Felt & RBM
Reinforced Bituminous Membrane (RBM) & Felt Roofing
Reinforced bituminous membrane (RBM), also known as torch-on felt, is a flat roof covering made of bitumen-coated reinforced sheets bonded to the roof in layers with a gas torch. It is the workhorse of British flat roofing and the core system Pluvian Roofing installs across London.
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What RBM actually is
Modern RBM is a long way from the felt of decades past. It is a reinforced bituminous membrane: a polyester or glass fleece core coated in polymer-modified bitumen, applied in a built-up system, typically an underlay and a mineral or plain cap sheet over a suitable base. The result is a thick, resilient, repairable waterproof layer.
Why we specify it as our core system:
- A proven track record on British flat roofs over many decades
- Robust: it takes foot traffic and weather better than thin single-ply
- Fully bonded, so water cannot track beneath the membrane unseen
- Repairable: local damage can be fixed without renewing the roof
- Compatible with most existing felt and asphalt roofs for overlays
Overlay or strip and renew?
If your existing felt or asphalt roof is dry underneath and structurally sound, an overlay can be the smart option: a new RBM system installed over the prepared existing covering, avoiding the cost, disruption and waste of a full strip. Where the deck or insulation is wet or the roof has simply reached the end of its life, we strip back and rebuild the covering properly. The survey tells us which is right, and we price both honestly. And if the roof only has a localised fault, it may need nothing more than a flat roof repair.
Where torch-on meets modern safety
An open flame has no place near timber cladding, eaves boxes or other combustible details. In line with the industry's Safe2Torch guidance, we design those areas out of the torching zone and finish them with cold-applied liquid waterproofing instead: no flame, and a continuous, joint-free finish on the most leak-prone parts of the roof, such as upstands, edges and penetrations.
What a job with us looks like
- Survey. We inspect the roof, deck, details and drainage, and report what we find in plain English.
- Specification. You get a written specification and price: the system, the build-up, and how details will be handled.
- Installation. Tidy, safe work with the site left clean, whether it is a garage roof or an occupied block.
- Handover. We walk you through what was done, with photos of the finished details.
We install RBM on extensions, garages and dormers as well as larger residential and commercial roofs across South London.
Book a surveyFrequently asked questions
What is torch-on felt?
Torch-on felt is the common name for reinforced bituminous membrane (RBM): sheets of polyester or glass fleece coated in polymer-modified bitumen, bonded to the roof with a gas torch. It is applied in overlapping layers, typically an underlay and a cap sheet, to form a continuous waterproof covering.
How many layers does an RBM roof have?
Usually two or three. A two-layer system is an underlay plus a cap sheet; a three-layer build-up adds a vapour control layer or a second underlay beneath them. The right build-up depends on the deck, the insulation and how the roof will be used.
Can you overlay a new membrane over an existing flat roof?
Yes, where the existing covering is dry underneath, well bonded and the structure can take it. A survey confirms this before any work is priced. An overlay avoids the cost, disruption and waste of stripping a roof that still has a sound base.
Is torch-on work safe near timber and other combustible details?
An open flame is never used near combustible details. In line with the industry Safe2Torch guidance, areas near timber cladding, eaves boxes and similar details are finished with cold-applied liquid waterproofing instead, which needs no flame at all.
How long does a felt roof last?
It depends on the system, the quality of the detailing and how the roof is looked after, so treat any single number with caution. Manufacturers publish design lives and guarantee terms for their RBM systems, and documented maintenance inspections are usually a condition of keeping those guarantees valid.