Reinforced bitumen membraneRBM

A reinforced bitumen membrane applied by melting the underside of the sheet with a gas torch so that it bonds to the substrate or preceding layer as it is rolled out. Torch application requires strict hot-works controls, and flame-free alternatives such as self-adhesive membranes are often specified where fire risk must be eliminated.

Bitumen blended with polymers, most commonly styrene butadiene styrene (SBS) for elastomeric flexibility or atactic polypropylene (APP) for plastomeric heat resistance, to improve low-temperature flexibility, fatigue resistance and durability compared with unmodified bitumen. It is used in roofing membranes and also as a binder in road surfacing.

A dense, voidless waterproofing material composed of graded limestone aggregate bound with bitumen, heated and applied in molten form by hand float to roofs, balconies, tanking and flooring. It sets to a seamless surface and has a long history of use in UK construction.

The overlap between adjacent sheets of roofing membrane or underlay where they are joined and sealed, by heat welding, solvent welding, adhesive, tape or bitumen bonding depending on the material. Lap integrity determines the watertightness of sheet systems, so minimum lap widths and the staggering of end laps are specified in system guidance.

A roof with a very low pitch, in UK practice generally taken as no more than 10 degrees to the horizontal. Flat roofs rely on a continuous waterproof covering rather than overlapping units, and must be laid to falls so that rainwater drains to outlets.