This Standard covers three types of asphalt-coated glass-base sheets (asphalt-impregnated and -coated glass-roll roofing) surfaced with insoluble inorganic mineral matter: (a) Type I: a lightweight base sheet intended primarily for use as an underlayment for shingles. (b) Type II: a standard-weight roofing base sheet intended for use as the first ply in a roofing membrane over insulation. (c) Type III: a venting base sheet surfaced with fine mineral matter on top and mineral granules on the bottom. This type is used in built-up roofing systems as a first ply to relieve pressure through lateral vapour movement. Type III felt can be unperforated or, if used for spot mopping, contain perforations of a size specified in Table 1.