Four Ply Gravel Surface Built-up
A built-up roofing consists of plies or layers of roofing felt bonded together on site with hot bitumen. It is laid down to conform to the roof deck and to seal all angles formed by projecting surfaces so that it constitutes a single-unit flexible waterproof membrane.

Three or Four Ply Smooth Built-up
A roof consisting of multiple plies of roof felts laminated together with bitumen. Built-up roof material can consist of bitumen-saturated felt, coated felt, polyester felt or other fabrics. A surfacing is generally applied and can be asphalt, aggregate, emulsion or a granule-surfaced cap sheet.