On Plywood

As remodeling contractors, many of the homes that we venture into 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 may have a tongue and groove to allow the material to be joint 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 and is frequently run diagonally to improve the shear 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 generally one inch thick and can range from four to twelve 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 is still used for siding, its use has been widely diminished by plywood siding or other types of sheet goods. Moreover, plywood and other sheet goods have universally replaced milled lumber for roof and wall sheathing. There are various reasons for this metamorphosis which began some forty plus years ago. The primary reason is cost. Additionally, today there are environmental concerns which make plywood and other sheet goods such as oriented strand board popular choices.

First, what is plywood? Simply stated, plywood consists of an odd number of wood veneers glued together in cross-laminated layers. The veneer is created by placing sections of a debarked tree on a “Paul Bunyun” sized lath which uses a blade to “peel” the veneer in continuous sheets to the point where a four to six inch core remains. The wood veneer is cut into sections, cross-laminated for strength, glued under pressure and trimmed to size. Whereas most sheet goods are four by eight feet, there are a myriad of sizes. Needless to say, the number of layers has a great deal to do with finished thickness which can range from 1/8 inch to 1 1/8 inches.

Plywood can be manufactured from more than seventy 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 next strongest in Group 2, and so on. 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.

APA performance standards deal exclusively with how a product must perform in a designated application rather than from what or how the product must be manufactured. 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 intended 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 as an Exterior panel and erroneously used in applications for which it does not possess the required resistance to weather. “CDX” should only be used for applications as outlined under Exposure 1 criteria. Exposure 1 panels have a fully waterproof bond and are designed for applications where long construction time may delay permanent protection or where high moisture conditions may be encountered in service. Exposure 1 panels are made with the same adhesives used in Exterior panels. However, because other compositional factors may affect bond performance, only Exterior panels should be used for permanent exposure to the 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, product must be specified by grade and species preference.

As with plywood, siding is also identified by a grading system with four basic classifications within the system. Each class is further divided into grades according to categories of repair and appearance characteristics. Depending on species, type of repair, finishing, etc., premium appearance products may be found in all grades. Some grades may be difficult to obtain in some areas.

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