# Reinforced Bitumen Membrane (RBM)

> The UK and European term for factory-made bitumen roofing sheets with polyester or glass reinforcement, laid in multi-layer systems on flat roofs.

- URL: https://roof.wiki/reinforced-bitumen-membrane/
- Topic: Low-Slope & Flat Roof Systems
- Also known as: RBM; torch-on felt; felt roof; built-up felt; bituminous membrane
- Regions: Europe
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Reinforced Bitumen Membrane (RBM)." roof.wiki. https://roof.wiki/reinforced-bitumen-membrane/

## Key facts

- **Region:** UK and Europe
- **Common polymers:** SBS and APP
- **Typical system:** Underlay plus mineral-surfaced cap sheet
- **UK design code:** BS 6229

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.

## Sources

1. BS 6229:2018 and flat roofing systems — IKO. https://ikogroup.co.uk/?p=820
2. What is the minimum drainage falls for a single ply flat roof? — IKO. https://ikogroup.co.uk/?p=829
3. Falls and drainage — Bauder. https://www.bauder.co.uk/roof-systems/refurbishment-projects/solving-the-challenge/improving-drainage-falls
4. Who Put the Modified in Modified Bitumens? — Buildings. https://buildings.com/feature/article/10187932/who-put-the-modified-in-modified-bitumens