©2008, 2014 by Raymond Alexander Kukkee
Plywood with Surface Defect that is NOT Marine Grade Quality
As a DIYer, do you know the difference between Marine Grade Plywood and the more common Pressure Treated Plywood? Plywood is plywood, right? Wrong. The difference can sink or float your boat.
“Wood is good” unless it fails to serve the purpose for which it is intended. To understand the differences between grades of plywood, and the meaning of the various designations, is to choose wisely and appropriately. The saying ” Be fooled at your peril” applies to many things, but when considering the construction and end uses of various types of plywood, if you are fooled into using the wrong grade, the results can be unnecessarily expensive, or even disastrous.
Pressure-treated plywood, often called “Wolmanized” or P.T. plywood, is not ” Marine grade” plywood, and those designations do not make the two products arbitrarily interchangeable.
Pressure treated plywood is common plywood that has been subjected to pressure treatment with chemicals to prevent the wood from decaying, or rotting. To some degree, it also discourages insect damage because of the chemicals involved . Pressure treated plywood, however, is not suitable for marine use. The treatment of plywood with copper and arsenic compounds under pressure simply does not make the plywood waterproof. Worse yet, continuous exposure to water will leach the preservative chemicals from the pressure-treated wood.
It is worth repeating, pressure treated plywood is ordinary, interior-grade plywood that has been chemically-treated, and it is often made with softer woods to enable the penetration of the wood treating chemicals, with no special care effected to eliminate all gaps or voids.
"Ordinary Plywood" and Exterior Grades vs. Marine Grade G1S plywood, (good-one-side) is plywood with one side graded “Select” to show no defects or gaps on the surface only, and is an aesthetic consideration.
Exterior grade plywood is made with water-resistant glue, but the exterior shell is the only layer that is made void-free. There may be gaps, voids and the resulting points of weakness in the interior layers. When you cut a sheet of exterior grade plywood, you may expose a gap on the cut surface.
Marine grade plywood, on the other hand, is a different creature. Marine grade plywood is assembled gap and void-free in all layers, and laminated under pressure with special, water-proof glue that holds the various layers together.
Why is Marine Grade Plywood Better?
When plywood is immersed, or soaked in water, the water has absolutely no effect on the glue or the strength of the lamination of marine grade plywood. Marine grade plywood will not easily delaminate, bubble, buckle, or warp. Upon cutting marine grade plywood, no voids or gaps will be discovered on the cut edges. It is also usually constructed of harder woods such as Douglas Fir, or Western Larch.
Marine grade is a superior grade of plywood, and a substantially better product. Do choose carefully when selecting plywood for marine use. Although it is more expensive, marine-grade plywood, when finished appropriately, will outlast pressure-treated plywood by far. The ordinary glues used in ordinary plywood , pressure-treated or not, will eventually fail with continuous exposure to water and moisture.
Product Failure, Economics and Safety When the transom on your boat fails in the middle of the lake and the motor falls off, the wisdom of having saved fifty dollars by buying cheaper pressure-treated plywood instead of marine grade plywood will come to question rather quickly.
In that critical application, and other important structural applications, let us suggest that “the RIGHT wood IS good”, —and marine grade plywood is best.
Now you know the difference between marine grade and pressure treated plywood.
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Is that Incoming I hear?
tags #marine grade plywood #pressure-treated plywood #differences #DIY #skills
photo credit: mike (morguefile)
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Is marine-grade plywood as safe as pressure-treated plywood from insects and rot?
Hi Charlie,
Good question!
The higher-quality construction of marine-grade plywood usually includes the use of higher-grade woods like Western fir, Larch, and Tamarack which are naturally more resistant to insects and decay than ordinary wood. Marine grades are also gap-free.
Pressure-treated construction plywood is usually constructed of Spruce, poplar, or other woods not naturally-resistant to insects or rot, and gaps may occur in the layers.
Hands down, Marine grade plywood is usually a stronger, less problematic and a better choice–but also more expensive.
To complicate the issue, much plywood is now outsourced from Asian and other offshore markets. For any critical application, verification of the material and performance specifications of the product you choose is essential. ~R
What about layering fiberglass and plywood? If the plywood is completely encapsulated in fiberglass/epoxy would “regular” plywood be acceptable? I assume treated plywood will also “gas-off” and would not be desirable.
Bad idea. The lamination of regular plywood is not water resistant, once it is coated with fibreglass/epoxy you would not be able to monitor it’s condition. Treated plywood will last longer, under the same conditions, but is still out of sight. For all the extra trouble, marine grade plywood remains the best option. For the record, any and all plywood will ‘off-gas’