When plywood or OSB is used for roof sheathing, it is laid with the face grain at right angles to the framing (Fig. 68). Sheathing-grade structural wood panels (SWP) is used for this purpose. To obtain a good tie across the roof framing, the end joints of the panels should be staggered on the framing members. The edges of the panels should be separated by at least an '/s in. (2 to 3 mm) to prevent buckling when minor expansion occurs during wet weather.
The thickness of the plywood, OSB or structural wood panel used for roof sheathing depends to some extent on the spacing of the rafters, roof joists or trusses and whether or not the edges of the sheets are supported. To prevent damage to the roof covering when thinner panels are used, the joints running across the framing should be supported by 2 x 2 in. (38 x 38 mm) blocking, nailed securely
Application of structural wood panel roof sheathing.
nails 6" (150 mm) o.c. grain direction double top plate nails 12" (300 mm) o.c. metal H-clip or blocking Vs" (2-3 mm) space nails 6" (150 mm) o.c. grain direction double top plate between the roof framing members, or by metal H-clips inserted between sheets. The latter method is widely used because the installation is simple and economical. Minimum thicknesses for plywood and other roof sheathing are shown in Table 32. Table 18 identifies the nail and staple requirements for roof sheathing. Staple fastening for 3/8 in. (9.5 mm) roof sheathing must be 1/16 in. thick, 11/2 in. long with a 3/8 In. crown (1.6 mm thick, 38.1 mm long with a 9.5 mm crown), and be driven with the crown parallel to the framing. (See Table 33.) Thicker roof sheathing, no less than 5/8 In. (15.5 mm), is required for built-up roofing on a flat roof where It is used as a walking deck. In these cases, Table 17 should be used to determi ne the minimum thickness for the sheathing.
Under materials requiring solid and continuous support, such as asphalt shingles and built-up roofing, lumber sheathing must be laid closed (Fig. 69B). The boards are usually 3/4 in. (19 mm) thick, but this thickness may be reduced to »/16 in. (17 mm) where supports are spaced at 16 in. (400 mm) on centre. Boards 8 in. (184 mm) or less wide are nailed to the framing members with two 2 in. (51 mm) nails per bearing. Those wider than 8 in. (184 mm) should be nailed with three 2 in. (51 mm) nails per bearing. Boards wider than 12 in. (286 mm) should not be used for roof sheathing. For a wood shingle roof, the roof boards may be spaced the same distance apart on centre as the shingle exposure. This method (Fig. 69A), commonly used in damp climates, permits freer movement of air around the boards and under shingles, thus reducing the possibility of decay.
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