The short answer: reinforced bituminous membrane (RBM) is a flat roof waterproofing sheet made of a strong synthetic carrier fleece coated both sides with polymer-modified bitumen, installed in bonded, overlapping layers. Known colloquially as torch-on felt, it is the system underneath most modern British flat roofs.
What does the name actually mean?
Each word earns its place. Reinforced: a carrier of polyester or glass fleece runs through the sheet's core, giving it tensile strength and dimensional stability the old unreinforced felts never had. Bituminous: the waterproofing itself is bitumen, upgraded with polymers, commonly SBS for elasticity or APP for heat and ultraviolet tolerance, so the sheet flexes and endures rather than turning brittle. Membrane: the layers fuse into a single continuous waterproof skin rather than remaining a stack of loose sheets. Which carrier and which polymer a given sheet uses, and what that changes on a roof, is the subject of types of roofing felt.
How is an RBM roof constructed?
As a build-up, engineered upward from the deck, the boarding the whole roof rests on. A typical warm roof, with its insulation above that deck, runs:
- Vapour control layer on the deck
- Insulation boards above it
- Underlay ply, the first waterproofing layer, bonded by heat or fixings
- Cap sheet finished in mineral granules that shield the bitumen from the sun
Some specifications add a third waterproofing ply or vary the fixing method, but the principle holds: each layer has one job, and the laps of successive plies are staggered so no seam lines up with another.
Why did the industry move on from old-style felts?
Because the old material failed predictably. The rag-fibre and early glass-fibre felts of the postwar decades relied on oxidised bitumen with no meaningful reinforcement: they embrittled in sunlight, cracked with thermal movement and tore under stress, which is why so many garage roofs from that era lasted barely a decade. Polymer modification and strong carriers addressed both weaknesses at once, and published system guarantees lengthened accordingly. The nickname "felt" survived; the material under it changed almost completely.
How is RBM installed?
Most commonly by torch: the installer heats the underside of the roll so its bitumen melts and fuses to the surface below as the roll advances, creating a fully bonded covering with no loose layers in it. Where open flame is inappropriate, and the industry Safe2Torch guidance defines those zones firmly, self-adhesive and hot-air-welded variants of the same membranes carry the detail work, or the specification hands those areas to a cold-applied liquid system. Skill matters more here than in almost any other roofing trade: the same roll can become a roof that outlives its guarantee or one that is leaking within a few winters, depending on who holds the torch.
Where does RBM sit against single ply and liquid?
Single ply membranes, PVC, TPO, EPDM, are one factory sheet thick, light and fast to lay across big simple roofs, but everything depends on one layer and its welded seams. RBM's two-ply redundancy and its tolerance of foot traffic make it the robust choice for refurbishment and smaller commercial work, and repairs blend invisibly into the original. Liquid systems beat both around complex details, which is why mixed specifications are routine. For portfolio owners weighing systems across many buildings, this is standard commercial roofing territory: the right answer is usually per roof, not per brand loyalty.
How do you recognise RBM on an existing roof?
Look for the granules and the seams. A surface finished in fine mineral chippings, green, grey or brown, with parallel overlap lines at regular sheet widths, is the signature of a capped RBM system; shiny black bitumen showing through means the mineral armour is wearing off and the sun is reaching the waterproofing. At the edges you can often count the layers of past coverings stacked like tree rings, which quietly documents the roof's overlay history.
Distinguishing it from its neighbours is straightforward: single ply sheets look like smooth uniform fabric with no granules, their joints hot-air welded on PVC and TPO and taped or bonded on EPDM, while liquid systems show roller texture, no seams at all and visible reinforcement at the details. Knowing which family is up there changes what any later repair or overlay can assume.
What should a specifier check?
Three things before any brand talk. That the quotation names a full system from one manufacturer, carrier weights and all, rather than mixing sheets ad hoc, because guarantees attach to systems. That the guarantee's length comes with its conditions, since documented maintenance is generally among them. And that the detailing package, upstand heights, outlet connections, movement joints, is drawn, not left to site improvisation. The membrane rarely fails; the gaps in the paperwork do.