediately visible. The only way of truly knowing is by peeling back the layers and inspecting. But all things considered, you shouldn’t go out buying (or building for that matter) without prior research. In the United Kingdom some building suppliers, for example Jewsons, have marine plywood listed on their website. Further investigation however, shows that the marine plywood is in fact not suitable for boat building (their words). Yeh that’s right a product labelled as marine plywood that is not suitable for boat building – like a kitchen knife not suitable for chopping vegetables or a TV that doesn’t show sports! You should always research or run your own tests on a few samples before burning your supplies budget on a bunch of a useless sheet materials.

To summarise you should always research or run your own tests on a few samples before burning your supplies budget on a bunch of a poor quality ply sheets. With boat building,you need to get as high a quality as you can afford but make sure its actual quality your buying and not just a name. Don’t think that plywood is poor quality because it comes from abroad, plywood is manufactured all over the world tropical woods being some of the best? Above is a picture of sundried eucalyptus plywood basking in the sun in full glory.

How Marine Plywood Is Manufactured

Photos Of Plywood Delamination And Rot

Photo A

Photo A shows the direct result of not sealing the end of a fibreglass boat deck where it meets the well. The end grain has been exposed meaning water has crept between the plywood and fibreglass over time. The end result is the entire floor is now spongy and most likely rotten. The floor will need to be cut out and replaced.

Photo B –

Photo B shows the direct result of water ingress on a plywood cabin side. The top layer of paint is still bonded incredibly well to the plywood but unfortunately the plywood itself has split apart. This may have been caused by a window that sits above this area. Water has got in over time through a poorly window bedded frame.

Photo C

Photo C is a perfect example of plywood delamination at it’s worst. The damage most like occurred where people have been grabbing onto the arch to get onto the boat. Once a crack or break in the substrate occurred water has then been allowed to permeate into the walls. The underlying floor has also been effected majorly.

Photo D

Photo D is a piece of plywood from the floor of a boat wreck. Note that even though the plywood has been snapped and left exposed to rain and wind it still holds its integrity. This was located underneath a faux teak floor meaning that it is most likely the original plywood used to build the floor of the boat. Perhaps this is why it still in better condition than it’s modern counterparts that surround it.

Photo E

Photo E shows us an example of plywood delaminating even while the boat sits out of the water. The above plywood was labelled as “marine”. This second world war torpedo boat was recently restored , the ends of the plywood were sealed but the water still permeated into the floor causing it to rot. The result is a floor that feels spongy to touch.

Photo F

In Photograph F above we can see that the underlying plywood has started to delaminate. The paint is completely failing to adhere to the plywood substrate and subsequently peeling off. Once water is allowed to permeate the plywood it begins to rot. This causes the paint to flake off in large chunks, along with layers of rotten ply.

Tony – Wooden Boat Builder from São Paulo, Brazil. “I asked for samples from 3 major manufacturers here in Brazil. My testing involved boiling for 3 hours – 4 times. Between boiling sessions, samples were frozen for one night. At the end, a microwave session until starting to smoke. All samples marked wbp survived the tests. The final choice was based on a higher number of plys /veneers per thickness. But the test of time will be much harder, and I don’t believe any plywood will survive it without constant verification and maintenance.”

Tertius – Wooden Boat Builder from Knysna, Western Cape I also switched to exterior grade after faults in BS1088 grade. But yes…boil it a couple off times and look carefully for voids and overlaps.

It’s easier being a boatbuilder to fix and make an informative decision. After all, how long do I want it to last! A friend of mine used exterior grade pine ply. He took it for pressurised Tanalith treatment. Any faults in the board made it explode.

Robert from Portland, Connecticut, US – You can use resin thinned down with acetone so that it becomes very watery seeping deep into the pores of the wood and sealing them. Then lightly sand and repeat until the wood grains are filled with the resin. Coating plywood like this will keep water from being able to enter. This is a trick used by manufactures to seal plywood that works quite well.