Abstract: A built-up roofing structure (10) is provided which is characterized by low dead weight, enhanced fire retardancy, and ease of construction. The structure (10) includes a lowermost deck (12) with integrated, insulative sections (24) applied thereover an adhered in place by a novel fire retardant mastic (27); the sections (24) each include a preformed expanded foam layer (14) covered by a roofing board (16). A modified bitumen membrane (18) is applied over and completes the roofing structure (10). The improved mastic includes asphalt, low volatility mineral spirits and a fire retardant additive such as an intumescent glass, and particularly a borosilicate glass. The mastic is advantageously applied using a spreader apparatus (30) having an elongated, tubular, apertured mastic delivery bar (32, 58) and a spreader means (46, 62) with a plurality of separate, spaced apart, lightweight trailing spreader chains (48).