The short answer: RBM suits robustness, repairs and the human scale of most British roofs; single ply earns its keep on large, simple commercial fields where speed and weight rule. We install RBM, torch-on and liquid systems ourselves, survey and advise across all of them, and this piece explains how the choice is really made.
What are the two systems?
RBM, reinforced bituminous membrane, is the built-up approach: plies of reinforced, polymer-modified bitumen bonded to each other and the roof, redundancy included by design. Single ply is the opposite bet: one factory-made sheet, from the PVC, TPO or EPDM families, laid in large widths and either mechanically fastened through the insulation or adhered, with joints hot-air welded or taped rather than torched. One system builds thickness on the roof; the other arrives with its thickness decided in the factory.
Where does RBM genuinely win?
- Robustness in service. Multi-layer redundancy and a mineral wearing surface tolerate the boots, ladders and dropped tools a real roof meets.
- Repairability. Damage is patched with the same material, fused in; a decade later the repair is part of the roof.
- Proven detailing. Generations of British installers and published details mean fewer novel junctions.
- Domestic and refurbishment scale. On the extensions, terraces and mixed-shape roofs that fill this city, built-up bitumen is in its element.
Where does single ply genuinely win?
- Large, simple fields. Warehouse and shed roofs measured in thousands of square metres reward a wide sheet laid fast.
- Speed at scale. Fewer passes and no built-up layering make programme savings real on big projects.
- Weight. A thin single sheet asks less of a lightweight structure than a multi-ply bituminous build-up.
Where is each weaker?
Single ply concentrates its risk: one layer means punctures matter immediately, every seam depends on the welder's discipline that day, and repairs must match the sheet's chemistry or fail quietly. RBM carries its own costs: torch application demands the Safe2Torch discipline and flame-free methods near combustible details, and on a vast, empty field it simply cannot match a wide sheet for pace. Note the symmetry: each system is weakest exactly where the other is strong.
What happens at the details?
Roofs rarely fail in the middle, so the detail strategy deserves its own paragraph. Single ply handles corners, pipes and terminations with prefabricated pieces and site-welded patches, which is quick and consistent when done well, and entirely dependent on the operative when not. RBM details are hand-formed in the same material as the field, which rewards craft and shows its quality immediately. Both families increasingly delegate the genuinely awkward junctions to liquid-applied waterproofing, which conforms to shapes neither sheet enjoys; a modern specification often reads as a treaty between all three.
How do the two systems age?
Differently, and the difference matters for whoever inspects the roof in year fifteen. An ageing RBM roof telegraphs its condition: mineral loss, surface crazing and softening laps are visible warnings that arrive years before failure. Single ply tends to age quietly, holding its appearance while thickness slowly reduces and seams take up movement, so its problems more often arrive as a surprise finding rather than a gradual picture. Neither ages better in any absolute sense; they simply reward different inspection habits, and both repay the same thing: someone actually looking, on a schedule, with a camera.
Can a roof switch families at renewal?
Yes, and renewal is the only sensible moment to do it. A stripped deck is neutral ground: what goes back on it is a fresh decision, and a building whose use has changed, more roof plant, different fire strategy, new ownership with a different budget horizon, may genuinely be better served by the other family this time round. The switch has costs worth naming: details and edge trims designed for one system rarely transfer, the maintenance habits differ, and whoever repairs the roof in future needs to know what is now up there. Switching over an existing membrane, by contrast, is specification quicksand; compatibility between families is the exception, not the rule.
How do specifiers actually choose?
Five questions settle most specifications. How big and how simple is the field? Traffic: will trades walk this roof for plant access? Budget horizon: cheapest install or cheapest decade? Fire strategy: what does the building's risk profile say about hot works and membrane behaviour? And the warranty route: which manufacturer system, at which tier, with which conditions? Answer those honestly and the membrane family usually picks itself; brand loyalty is what people fall back on when they have not asked.
Where do we stand?
We are practitioners of RBM and torch-on systems and liquid-applied waterproofing, trained across flat roof systems and happy to say when a roof's honest answer is a system we do not install. A condition survey of the actual roof beats any allegiance, and for portfolio owners weighing this across estates, that system-agnostic advice is standard commercial roofing practice. The roof decides; we just make sure it gets asked.