Aggregate. Gravel, crushed stone, washed round river gravel slag
or mineral granules either (a) embedded in a conventional built-
up membrane's bituminous flood coat, or (b) laid on a membrane as
a protective ballast. Crushed stone should be avoided if
Alligatoring. Deep shrinkage cracks, progressing down from the
surface, in smooth-surfaced membrane coatings and sometimes in
bare spots of aggregate-surfaced membranes, a consequence of
photo-oxidative hardening, shrinkage due to loss of aromatics.
Asphalt. A dark brown to black, highly viscous, hydrocarbon
produced from the residuum left after the distillation of
petroleum, used as the waterproofing agent of built-up roofs and
other building components.
Atactic polypropylene. High molecular-weight, thermoplastic,
amorphous polymers used as a chemical ingredient to improve
flexibility, elasticity, and ductility of bitumen, used in some
single-ply modified bitumen membranes for torching applications.
Backnailing. Slippage-preventing technique of "blind" (i.e.,
concealed) nailing in addition to hot-mopping.
Ballast. Aggregate, concrete pavers, or other material designed
to prevent wind uplift or flotation of a loose-laid roof or
insulation system. Aggregate should be smooth, round, without
flat faces and sharp edges.
Base ply. A felt or mat, usually coated, placed as the first
non-shingled ply in a multi-ply built-up roof membrane.
Batten. Narrow metal or wood band used to anchor a membrane
against wind uplift.
Bitumen. Generic term for an amorphous, semisolid mixture of
complex hydrocarbons derived from petroleum or coal. In the
roofing industry there are two basic bitumens: asphalt and coal-
tar pitch. Before application, they are either (l) heated to a
liquid state, (2) dissolved in a solvent, or (3) emulsified.
Blindnailing. Nailing of felts with nail heads covered by
overlapping top felts.