As defined by the International Building Code: un∙der∙lay∙ment n. “One or more layers of felt, sheathing paper, nonbituminous saturated felt, or other approved material over which a steep-slope roofing covering is applied.” There are a multitude of material types, compositions and installation techniques that are available for underlayments for roof assemblies, with felt being the most commonly used one. There is no one size fits all for the type of roofing underlayment, since the guidelines of underlayment pertain to the pitch of the roof, and the various types of roofing material. Using an example of the most common type of roof material, which would be asphalt shingles and the most common roof pitch of 4/12 the underlayment requirements are using a 15# felt only.