Modified Bitumen Roofing

Modified bitumen, often shortened to "mod-bit", is a family of prefabricated roofing membranes made by blending asphalt with polymers that improve flexibility, elasticity and resistance to ageing. The modified asphalt is coated onto a reinforcing mat in the factory, and the resulting rolls are installed on site, typically in a two-ply system of base sheet and cap sheet.

Origins

Polymer-modified bitumens were developed in Europe in the 1960s. Atactic polypropylene (APP) modification was first reported in Italy in 1967, and SBS (styrene-butadiene-styrene) modification was developed in France and Germany for colder climates. According to a trade history published by Buildings, by the mid-1980s modified bitumen held nearly 90% of the roofing market in Italy and about 60% in France and Norway. The systems reached the US commercial market in the late 1970s.

Construction of the sheet

• Polymer: SBS (roughly 10-15% of the blend) gives rubber-like elasticity; APP (roughly 25-30%) gives a plastic, heat-resistant compound.
• Reinforcement: polyester mats for elongation, puncture and tear resistance; glass-fibre mats for dimensional stability and tensile strength. Some sheets combine both.
• Surfacing: mineral granules, metal foil, sand or film. Reflective granules and coatings are used for cool-roof cap sheets.
• Size: rolls are typically about 0.9-1.0 m (36-39 in) wide and 1.8-4.3 mm (70-170 mil) thick.

Installation methods

• torch-applied (heat-welded with propane torches or hot-air equipment)
• mopped in hot asphalt (mainly SBS)
• cold-applied adhesives, solvent- or water-based
• self-adhered sheets with factory-applied adhesive

Modified bitumen can also be combined with built-up plies to form hybrid roofs, and is widely used as flashing material on BUR roofs.

Uses beyond the field membrane

The same sheets are used for temporary roofs, vapour retarders, air barriers and below-grade waterproofing. In the UK and Europe the broadly equivalent products are called reinforced bitumen membranes (RBM) or torch-on felt.

Performance

Multi-ply systems provide redundancy at terminations and penetrations, where most leaks begin. Industry sources report US mod-bit roofs in service for more than 25 years, and many products hold FM Approvals listings for very severe hail.