Exterior Vs. Interior Plywood
Plywood is one of the most versatile of all building materials. Exterior plywood is used as a sheathing material to frame a home under construction to give it shape and structure. Interior plywood can transform a blank kitchen wall into a wall of cabinets. Both types are made from soft or hardwood veneers which are glued together under heat and pressure. The glue, types of wood, number of veneer plies and quality of the panels determine whether the plywood is suitable for exterior or interior applications. Does this Spark an idea?
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Exterior Plywood
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Exterior plywood, as the name implies, is constructed for outdoor use, and specifically designed to endure extreme weather conditions and moisture. Exterior plywood (also called WPB, for weather and boil proof) is typically made of five to seven plies of soft or hardwood veneers. The lumber used to make exterior plywood is pre-treated and waterproofed through a chemical soaking process. In addition to WPB, marine and structural exterior plywood is also made. Marine plywood is the most moisture-resistant of all types of plywood and both sides of the panels are free of imperfections. Structural plywood is the exact opposite. Both sides display knots and the surface is uneven. A common application for structural plywood is to build shuttering box forms to pour and mix concrete.
Interior Plywood
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Interior plywood is produced to use in non-structural applications in which a high-quality finish is desired. Unlike exterior plywood, both the facing and back panels are often of top quality and the highest grades. Aesthetics are more important for interior plywood because it is often used as the surface and structural material for indoor items such as wall sheathing, cabinetry and furniture. As a result, moisture resistance is not as important, so the adhesives used to bond plies for interior plywood are not moisture resistant. Interior plywood is also lighter than exterior plywood.
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Plywood Grades
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Plywood panels are graded based on the quality and appearance of the front and back panels. Grades range from A to D, and panels are given two letter grades. The first letter grade refers to the facing side, and the second letter grades the back side. The closer the grade is to “A” the better the appearance and quality of the panel. Most interior plywood used for furniture building and wall sheathing is often A or B-grade with minimal imperfections, knots and blemishes. Another common grade is CDX, which is the lowest grade possible and often assigned to structural plywood.
Similarities and Differences
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The type of glue used to bond the plies is a distinguishing characteristic between interior and exterior plywood. Exterior plies are glued with A Bond and B Bond glues, which are made from phenolic resins that do not deteriorate under extreme weather conditions. Interior plywood is glued with C&D Bond, which is a urea formaldehyde resin that does not stand up to moisture and extreme weather. You can use exterior plywood for indoor applications, but not the other way around. For example, exterior plywood is used as a subfloor material in humid climates and for basements because of its moisture-resistant properties. Interior plywood would be structurally inferior as a subfloor material and have a tendency to warp and buckle.
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References
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