Single ply

Single Ply Roof Repair & Liquid Overlay

Plenty of London roofs built in the last thirty years are single ply, and plenty of them are now at the age where the seams and details need attention. We repair those roofs and, where the evidence supports it, renew them with a liquid overlay rather than a strip-off. What we do not do is lay new single ply, and this page is straightforward about the line between the two.

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What we do and do not do with single ply

Our own trade is reinforced bituminous membrane, torch-on felt, and liquid systems applied cold. We are trained to survey flat roofs of every kind and we advise across all of them, which is a different thing from laying all of them. So: existing single ply roofs we inspect, repair and overlay. New single ply roofs we do not install, and no page here will suggest otherwise. If your roof needs one, our report says so and you take it to a contractor approved by the relevant manufacturer.

The comparison itself, how these membranes stack up against the built up systems we do lay, is worked through in our guide to RBM versus single ply, written from the same position: we survey both families and install one of them.

This page covers the thermoplastic membranes, PVC and TPO. EPDM is a rubber rather than a thermoplastic and responds differently to anything laid over it, so it is left out here deliberately rather than quietly lumped in; we survey it and advise on it, and whether an overlay suits a particular EPDM roof is decided on the roof.

How single ply roofs fail

These are thin, factory-made sheets doing a job that depends almost entirely on how they were joined and held down.

Seams. The sheet itself usually outlives the welds between sheets. A weld that was cold, contaminated or rushed on installation day can hold for years and then open, and a seam that has begun to peel will keep peeling.
Mechanical damage. A thin membrane is easily punctured by a dropped tool, a dragged ladder, a fixing left on the deck or an aerial installer in trainers. Damage of this kind is the most repairable of all, provided somebody finds it.
Shrinkage at upstands. Some membranes tighten slightly over their life, and a sheet pulling in drags at its perimeter. The evidence is at the edges: bridging where the covering no longer sits into the angle, strain at corners, and eventually a split at the top of an upstand.
Fixings and wind. Mechanically fastened roofs rely on fasteners through the membrane and into the deck. Fixings that have backed out, corroded or been placed into a deck that has since softened let the sheet lift and flutter, which fatigues the seams from underneath.
Details and penetrations. Prefabricated corners, pipe collars and outlet flanges are the fiddly parts of the system and they age faster than the open field.

Repair first, where repair is real

A punctured membrane with sound seams and a dry build-up beneath is a repair, not a project. Isolated damage is made good compatibly with the membrane on the roof, and the useful question afterwards is what caused it: a roof punctured by whoever services the plant will be punctured again next year unless something changes about access.

Where repair stops being honest is when the same roof keeps producing new failures in different places. Seams opening in several bays, widespread strain at upstands, or a membrane that has gone brittle underfoot are signs of a covering nearing the end of its service rather than an unlucky roof, and continuing to patch it is simply buying the same winter twice. That is the point at which the choice between refurbishing and renewing deserves a proper answer.

When a liquid overlay genuinely works

Overlaying an existing single ply roof with a cold-applied liquid waterproofing system, HYDRONYLON in our case, is attractive for the obvious reasons: the old membrane stays where it is, nothing is stripped, the deck is never exposed and the building underneath carries on working. Applied wet, the new layer runs over seams, around fixings and into details as one continuous surface, which suits a roof whose weakness is precisely its joints.

It works when the conditions are met. The existing membrane has to be firmly held down, sound rather than brittle, cleanable to a state something will bond to, and dry all the way through the build-up beneath. Adhesion is proved on the roof itself, on the actual membrane, before the specification is finalised: the material is identified, prepared as the system requires and tested, because two roofs that look identical from the ground can behave differently under preparation.

And when it does not

There are single ply roofs no overlay should go near, and saying so costs us work occasionally. Membranes that release plasticiser towards the surface may refuse to hold anything applied over them, or appear to hold and then let go later, which is worse. Some surfaces cannot be cleaned up to a state anything will stick to, however long is spent on them. A loose-laid or ballasted roof that was never fastened down does not become a candidate because somebody has swept the ballast off. And a membrane already shrinking hard at its perimeter is telling you it is under tension, which does not stop when something is laid on top of it.

Above all, insulation that has gone wet under the membrane changes the question entirely: the sheet is no longer what is wrong, and an overlay would shut the moisture in. The answer then is taking the roof off and rebuilding it, and we will price that instead. Which of these applies to your roof is established by a condition survey, not by the visit that quotes the work, and whatever guarantee attaches to the specified route carries is put down in writing beside the price.

Frequently asked questions

Do you install new single ply roofs?

No. The systems we lay are reinforced bituminous membrane, torch-on felt, and liquid waterproofing applied cold, and those are what we are trained and equipped for. Single ply is a different discipline with its own welding equipment, its own manufacturer approvals and its own detailing, and claiming competence we have not built would be dishonest. We survey single ply, advise on it, repair it and overlay it, and where a new single ply roof is genuinely the right answer we will say that too.

How do I tell PVC from TPO on my own roof?

Often you cannot from ground level, and it matters, because the two accept an overlay differently. PVC is typically softer and more flexible to the touch and its seams look chemically fused; TPO tends to be stiffer with heat-welded seams. Confirming which you have is part of the survey, and where it is ambiguous the material gets identified properly rather than assumed.

Can any single ply roof take a liquid overlay?

No, and that is the honest headline. Suitability depends on the membrane type, its age and condition, how firmly it is held down and how it behaves under preparation and adhesion testing on the day. Some membranes take an overlay readily, some need specific preparation, and some should simply be replaced rather than built on.

What is plasticiser migration and why does it matter?

Some flexible membranes contain plasticisers that keep them supple, and over the years those can move towards the surface. Anything applied on top may then struggle to hold, or may hold at first and let go later. It is one of the reasons adhesion is tested on the actual roof rather than assumed from the material name.

What about EPDM rubber roofs?

We survey EPDM and advise on it, but it sits outside this page on purpose: it is a rubber membrane rather than a thermoplastic and it behaves differently under an overlay, so whether one suits a given EPDM roof is a question settled on site with the roof in front of us, never on a web page.

Single ply roof past its best?

Tell us roughly when it went on, what the seams look like now and whether anything is standing on it. We will take it from there.