Table 3-3.-Lumber Sheathing Specifications and Estimating Factor
Plywood extension beyond the end wall is usually
governed by the rafter spacing to minimize waste. Thus,
a 16-inch rake (gable) projection is commonly used
when rafters are spaced 16 inches OC. Butt joints of the
plywood sheets should be alternated so they do not occur
on the same rafter.
DETAILS AT CHIMNEY AND
VALLEY OPENINGS
Where chimney openings occur in the roof
structure, the roof sheathing should have a 3/4-inch
clearance on all sides from the finished masonry. Figure
3-7 shows sheathing details at the valley and chimney
opening. The detail at the top shows the clearances
between masonry and wood-framing members.
Framing members should have a 2-inch clearance for
fire protection. The sheathing should be securely nailed
to the rafters and to the headers around the opening.
Wood or plywood sheathing at the valleys and hips
should be installed to provide a tight joint and should be
securely nailed to hip and valley rafters. This provides
a smooth solid base for metal flashing.
ESTIMATING SHEATHING MATERIAL
To figure the roof area without actually getting on
the roof and measuring, find the dimensions of the roof
on the plans. Multiply the length times the width of the
roof, including the overhang. Then multiply by the
factor 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 and
30 feet wide (including the overhang), and the roof has
a rise of 5 1/2 inches: 70 feet x 30 feet = 2,100 square
feet. For arise of 5 1/2 inches, the factor on the chart is
1.100:2,100 square feet x 1.100=2,310 square feet. So,
the total area to be covered is 2,310 square feet. Use this
total area for figuring roofing needs, such as sheathing,
felt underpayment, or shingles.
Lumber Sheathing
To decide how much lumber will be needed, first
calculate the total area to be covered. Determine the size
boards to be used, then refer to table 3-3. Multiply the
total area to be covered by the factor from the chart. For
example, if 1- by 8-inch tongue-and-groove sheathing
3-7
