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All about plywood

As remodeling contractors, we encounter many homes that were built prior to the advent of plywood. Accordingly, the floor boards, roof sheathing and wall sheathing consist of an assembly of dimensional lumber (boards) assembled to satisfy the desired configuration.

In some cases, as with floor boards (subflooring), the material might have a tongue and groove to allow it to be joined for added integrity. What's more, it is typically thicker than the material used on walls and roofs. The material used in these locations doesn't need to be as thick as flooring, and is frequently run diagonally to improve the strength of a wall or roof.

In addition to this type of lumber being used in the rough framing of a building, better grades of the same material were used for interior and exterior surfaces and trim. Lap siding, beveled siding and board-and-batten siding are just a few of the many popular choices used to finish the exterior of a building. This material is usually 1 inch thick and can range from 4 to 12 inches in width. There are almost as many species as there are styles and patterns with cedar, pine, redwood and fir the most popular.

While milled dimensional lumber still is used for siding, its use has been greatly diminished by plywood siding or other types of sheet goods. Moreover, the latter have universally replaced milled lumber for roof and wall sheathing. There are various reasons for this metamorphosis which began some 40-plus years ago. The primary one is cost. Additionally, today there are environmental concerns that make plywood and other sheet goods, such as oriented strand board, popular choices.

What is plywood? Simply stated, it consists of an odd number of wood veneers glued together in cross-laminated layers.

Plywood can be manufactured from more than 70 species of wood. These species are divided, on the basis of bending strength and stiffness, into five groups. Strongest species are in group 1. The group number that appears in the trademark on some American Plywood Association (APA) trademarked panels, primarily sanded grades, is based on the species of face and back veneers.

The APA trademark appears on each sheet of plywood or other panel products, such as composites and oriented strand board. In addition to the species group, the APA trademark defines the veneer grade, the moisture exposure rating, panel grade, thickness, span rating, product standard number and mill number.

Veneer grade defines veneer appearance in terms of natural unrepaired growth characteristics and the number and size of repairs allowable during manufacture. Of the six veneer grades, the highest quality are N and A. The minimum grade of veneer permitted in exterior plywood is C-grade. D-grade veneer is used only for backs and inner plies of panels intended for interior use or applications protected from exposure to permanent or severe moisture.

Panels with B-grade or better veneer faces are sanded smooth in manufacture to fulfill the requirements of their intended application such as cabinets, shelving and furniture. APA-rated sheathing panels are unsanded, since a smooth surface is not required for their use. Other APA panels underlayment, rated Sturd-I-Floor, C-D Plugged, and C-C Plugged require only touch-sanding for sizing to make the panel thickness more uniform.

APA-trademarked panels may be produced in four exposure durability classifications; Exterior, Exposure 1, Exposure 2 and Interior. All-veneer APA-rated sheathing, Exposure 1, commonly called "CDX" in the trade, is frequently mistaken for an exterior panel and erroneously used in applications for which it does not possess the required resistance to weather. Only exterior panels should be used for permanent exposure to weather.

APA-rated sidings include a wide variety of surface textures and patterns, most of them developed for optimum performance with stain finishes. Actual dimensions of groove spacing, width and depth may vary with the manufacturer. Where the characteristics of a particular wood species are desired, the product must be specified by grade and species preference.