d) Minimize possible chemical reactions between resins
in wood decks and asphalts in asphalt shingles or roll roofing.
Underlayments for Steep Roofing. Underlayments
generally consist of one layer, non-perforated asphalt saturated
felt for slopes of four inches per foot and higher. For lesser
slopes, two layers of the same felt may be required and should be
applied with a 19-inch (483-millimeter) overlap. Modified
bituminous membranes are used at cold eaves in cold regions.
Underlayment for Wood Shake, Clay Tile, and Concrete
Tile Roofs. Underlayment is required only in the following
In valleys or eaves where ice dams may form;
Where blowing dust or sand occurs regularly;
In hurricane zones.
Regardless of the type of underlayment required, or the
slope of the roof, in locations where the January mean
temperature is 30 degrees F (-1 degree C) or less, two plies of
No. 15 felt, set in hot asphalt or mastic, or an adhered bitumen
membrane should be applied as the underlayment starting from the
eaves to a point 24 inches (610 millimeters) inside the inside
wall line of the building to serve as an ice shield.
Wood shake roofs of less than 4 inches (33 percent)
slope require other measures; i.e., one layer asphalt saturated
felt underlayment nailed to wood decks followed by one layer of
the same type of felt applied with hot moppings of asphalt, plus
construction of a vented lattice work of boards above the
structural deck. For other measures designed to improve water-
resistance and overall roof system performance, refer to the NRCA
Steep Roofing Manual.