The bottom or first ply in a roofing membrane when additional plies are to be subsequently installed.
An asphalt-saturated and/or coated felt installed as the first ply with 4 inch laps in a built-up roof system under the following felts which can be installed in a shingle-like fashion.
Initial layer in a multi-layer system of the type of built-up roofing or a first layer of wallboard in a multi-ply construction.
the lower most ply of roofing material in a roof assembly.
the lowermost ply of roofing in a roof membrane or roof system.
the lowermost ply of roofing material in a roof membrane assembly.
the base ply is the first ply when it is a separate ply and not part of a shingled system.