Most owners of a tired flat or low-pitched roof ask the same thing early on: can the new covering simply go over the top? Sometimes it can. A unit that has to keep trading, a roof with a sound deck and dry insulation, or an asbestos-containing sheet that is better left undisturbed can all point towards an overlay, which some trades call a recover or over-roofing.
This guide explains the decision as we make it: what the existing roof has to prove, what the extra layer adds in weight and thickness, how the regulations treat it, and the signs that tell us to strip instead. It sits beside our pricing guide to commercial roof costs in Stroud, and both belong to the wider commercial roofing service.
What an overlay is, and what it is not
A strip-and-replace job removes the old covering, lets the insulation and deck be inspected or renewed, and then puts down a fresh build-up. In an overlay the existing build-up stays. A new membrane or sheet system is laid across it, usually with a separating or levelling layer between old and new, and the edges, upstands and outlets are reworked to receive the added thickness.
That is different from a repair, which treats a defect in one place, and from a coating, which paints a protective skin onto a surface that is already sound; our guide to commercial roof coatings covers that route. An overlay is a complete new waterproofing layer, so its performance depends on everything beneath it. If the old deck is soft, the new layer sits on a soft deck. If the insulation is saturated, the new layer traps the water.
Where the idea comes from
The appeal is plain. No strip means no exposed building during the work, less waste carried off, and a shorter programme. MyBuilder's 2026 re-roof guide lists disposal at £100 to £1,000, and that line, with the labour to uncover and re-lay, is what an overlay can reduce. Whether it is the right answer comes down to the evidence, and the evidence comes from the survey.
The tests an old roof has to pass
We do not recommend an overlay on appearance. A roof can look fair from the ground and still fail every one of these checks.
- The deck is sound. We walk it and probe it. Flexing, spongy patches, rusted fixings or rotten boards rule out an overlay in that zone.
- The insulation is dry. This is the test most often skipped, and it matters most. It is covered in the next section.
- The falls work. Water must leave the roof. If the finished surface will hold standing water, the new layer will not cure that.
- The edges can take another layer. Upstands, parapets and rooflight kerbs need height for the new membrane to turn up and be sealed, and outlets must still sit lower than the roof surface.
- The structure can carry it. Every added layer weighs something.
- Nothing hazardous is hidden. If the old sheets may contain asbestos, that comes first.
A roof that passes all six is a real candidate. One that fails the first two is almost always a strip. The rest can often be corrected as part of the work, at a price shown in the quote.
Wet insulation: the question that decides most overlays
Flat commercial roofs commonly have rigid insulation under the membrane. If water has been getting in through a failed seam, a split or a blocked outlet, that insulation takes it up. Laid over the top, a new membrane seals the dampness in, and the board stays wet, heavy and poor at its job for years. A flat roof with a slow leak can look intact on top and be wet underneath, so the surface alone proves very little.
How we find out
A survey of the whole roof looks at seams, laps, penetrations and outlets, then locates likely wet zones. MyBuilder's 2026 inspection guide gives thermal surveys at £300 to £500, drone surveys at £200 to £400 and a standard inspection at £150 to £250. Where a thermal image or a patch of soft surface raises doubt, we cut an inspection opening, photograph the layers and measure the thickness. Those photographs go to you with the report, so you see what we saw.
What happens if some of it is wet
Wet zones are cut out and renewed to the same thickness, then the overlay runs across the whole roof. This is a hybrid: partial strip, full overlay. It is common on felt roofs that have failed at a few seams, and the quote allows for the cut-out area, so the cost is clear. If the wet areas are too extensive, stripping costs less than patching, and the report tells you.
Our page on ponding on commercial roofs explains why standing water is such a reliable warning sign.
Weight, thickness and what the regulations say
A new layer adds load. Where the existing covering is felt or single-ply, the extra weight of another membrane system is modest, but a ballasted or stone-covered roof can add more than expected. Where a roof is metal sheet and the plan is to fix a second sheet or an insulated panel over it, the load on the purlins (the horizontal rails that support the sheets) and on the fixings changes too.
According to the Planning Portal, a covering that adds 15 per cent or more to the roof's weight needs Building Regulations approval, and it recommends consulting a structural engineer or surveyor first. Approved Document A speaks of a significant change in roof loading when the load rises by more than 15%, so we quote both and ask building control which applies.
Fire performance and thermal upgrades
The Planning Portal also lists a new covering whose fire performance differs significantly from the existing one as needing approval. A change from felt to a membrane with a different fire classification falls into that question, so we check the product's classification before specifying.
Thermal upgrades are the next trap. Approved Document L for dwellings lists a new membrane on a flat roof among the renovations of a thermal element, and says the whole element should be improved when more than 50% of its surface is renovated. Other building types follow their own volume of the guidance, and we have not quoted its figures here. An overlay may or may not count as renovation for the purpose, and the answer turns on the building, so ask your building control body early. Our guide to Building Regulations and Part L for commercial roofs sets out the questions to put.
Overlaying felt, metal sheet and cement sheet
The existing covering changes the answer, so we take them one at a time.
Built-up felt
Felt roofs are the most common overlay candidates. Less puts the life of a felt roof at 10 to 20 years, and says a good job can last 20 while a poor one can go within 10. A felt roof that has simply aged, with a sound deck and dry insulation, can take a new membrane. One with blisters over wet insulation cannot. See our page on built-up felt for repair and replacement options.
Single-ply
An old single-ply membrane is a harder base because new membranes do not always bond or weld compatibly with old ones, so a separating layer is common. The manufacturer's data sheet for the new system states which substrates it accepts; we work from that rather than from habit. Our guide to single-ply membranes covers the products.
Metal sheets
Corroded profiled sheets can sometimes be overclad with new insulated sheets or have a refurbishment system fixed over them. The rust has to be stopped first, and the fixings have to hold in sound steel. See our guide to cut-edge corrosion.
Cement-based sheets
Older cement sheets may contain asbestos. HSE guidance treats careful removal without breakage as non-licensed work. Because covering the sheets avoids breaking them, an overlay is sometimes worth weighing, but only after the material has been identified. Our page on asbestos on commercial roofs sets out the steps. Fixings driven through such sheets disturb them, so the method is settled with specialist advice before any fixing goes in.
Costs, claims and leases when you overlay
MyBuilder's 2026 flat roof guide gives replacement ranges of £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM (a synthetic rubber membrane), £95 to £150 for GRP (glass-reinforced plastic, usually called fibreglass) and £95 to £140 for PVC single-ply. Those are national figures for replacement, and they carry no allowance for your access or your roof's details. An overlay removes some lines and adds others, such as separating layers, edge raising and kerb extensions. We price it on the survey, so the saving or the extra shows up in the quote itself. Our cost guide compares the whole range of work.
Insurance and later claims
If a claim is under way, do not overlay before the insurer or its loss adjuster has seen the damaged roof, because the evidence disappears under the new layer. Photographs and a written report from the survey help, and our guide to commercial roof insurance and dilapidations covers the process.
Leased buildings
A tenant who wants to overlay may need the landlord's agreement, and a landlord who overlays may recover cost through the service charge, depending on the lease. Photograph the roof layers before they are covered, because whoever inherits the roof next will want to know what lies beneath.
Guarantee
Our work carries a 10-year workmanship guarantee in writing, and any warranty from the material manufacturer has terms of its own. Check that its conditions allow the system to go over your existing roof.
When stripping is the better answer
Overlaying has limits, and good surveys find them. We recommend stripping when:
- the insulation is wet across more than a few small patches;
- the deck is rotten, rusted through or spongy over a wide area;
- a roof has already been overlaid and a further layer would exceed what the structure or the edge details can take;
- the falls are wrong and the deck has to be reshaped;
- the building control body requires the full thermal element to be upgraded and the existing build-up cannot be reworked.
A roof overlaid more than once is hard to diagnose, because leaks travel between layers and appear far from their source. At that stage the plain advice is to take everything off and begin again.
We would rather tell you that after the survey than after the overlay. The written quote can cover the strip option and the overlay option where both are sensible, which lets you weigh one against the other with figures to hand. We photograph the roof as the work goes on, and our crew takes the rubbish away at the end.