Sheathing should be applied as soon as the roof framing is finished. Sheathing provides additional strength to the structure, and provides a base for the roofing material.
Some of the materials commonly used for sheathing are plywood, OSB, waferboard, shiplap, and common boards.
When composition shingles are used, the sheathing must be solid.
If wood shakes are used, the sheathing board may be spaced.
When solid sheathing is used, leave a 1/8” space between panels to allow for expansion.
The next slide presents an overview of the steps that need to be followed to install roof sheathing using plywood or other 4 × 8 sheet materials.
Steps to Install Roof Sheathing
(using plywood or other 4 × 8 sheet materials)
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Measure up from where the finish fascia will be installed. Chalk a line at that point, lay the first sheet down, and nail it. Install H-chips midway between the rafters or trusses before starting the next course.
Apply the remaining sheets. Stagger the panels by starting the next course with a half sheet. Let the edges extend over the hip, ridge, and gable end. Cut the extra sheathing off with a circular saw.
Once the sheathing has been installed, an underlayment of asphalt-saturated felt or other specified material must be installed to keep moisture out until the shingles are laid. For roofs with a slope of 4” or more, 15 pound roofer’s felt is commonly used.
Materials such as coated sheets or heavy felt that can act as vapor barriers should not be used. They can allow moisture to accumulate between the sheathing and the underlayment.
The underlayment is applied horizontally with a 2” top lap and a 4” side lap. A 6” lap should be used on each side of the centerline of hips and valleys. A metal drip edge is installed along the rakes and eaves to keep out wind-driven moisture.
In climates where snow accumulates, a water-proof underlayment, should be used at roof edges and around chimneys, skylights, and vents. This underlayment has an adhesive backing that adheres to the sheathing. It protects against water damage that can result from melting ice and snow that backs up under the shingles.