Reinforced Bitumen Membrane (RBM)

Reinforced bitumen membrane, abbreviated RBM and often still called "felt" or "torch-on felt", is the British and European name for bituminous roofing sheets consisting of a reinforcing carrier coated with bitumen. Modern RBMs are mostly polymer-modified with SBS or APP, the same technologies known in North America as modified bitumen.

Evolution

Traditional British built-up felt roofs used oxidised bitumen sheets with organic or glass-fibre bases, laid in three layers in hot bitumen and often finished with mineral chippings. Flat roofs of this era earned a reputation for short lives in some cases. Today most sheets are polymer-modified on polyester or composite carriers, which are tougher and more flexible.

Typical system

An RBM roof is usually a two- or three-layer warm-roof build-up:

1. vapour control layer on the deck
2. insulation, often tapered
3. underlay or base sheet, sometimes partially bonded to allow vapour to disperse
4. cap sheet with a mineral or slate-granule finish for UV protection

Manufacturers supply complete systems combining underlays, insulation and cap sheets.

Application methods

• Torch-on: heating the underside with propane torches; the most common method in the UK
• Pour and roll: rolling the sheet into hot bitumen
• Self-adhesive and cold-adhered systems, chosen where hot works are restricted

Design

RBM systems in the UK are designed to BS 6229, with a minimum finished fall of 1:80, upstands of at least 150 mm (6 in), and falls created by structure, firrings or tapered insulation. Bauder and other manufacturers note that tapered insulation works with RBM, single-ply and liquid systems alike.