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Chapter 3 Roof Construction and Trim Carpentry
Table 3-1.-Plywood Roof Sheathing Application Specifications

Builder 3&2 Volume 02 - Construction manual for building structures
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Open sheathing can be used under wood shingles or shakes  in  blizzard-free  areas  or  damp  climates.  Open sheathing usually consists of 1- by 4-inch strips with the on-center (OC) spacing equal to the shingle weather exposure, but not over 10 inches. (A 10-inch shingle lapped 4 inches by the shingle above it is said to be laid 6 inches to the weather.) When applying open sheathing, you should lay the boards without spacing to a point on the roof above the overhang. Nailing Nail lumber roof sheathing to each rafter with two 8-penny (8d) nails. Joints must be made on the rafters just as wall sheathing joints must be made over the studs. When  tongue-and-groove  boards  are  used,  joints  may be made between rafters. In no case, however, should the joints of adjoining boards be made over the same rafter space. Also, each board should bear on at least two rafters. PLYWOOD Plywood  offers  design  flexibility,  construction  ease, economy, and durability. It can be installed quickly over large areas and provides a smooth, solid base with a minimum number of joints. A plywood deck is equally effective 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 sheathing materials in use today. It comes in 4- by 8-foot sheets in a  variety  of  thicknesses,  grades,  and  qualities.  For sheathing work a lower grade called CDX is usually used. A large area (32 square feet) can be applied atone time. This, plus its great strength relative to other sheathing  materials,  makes  plywood  a  highly  desirable choice. The thickness of plywood used for roof sheathing is determined by several factors. The distance between rafters (spacing) is one of the most important. The larger the spacing, the greater the thickness of sheathing that should be used. When 16-inch OC rafter spacing is used, the minimum recommended thickness is 3/8 inch. The type of roofing material to be applied over the sheathing also plays a role. The heavier the roof covering, the thicker  the  sheathing  required.  Another  factor determining  sheathing  thickness  is  the  prevailing weather. In areas where there are heavy ice and snow loads, thicker sheathing is required. Finally, you have to consider allowable dead and live roof loads established by calculations and tests. These are the controlling factors in the choice of roof sheathing materials. Recommended spans and plywood grades are shown in table 3-1. Installation Plywood  sheathing  is  applied  after  rafters,  collar ties, gable studs, and extra bracing (if necessary) are in place.  Make  sure  there  are  no  problems  with  the  roof frame. Check rafters for plumb, make sure there are no badly deformed rafters, and check the tail cuts of all the rafters for alignment. The crowns on all the rafters should be in one direction—up. Figure 3-2 shows two common methods of starting the application of sheathing at the roof eaves. In view A, the sheathing is started flush with the tail cut of the rafters.  Notice  that  when  the  fascia  is  placed,  the  top edge of the fascia is even with the top of the sheathing. In view B, the sheathing overlaps the tail end of the rafter by the thickness of the fascia material. You can see that the edge of the sheathing is flush with the fascia. If you choose to use the first method (view A) to start the sheathing, measure the two end rafters the width of the plywood panel (48 inches). From the rafter tail ends, and using the chalk box, strike a line on the top edge of all the rafters. If you use the second method, Figure 32.—Two methods of starting the first sheet of roof sheathing at the eaves of a roof: A. Flush with rafter; B. Overlapping rafter. 3-2







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