Open sheathing can be used under wood shingles orshakes in blizzard-free areas or damp climates. Opensheathing usually consists of 1- by 4-inch strips with theon-center (OC) spacing equal to the shingle weatherexposure, but not over 10 inches. (A 10-inch shinglelapped 4 inches by the shingle above it is said to be laid6 inches to the weather.) When applying open sheathing,you should lay the boards without spacing to a point onthe roof above the overhang.NailingNail lumber roof sheathing to each rafter with two8-penny (8d) nails. Joints must be made on the raftersjust as wall sheathing joints must be made over the studs.When tongue-and-groove boards are used, joints maybe made between rafters. In no case, however, should thejoints of adjoining boards be made over the same rafterspace. Also, each board should bear on at least two rafters.PLYWOODPlywood offers design flexibility, construction ease,economy, and durability. It can be installed quickly overlarge areas and provides a smooth, solid base with aminimum number of joints. A plywood deck is equallyeffective under any type of shingle or built-up roof.Waste is minimal, contributing to the low in-place cost.Plywood is one of the most common roof sheathingmaterials in use today. It comes in 4- by 8-foot sheets ina variety of thicknesses, grades, and qualities. Forsheathing work a lower grade called CDX is usuallyused. A large area (32 square feet) can be applied atonetime. This, plus its great strength relative to othersheathing materials, makes plywood a highly desirablechoice.The thickness of plywood used for roof sheathing isdetermined by several factors. The distance betweenrafters (spacing) is one of the most important. The largerthe spacing, the greater the thickness of sheathing thatshould be used. When 16-inch OC rafter spacing is used,the minimum recommended thickness is 3/8 inch. Thetype of roofing material to be applied over the sheathingalso plays a role. The heavier the roof covering, thethicker the sheathing required. Another factordetermining sheathing thickness is the prevailingweather. In areas where there are heavy ice and snowloads, thicker sheathing is required. Finally, you have toconsider allowable dead and live roof loads establishedby calculations and tests.These are the controlling factors in the choice ofroof sheathing materials. Recommended spans andplywood grades are shown in table 3-1.InstallationPlywood sheathing is applied after rafters, collarties, gable studs, and extra bracing (if necessary) are inplace. Make sure there are no problems with the roofframe. Check rafters for plumb, make sure there are nobadly deformed rafters, and check the tail cuts of all therafters for alignment. The crowns on all the raftersshould be in one direction—up.Figure 3-2 shows two common methods of startingthe application of sheathing at the roof eaves. In viewA, the sheathing is started flush with the tail cut of therafters. Notice that when the fascia is placed, the topedge of the fascia is even with the top of the sheathing.In view B, the sheathing overlaps the tail end of the rafterby the thickness of the fascia material. You can see thatthe edge of the sheathing is flush with the fascia.If you choose to use the first method (view A) tostart the sheathing, measure the two end rafters the widthof the plywood panel (48 inches). From the rafter tailends, and using the chalk box, strike a line on the topedge of all the rafters. If you use the second method,Figure 32.—Two methods of starting the first sheet of roofsheathing at the eaves of a roof: A. Flush with rafter;B. Overlapping rafter.3-2
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