Wood Roof Decks
Plywood Panel Roof Decks: Plywood roof deck panels are made from thin wood layers called veneers that are peeled from logs. The veneers are placed at right angles to each other, then glued together under heat and pressure. This cross lamination adds strength and stability to the plywood panels. These panels consist of a number of cross laminated layers that vary in number according to the panel's thickness.

Plywood panels shall comply with the U.S. Product Standard PS 1-83 for Construction and Industrial Plywood, and the American Plywood Association Standard APA PRP-108, Performance Standards and Policies for Structural Use Panels. Plywood panels shall be a minimum of 15/32" thick for roof decks. The correct thickness of the plywood panels required for a specific roof shall be determined by the design loads, including wind uplift and the distance between the roof deck supports.

OSB (Oriented Strand Board) Panel Roof Decks: Oriented strand board roof decks are composed of panels made from layers of compressed, glued wood strands. These strand layers are oriented at right angles to one another before being glued and formed into panels. Oriented strand boards shall be a minimum thickness of 15/32". The correct thickness of the oriented strand board panels required for a specific roof shall be determined by the design loads, including wind uplift and the distance between the roof deck supports.

Wood Plank Roof Decks shall be constructed from dimensional lumber such as 2" x 6" tongue and groove boards, and are normally supported by wood beams, often glue laminated timber and solid lumber joists or purlins. Wood planks may be single or double tongue and groove, straight edged, ship lapped, or grooved for spleens on longitudinal edges. Wood plank decks shall have a minimum thickness of 1". The correct thickness of wood planks required for a specific roof shall be determined by the design loads, including wind uplift, and the distance between the roof deck supports.

Wood Panel Roof Deck Requirements: Wood panel roof decks shall be rated for structural roof sheathing requirements. For particularly demanding installations, such as prefabricated panelized roof decks where cross panel strength and stiffness, or shear properties are critical, the designer shall specify panels meeting the APA Standard for Structural I Rated Sheathing.

Most building codes require a label on wood panels to assure that the panel complies with the criteria set forth in the Department of Commerce Standard PS 1-83 for all veneer plywood, or the American Plywood Association (APA) Performance Standard PRP-108 for oriented strand board, oriented wafer board, and all veneer plywood. The APA Standard may apply to panels that do not comply with PS 1-83 in certain respects. Wood panels installed, as roof sheathing shall meet the requirements set forth by APA for performance rated roof sheathing. The panels shall have the APA trademark and have a smooth surface upon which to apply the Fields roof system.

Illustration 9: Wood panel markings

Label A

Label B

Illustration 9 is printed on all veneer plywood conforming to PS 1-83. Illustration 9 is found on wood panels conforming to PRP-108, which includes oriented strand board, oriented wafer board and also all veneer plywood that does not comply with PS 1-83 in certain respects.

A label similar to one of these shall be found on each wood panel for roof sheathing. The Span Rating is of particular significance. The first number of the Span Rating, which is 32 in the above examples, indicates the maximum recommended center to center spacing of the roof deck supports in inches when the panel is installed as roof sheathing with the long dimension of the panel running perpendicular to the supports. The second number of the Span Rating, which is 16 in the examples, indicates the maximum recommended spacing of the supports in inches when the panel is installed for sub flooring in double layer construction with the long dimension of the panel running perpendicular to the supports. The Span Rating of 32/16 in these labels means that the wood panel may be installed either for roof sheathing over supports that are spaced 32" on center or for sub flooring over supports that are spaced 16" on center. In all cases, panels are assumed to be continuous over two or more supports.

Though the industry terminology refers to plywood sheathing with exterior glue as C-DX, the X, which refers to exterior glue, is not used in Illustration 9, Label A. The EXPOSURE 1 classification is typically an interior type of wood panel made with exterior glue. The designer shall specify the type of wood panels and the minimum allowed thickness.

End joints of wood panels shall be staggered. Wood blocking or clips shall be installed at all wood panel joints between the roof deck framing supports. Wood panels shall be attached to supports such as joists, beams, and purlins by annular threaded or ring shank nails, or by other approved shank fasteners, including pneumatically driven fasteners.

Potential fastener holding challenges, and dimensional stability due to the effects of moisture with oriented strand boards and wafer boards installed as roof decking may be experienced. Additionally oriented strand boards and wafer boards may cause rigiding, buckling, and wrinkling in organic fills and roof membrane systems.

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