Table 3-3.-Lumber Sheathing Specifications and Estimating FactorPlywood extension beyond the end wall is usuallygoverned by the rafter spacing to minimize waste. Thus,a 16-inch rake (gable) projection is commonly usedwhen rafters are spaced 16 inches OC. Butt joints of theplywood sheets should be alternated so they do not occuron the same rafter.DETAILS AT CHIMNEY ANDVALLEY OPENINGSWhere chimney openings occur in the roofstructure, the roof sheathing should have a 3/4-inchclearance on all sides from the finished masonry. Figure3-7 shows sheathing details at the valley and chimneyopening. The detail at the top shows the clearancesbetween masonry and wood-framing members.Framing members should have a 2-inch clearance forfire protection. The sheathing should be securely nailedto the rafters and to the headers around the opening.Wood or plywood sheathing at the valleys and hipsshould be installed to provide a tight joint and should besecurely nailed to hip and valley rafters. This providesa smooth solid base for metal flashing.ESTIMATING SHEATHING MATERIALTo figure the roof area without actually getting onthe roof and measuring, find the dimensions of the roofon the plans. Multiply the length times the width of theroof, including the overhang. Then multiply by thefactor shown opposite the rise of the roof in table 3-2.The result will be the roof area.For example, assume a building is 70 feet long and30 feet wide (including the overhang), and the roof hasa rise of 5 1/2 inches: 70 feet x 30 feet = 2,100 squarefeet. For arise of 5 1/2 inches, the factor on the chart is1.100:2,100 square feet x 1.100=2,310 square feet. So,the total area to be covered is 2,310 square feet. Use thistotal area for figuring roofing needs, such as sheathing,felt underpayment, or shingles.Lumber SheathingTo decide how much lumber will be needed, firstcalculate the total area to be covered. Determine the sizeboards to be used, then refer to table 3-3. Multiply thetotal area to be covered by the factor from the chart. Forexample, if 1- by 8-inch tongue-and-groove sheathing3-7
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