Felt & Torch-On Roofing (RBM) in London

Suitability

Where the roof in front of you suits it

Most of them, on this side of London, and not by accident. The building stock here is full of rear additions, garages, dormers, bay tops, porches and small back extensions, all of them modest in area, all of them reached over somebody's garden, and most of them already carrying a bituminous covering of some age. A built-up bitumen system suits that work because it goes down over what is there without a tear-off where the survey allows one, it takes the odd geometry those roofs come with, and it does not need a factory-width sheet to be economic.

It suits the larger end too, for a different reason: on a block, a school roof or a warehouse field, the covering will be walked on by whoever services the plant, and a thick multi-layer skin tolerates that in a way a thin sheet does not. Where it is genuinely not the answer, we say so in the report rather than fitting the roof to the system.

Two things have to be settled before any of that matters. The first is what the felt is being bonded to: the deck, the falls built into it and the kerbs around anything coming through are decided before a roll is opened, because no covering compensates for boarding laid level or laid wet. The second is the openings. Where a rooflight sits in the field, the covering climbs the kerb and terminates beneath the frame as part of the same sequence, not added afterwards. On the smallest jobs of all, a garage roof follows its own logic, and a brand new roof over an extension or dormer is priced from drawings rather than from a strip.

Workmanship

What separates a proper torch-on job from a cheap one

Almost never the open field. Anyone can roll a sheet across a flat area; the money and the skill go into the metre nearest every edge, and that is exactly where a cheap job saves them. The things worth looking for:

• Upstands taken to a proper height and terminated. Not run up a wall and left, but carried high enough to beat driven rain and closed with a fixed termination or a flashing that keeps water out of the top edge.
• Angle fillets at every internal corner. A membrane bridged across a sharp right angle is holding a fold, and folds crack. The fillet turns the angle into a curve the sheet can actually follow.
• Laps bonded, not just heated. A properly torched lap shows a small consistent bead of bitumen squeezed out along its length. No bead means the sheets are lying together rather than welded, and that lap will open.
• Details cut and welded, not dressed and hoped. Outlets, pipes and kerbs each need their own pieces, worked in separately. Anything wrapped in one continuous sheet has strain built into it from day one.
• Falls that actually clear. A new covering laid over a deck that ponds will pond, and standing water finds every weak lap it sits on. If the falls are wrong, that is a decking conversation before it is a membrane one.

Paperwork

What a specification means in practice

It means you know what you are buying before anyone lights anything. A written specification names the system rather than saying "felt", states the build-up layer by layer with what goes down first, says how each detail type will be formed, and defines the extent of the work so the boundary between what is included and what is not is on paper. It also separates the known from the provisional: anything that cannot be established until a covering is lifted is listed as such with what happens if it is found.

That document is what makes two quotations comparable. Two prices for "a new felt roof" are not comparable at all, because one of them may assume a two-layer build where the other assumes three, or may plan to dress a detail the other intends to form properly. The lines that make two sheets genuinely comparable, carrier, polymer, thickness and fixing method, are set out in types of roofing felt. Ours is drawn up so somebody else could price against it, which is the only real test of whether a specification says anything.

The decision

Reroof or overlay

One sentence, because the decision belongs elsewhere: if what sits under the covering is dry and sound, a new system can usually go over the top, and if it is not, the roof comes off, with the refurbishment options guide setting out how that call is made. Where the covering already up there is felt, what has to be true before anything goes over it is set out on overlaying an existing felt roof.

Why felt

Why felt is still the sensible default for a new covering

Because of what happens after installation day. Every flat roof gets damaged eventually, by a dropped tool, an aerial fitter, a ladder or a contractor who needed to run a cable. A built-up bitumen roof takes a local repair without drama: the damaged area is cut out and new material welded in, by any competent roofer, with materials available anywhere. Systems that depend on a particular welding kit or a manufacturer-trained installer are fine until the day you need one at short notice.

Add the London context. These roofs are small, awkward to reach, and surrounded by other people's property, so a system that can be carried through a house and worked in sections earns its place over one that wants a crane and an open run. And they are almost all replacing something bituminous already, which means compatibility rather than a strip-out. None of that makes felt the right answer everywhere, and the survey is what settles it. That reasoning is why, on a roof being built or renewed outright, felt is what we reach for first. Where the job is an overlay rather than a new covering, the first system we put forward is the cold-applied liquid one, for reasons of weight and joints set out there.