MAXIMUM ULTIMATE DESIGN WIND SPEED, V ult ≥ 140 MPHįor roof slopes of four units vertical in 12 units horizontal (4:12) or greater, underlayment shall be one layer applied in the following manner: underlayment shall be applied shingle fashion, parallel to and starting from the eave and lapped 4 inches. ![]() MAXIMUM ULTIMATE DESIGN WIND SPEED, V ult < 140 MPH TABLE 905.1.1(1) UNDERLAYMENT TYPES ROOF COVERING Cap nail shank shall have a length sufficient to penetrate through the roof sheathing or not less than 3/ 4 inch (19 mm) into the roof sheathing. The cap nail shank shall be not less than 0.083 inch (2.11 mm) for ring shank cap nails and 0.091 inch (2.31 mm) for smooth shank cap nails.Minimum thickness of the outside edge of plastic caps shall be 0.035 inch (0.89 mm). Power-driven metal caps shall have a thickness of not less than 0.010 inch (0.25 mm). Metal caps shall have a thickness of not less than 32-gage sheet metal. Underlayment shall be attached using metal or plastic cap nails with a nominal cap diameter of not less than 1 inch (25 mm).The underlayment shall be attached with corrosion-resistant fasteners in a grid pattern of 12 inches (305 mm) between side laps with a 6-inch (152 mm) spacing at side and end laps.End laps shall be 4 inches (102 mm) and shall be offset by 6 feet (1829 mm). Starting at the eave, apply 36-inch-wide (914 mm) strips of underlayment felt, overlapping successive sheets 19 inches (483 mm). Apply a 19-inch-wide (483 mm) strip of underlayment parallel with the eave. ![]() ![]() As an alternative, two layers of underlayment complying with ASTM D226 Type II or ASTM D4869 Type III or Type IV shall be permitted to be installed as follows in 3.1-3.4:.An approved underlayment for the applicable roof covering for maximum ultimate design wind speeds, V ult, less than 140 miles per hour shall be applied over the entire roof over the 4-inch-wide (102 mm) membrane strips. As an alternative, a minimum 4-inch-wide (102 mm) strip of self-adhering polymer-modified bitumen membrane complying with ASTM D1970, installed in accordance with the manufacturer's installation instructions for the deck material, shall be applied over all joints in the roof decking.As an alternative, self-adhering polymer-modified bitumen underlayment complying with ASTM D1970 installed in accordance with both the underlayment manufacturer's and roof covering manufacturer's instructions for the deck material, roof ventilation configuration and climate exposure for the roof covering to be installed, shall be permitted.
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