The reduction in thickness and the absence of gravel reduce dead load on the structural deck. The modified bituminous membrane is adhered to the roof deck or the insulation by applying a suitable asphalt adhesive or by melting the underside of the membrane, as it is unrolled, with a multiple-nozzled propane torch. The edges of the laps are finished with a trowel and a hand-held torch or contact cement to ensure adhesion and prevent water penetration. Its weatherability is about the same as that of regular bitumen. To prevent deterioration caused by ultraviolet light, the membrane is surfaced with mineral slate or granules, or given a special coating or reflective facing such as embossed aluminum, copper or stainless steel.
Granules lost by erosion and freeze-thaw cycles increase the area of exposed bituminous surfaces.
As part of an annual inspection and maintenance program, the granules should be replaced by torch-heating the exposed areas. If an incompatible asphalt has been used for adhesion, joint delamination may occur due to the brittleness of deteriorated materials, This can also occur with torched joints that are overheated. Lap joints can slip if excessive asphaltic adhesive has been applied or if the adhesive is too soft. The performance of roofing membranes depends on the design, the properties of the materials, the workmanship at the site and the exposure conditions. Current material standards are prescriptive and define the physical characteristics considered necessary for proper in-situ performance and durability of the products.