Torch-on membrane

A sheet waterproofing material comprising a polyester or glass fibre carrier coated with oxidised or polymer-modified bitumen, normally installed in two or more bonded layers on flat roofs. It is the modern UK term for what was traditionally called built-up felt roofing.

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.

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 localised element of a roof or below-ground waterproofing system where the continuous field membrane is interrupted or terminated, such as upstands, penetrations, outlets, movement joints and edges. Details demand specific design and careful workmanship, and they are where most waterproofing failures originate.