An overlay leaves the existing roof in place and fits a new weatherproof layer above it. On a large industrial unit or warehouse the attraction is obvious: no stripping, no skips, no open roof over stock and machinery, and a shorter programme. The risk is just as clear. A new layer does not repair what is wrong underneath, and a roof that is wet, rotten or overloaded stays that way beneath its fresh surface. The honest answer to whether overlay beats stripping is that it does for some roofs and not for others, and the dividing line is the condition of the existing sheets, the insulation, the fixings and the structure. This guide covers the two common cases, profiled metal sheeting and built-up felt, sets out the checks that decide the question, and describes what each approach involves. It belongs with the rest of our commercial roofing guidance.
Overlay defined, and how it differs from a coating
The word covers several methods, so it helps to be exact. On a metal roof, an overlay typically fixes a new set of sheets over the old ones. Because the old profile is rarely flat enough to carry the new sheets directly, support brackets or sub-frames are fixed through the old sheets to the purlins beneath, and the new sheet is clipped to that framework. Insulation can be added in the gap, which is how an overlay can also improve thermal performance. On a felt roof, the overlay is a new membrane, such as single-ply or another bituminous layer, laid over the old surface once it has been prepared.
Do not confuse it with a coating. A coating is a liquid-applied or sprayed layer that seals and protects a sound surface, and it suits roofs that are tired but intact. An overlay is a new roof covering with its own structure and fixings. Nor is it a cover-up for a failed roof: if the existing sheets are rusted through, if the felt is saturated, or if the purlins cannot take the new fixings, nothing laid on top will last.
Overlay is also a recognised part of how industrial roofs are renewed in practice, not a shortcut. It is a specific tool with limits, and a survey exists to find out whether those limits have been reached.
Why an overlay can beat stripping
When the conditions are right, there are real advantages, and they explain why building owners ask about it.
The building stays in use
Stripping a roof opens the building to the weather. Production lines, stock and offices below are exposed to any shower that arrives faster than expected. With an overlay the old roof acts as a temporary cover while the new one goes on, so the exposure is far smaller and the business can often continue trading.
No waste stream for the old covering
Stripping produces a large volume of waste that must be taken away and disposed of properly. The cost guides we use list disposal as a distinct item in a roof replacement, and on a big roof it is a significant one. An overlay avoids most of it.
Asbestos is left undisturbed
Many older industrial buildings have fibre-cement sheets that contain asbestos. Under HSE guidance, taking off asbestos cement sheeting carefully and intact falls in the non-licensed category. Once the sheets are going to be smashed up and release heavy dust, the job becomes notifiable. A sound sheet left in place under a new overlay is not being broken up at all, which can make the overlay the safer and quieter route. How the new roof is held down still matters. HSE task sheet a14 advises fixing the new material to the existing purlins and keeping drills out of the asbestos cement, and where a hole cannot be avoided it refers to sheet a9 on drilling asbestos cement. The detail needs specific advice, which is covered under asbestos in industrial roofs. We never describe our work as licensed.
A shorter programme
With no strip, the weather window the job needs is smaller, and in a wet winter that matters. An overlay does not remove the need for planning, but it shortens the period of risk.
The tests an existing roof must pass
These are the checks we make before recommending an overlay. Failing any one of them tips the decision towards stripping.
- Is the covering sound enough to carry the work? Metal sheets must be free of through-rust and holed panels at the fixings, since new brackets pass through them. A felt surface must be firmly adhered and not blistered across large areas.
- Is the insulation dry? Wet insulation under felt will stay wet under an overlay. We take samples or use moisture surveys to find out. Under metal sheets, look for staining and signs of condensation on the underside.
- Can the structure take the load? An overlay adds weight, and so do its brackets and framing. A new covering that raises the roof's weight by 15 per cent or more needs Building Regulations approval and a structural check, according to the Planning Portal. For lightweight steel-framed units that margin is easily used up.
- Are the fixings sound? Corroded fasteners and failing purlin connections cannot be built on.
- Is there room at the edges? Gutters, parapets, rooflight kerbs and upstands all have fixed heights. A new layer raises the roof surface, so each of these details must be reworked to suit.
If every item passes, overlay is a credible option. If one fails but can be cured, such as a handful of rusted sheets that can be replaced first, it may still be the better route. If several fail at once, stripping is the better course, and that is what we recommend.
Overlaying profiled metal roofs
Profiled steel sheeting on industrial units has often served for a long time. Typical published life for plastisol-coated steel sheet is 15 to 25 years to first maintenance, and in excess of 40 years overall in a typical environment, if installed and maintained correctly. Sheets near the coast or in harsh industrial air wear faster. Those figures help explain why so many metal roofs reach the stage where they leak at laps and fixings but are otherwise sound, which is the stage at which an overlay makes sense.
The commonest weakness is cut-edge corrosion, where the coating has failed along a cut edge and rust spreads under the paint. It starts at the laps, the eaves and the penetrations. Our guide on cut-edge corrosion explains it in detail. Light corrosion can be treated and sealed, but once the steel is losing thickness the sheet no longer holds a fixing, and overlaying it simply transfers the problem.
A metal overlay involves several steps in order. The old roof is cleaned and surveyed in detail. Defective sheets and fixings are replaced. Support brackets are set out and fixed through to the purlins, with the spacing confirmed by calculation for the wind load. Insulation is laid in the cavity, and the new sheets are fitted, with new flashings and ridge details. Rooflights are dealt with as a separate item, since their kerbs must be raised and their glazing is a common leak point.
Wind uplift matters on big roofs. The fixings are the part that resist it, and a poorly fixed overlay is a hazard, not a saving. We specify fixings and spacing for the building and its exposure, not by habit.
Overlaying felt and flat commercial roofs
On a flat roof the overlay question is different, because the thing that fails is usually the membrane, and the thing that decides the answer is the moisture in the layers below. If the existing built-up felt is dry, adhered and free from large blisters, a fresh membrane can be laid above it once the surface is cleaned, loose areas are cut out and patched, and blisters are slit and dried. A new layer of single-ply, bituminous felt or similar is then installed, with fresh details at upstands and outlets. Our guide on built-up felt roofing sets out how to tell sound felt from tired.
If the felt is wet, the case for overlay disappears. Trapped moisture in insulation beneath will turn to vapour in warm weather and lift the new membrane, and the insulation itself loses much of its value. Wet material must come out. A partial approach is sometimes sensible: strip only the saturated areas, replace the insulation there, and overlay the rest. A moisture survey tells you where the line falls.
Compatibility is a technical point worth getting right. Not every new membrane can go over old bitumen. Some single-ply products need a separating layer, and some adhesives attack bitumen. The manufacturer's guidance for the specific system decides the build-up, and we follow it.
Building Regulations are relevant again. Re-covering a roof is normally outside approval when it affects less than 25 per cent of the building envelope and less than 50 per cent of the roof. A whole-roof overlay will pass those limits, and an insulation upgrade can become part of the scope. Overlay offers an opportunity here, since new insulation can often be added on top of the old build-up as part of the system.
Edges, drainage and the details that make or break it
Most failures of overlay roofs begin at the details, not on the open surface. Raising the roof by the thickness of a new layer changes every junction, and each must be reworked with care.
Valley gutters are the first example. On an industrial unit with a twin-span roof, the valley gutter between the spans is the most heavily loaded part of the roof, because it collects the water from both slopes. If it is already near the end of its life, an overlay over the slopes leaves the gutter unaddressed, and it fails again within a few seasons. Our note on industrial valley gutters deals with the options for lining or replacing them, and the decision about the gutter is best made before the overlay is chosen, not after.
Rooflights and smoke vents, as noted, need raised kerbs. Parapets need new cappings and flashings to match the new height. Rainwater outlets must be lifted or replaced so that water still drains freely. Where the roof meets a wall, flashings must be reformed.
Falls matter on flat roofs. The flat roof standard, BS 6229:2018, asks for a finished fall of 1:80 or better, and an overlay cannot correct hollows in the old surface unless tapered insulation is used. If the existing roof ponds, the new membrane will pond as well, and standing water shortens its life.
On listed or historic commercial buildings, an overlay changes the profile and the height of edges, and it may change the appearance. The consent position has to be checked first, as the guide to listed commercial roofs in Bath sets out. For a typical modern unit, those constraints do not arise.
Comparing systems and getting a price
It helps to see the options side by side before choosing. Our overview of commercial roof systems describes the main families, and the table below summarises how the routes compare in principle.
| Route | Suits | Main risk |
|---|---|---|
| Repair | Isolated failures on a sound roof | Repeat visits if the cause is general |
| Coating | Intact but weathered surfaces | Hides faults if the surface is not sound |
| Overlay | Sound structure, dry insulation, adequate edge heights | Building on an unseen defect |
| Strip and replace | Wet insulation, failed sheets, overloaded structure | Exposure during the works and disposal cost |
No tidy price list exists for any of this. The only commercial figures we can cite come from a single low-confidence contractor answer on MyJobQuote (July 2026): torch-on at £80 to £120 per square metre, EPDM at £90 to £130, single-ply PVC or TPO at £100 to £140, with a further £20 to £40 per square metre if fresh insulation is required. Read them as national typical ranges, not as what we would charge, and they leave out brackets, framing and edge work. Pricing for your building follows the survey.
The survey covers the roof, the underside where we can reach it, the gutters and the details. It ends with a recommendation between repair, overlay and strip, with reasons and photographs of your own roof. Where overlay is the right call, the written price lists the framework, insulation, new covering, details and access. When the answer is different, the report says that instead. Workmanship is covered by our 10-year written guarantee.