Which plywood is most waterproof?

Your best choices are either ACX or marine grade plywood. Both are made with waterproof permanent phenolic (WBP) glue, but there are some important differences between ACX vs marine grade plywood.

clp-inc.com - ACX vs Marine Grade Plywood

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When you need plywood for your construction project, you may be confused about the best material to use. Plywood is available in many different types of wood, various thicknesses and in a wide range of quality and price points. For exterior projects or projects exposed to weather conditions, you need to use plywood that is suited for those conditions. Your best choices are either ACX or marine grade plywood. Both are made with waterproof permanent phenolic (WBP) glue, but there are some important differences between ACX vs marine grade plywood.

ACX Plywood

Plywood is graded using letters. The “A” in ACX means the front of the plywood is the highest grade. It is sanded smooth and paintable. It may have some neatly made manufacturer repairs, but you should find it to be free of major repairs and knots. The “C” in “ACX” refers to the back of the plywood. The “C” rating means the back of the plywood is a lower grade. It may have some defects on the back of it. The thickness of the back veneers can be a little thinner for cost-savings. It may also have tight knots sized up to 1-1/2 inches and knotholes to 1 inch. It may also have some splits and discoloration. In ACX plywood, each core layer can consist of several pieces of core veneers. Also, both end joints and edge joints are allowed. It can be made of hardwood of high density or mixed light hardwood. The layers are glued with waterproof glue. Often, the glue is the same high-quality waterproof glue as used in marine plywood. ACX plywood is intended for indoor or outdoor applications where resistance to weather and water is necessary.

Marine Grade Plywood

Marine plywood is the highest grade of plywood. Marine plywood is available in the following grades: A-A, A-B, and B-B. It is usually made from Western Larch or Douglas Fir wood. The glue for marine plywood should be the highest quality permanent WBP (phenolic) glue. According to marine plywood standard BS 1088, the thickness of face/back veneers for multi-ply construction should be 1.3 to 3.8 mm. For three-ply construction, the thickness should be about 0.5mm. The quality of the veneers in marine plywood is better than that used in exterior grade plywood. There should be no defects (except sound pin knots) in the face or the back of marine plywood. In the core layers, each core layer of long grain should consist of only one piece of core veneer. Also, edge joints are allowed but end joints are prohibited. There can be no voids or holes hidden on the inner layers of marine grade plywood. Accordingly, it is denser and stronger than exterior plywood. The edges tend to cut and sand cleaner than the standard plywood.

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Marine plywood is intended for use in boat building or other marine applications. Because it has no voids, its strength is consistent across the wood and is less prone to cracking when it is being formed into a curve. Marine plywood is also used in aircraft construction, and it is excellent for exterior structures and outdoor patio furniture. Some of the outdoor furniture projects that marine grade plywood is best for include garden furniture including benches, tables and chairs, and structures such as porches, decks, planters, pergolas and gazebos. If a structure will be used in a humid climate or near water, marine plywood is preferable because it will withstand the conditions better and longer than standard plywood. In short, marine plywood can be used for all the jobs which can be done by exterior plywood; however, marine plywood is much more expensive than standard plywood.

How Do I Choose Between the Two?

The answer to this question depends on your project. For building a boat, true marine grade plywood is probably going to give you the finish and water resistance you need. On the other hand, if you are building an exterior structure, water resistance is not at the same level of importance (unless your structure will be oceanside or a commercial building) as it is in boat building, and the appearance is not a consideration if the plywood will be covered by exterior siding. Local building codes may dictate one type of plywood over the other. Marine grade plywood weighs more. Weight may be a factor for your project, also. Other factors need to be considered, as they will affect how the plywood will behave under the conditions it will be subjected to. Factors include:

The species of face veneer and appearance

Species of veneer core

Core construction

Glue

Speak to a Wholesale Lumber Supplier for More Information

In some situations, you may be able to save money by using ACX plywood, but in other cases, you will need to use premium marine grade plywood. For help selecting ACX vs marine grade plywood for your project, talk to the experts at Curtis Lumber, a wholesale lumber supplier. They can work with you to make sure you get the right plywood product for your project.

clp-inc.com - ACX vs Marine Grade Plywood