This specification covers inorganic fiber-reinforced organic felt, and inorganic fiber-based asphaltic and nonasphaltic felt underlayments for use as underlayment with steep-slope roofing products. The intent of this specification is to provide criteria for producing and evaluating underlayments with a significantly reduced tendency to wrinkle before or after the installation of steep roofing products. Materials shall be sampled and tested suitably to examine their conformance with performance requirements such as tear strength, pliability, behavior on heating, liquid water transmission, dimensional stability at low to high humidity conditions, and elongation.