Plenty of Stroud garages still wear a corrugated asbestos cement roof from the mid-twentieth century. The sheets are grey, mossy and often sound. Owners ask a fair question: if the roof is intact and nobody is touching it, why pay for removal when a new profiled sheet could simply be fixed on top? That is overcladding, sometimes called over-sheeting or overlaying.
It is a legitimate option in some cases and a poor one in others. This guide sets out how an overlay is built, which buildings suit it, where it creates problems for the next owner and what the alternatives cost. The rules on handling and removing the material are in our guide to asbestos garage roofs and the HSE rules, which this page does not repeat. For the service as a whole, see garage roof replacement.
What overcladding an asbestos roof involves
An overlay is a second roof built above the first. The old corrugated sheets stay put and act as a deck. New profiled metal sheets, usually box-profile or a coated steel sheet with an insulated backing, are carried over them on fixings that reach the purlins, the horizontal timbers or steel sections the old sheets are bolted to. The new roof sheds the rain, and the old one sits underneath it.
Two features make the job different from an ordinary re-cover. The first is that the new sheets must be supported without disturbing the old ones. Drilling through asbestos cement is the very thing that careful work tries to avoid, so the way the new roof is held up has to be worked out in advance. The second is that the old sheets are not simply buried: the roof edges, gutters, flashings and any rooflights all have to be rebuilt around a roof that is now thicker than it was.
Why people choose it
The attractions are plain. There is no stripping, no waste from the old sheets to bag and carry away, and a shorter spell with the roof open to the weather. For a garage that stands apart from the house and holds only a car and some tools, the sums sometimes favour it.
What it does not do
An overlay does not remove the asbestos. It hides it. Every later job on that roof, from a gutter replacement to a demolition, will have to deal with a roof that has asbestos cement under a newer skin.
When the sheets are sound enough to cover
Overcladding only makes sense if the old sheets are in good order. A survey looks for the following, working from inside the garage as well as from the outside.
- Cracks, particularly along the lines of the fixings and across the corrugations.
- Sheets that have sagged between purlins, which suggests the supports are weak or the sheets have lost strength.
- Corners that have crumbled or broken edges at the eaves.
- Heavy moss and lichen, which holds water against the surface.
- Sheets that rock, which means the bolts or hook fixings are loose or gone.
A roof that fails on these counts is not a candidate. Fixing new sheets on top of cracked, loose material puts people on a brittle surface and invites the exact breakage that turns routine work into something much more serious. HSE guidance draws the line between two kinds of job. Weathered cement counts as non-licensed work, provided the handling is careful and the sheets stay whole. Cement that is substantially damaged, or that a removal would shatter, falls into the notifiable category instead. Our HSE guide explains both categories.
If a sheet is already broken, nobody should walk on it, and nothing should be fixed to it. Leave it alone and get it assessed first.
Who tests what
Not every grey roof is asbestos. Where the material is uncertain, a sample goes to a laboratory. MyBuilder's 2026 asbestos guide gives £50 as the starting figure for a basic survey and £200 to £245 for a fuller one. A basic survey is usually enough to say whether a roof is asbestos cement; the fuller kind is for a building that is being altered or demolished.
The load and the structure underneath
Asbestos cement sheets are light compared with tiles, but an overlay adds a second layer. The new sheets, their backing, the fixings and any spacers all weigh something, and the garage's purlins and rafters were sized for one layer. The Planning Portal lists a new covering that would raise the roof covering's weight by 15 per cent or more as needing approval, and Approved Document A calls a load rise of more than 15 per cent significant. The two wordings differ slightly, so ask your building control body which applies to your job.
Whether an overlay crosses that line depends on the sheets chosen. A lightweight coated steel sheet adds modest load; a heavy composite panel adds more. Both go on top of whatever moisture, snow or maintenance loads the roof already carries. A purlin rotted where it sits on the wall, or a steel one rusted at its end, can give way under a second layer while the sheets above it still look fine.
Fire performance
The Planning Portal gives a second trigger: a covering whose fire performance departs markedly from the existing one. Metal and asbestos cement are different materials, so we raise that with building control before any sheets are ordered.
The check we make
Before pricing an overlay we look at the supporting structure from beneath, because the old sheets hide the purlins from above. A structure that cannot carry the extra layer rules out the overlay however sound the sheets look.
Condensation, access and other drawbacks
A metal sheet laid over an existing roof traps a layer of air between the two. In a garage that stays cold, little happens. Sheds and stores that are never heated are the best candidates for this reason. In a garage with a workshop, a damp store or a warm room below, moisture can condense on the underside of the new sheet and drip into the gap, or seep through the joints of the old roof into the building. Anti-condensation backing helps. It does not abolish the need to think about moisture, and a sealed void between two roofs is hard to inspect afterwards.
Fixings and flashings
Every fixing that passes through the new sheet has to find a purlin, and each one is a potential leak. Where the new roof meets a wall, the old flashing sits in a different place from the new one. Edge details are cut around the thicker build-up, and the finished roof can end up with a heavier profile than the building was designed to show.
Maintenance and the next owner
The old sheets do not disappear. A buyer's surveyor who opens the hatch will see them, and the buyer will want to know about the asbestos. Our answer on garage roofs and Building Regulations explains why paperwork matters, and the point holds here: hold on to the survey report, the invoice and a set of pictures that show what lies beneath the new sheets.
Sooner or later the overlay wears out as well. The eventual job is then larger, since two roofs have to come off and one of them is asbestos. The cost of that day is only postponed.
Removal and replacement: the other route
The alternative is to take the asbestos roof off and fit a new covering in one operation. HSE describes weathered but not substantially damaged cement as non-licensed work, provided it is handled with care and kept in one piece. The sheets are unbolted rather than smashed, lowered by hand and wrapped for disposal, never dropped. If a removal would break the material up, it moves into notifiable non-licensed work, which brings notification to the enforcing authority, medical examinations and a register of work.
On price, MyBuilder's 2026 asbestos removal guide gives a typical average of about £2,000, with the per-metre rate quoted as beginning at £50 per m². For a single garage roof, removal alone runs between £350 and £1,295, and for a double garage between £650 and £1,800. Those figures cover the removal, not the replacement roof. Garage roof costs for the new covering are set out in our costs guide, and a pitched garage roof is given at £2,500 to £4,500 in the same MyBuilder guide.
Pricing an overlay
What an overlay costs depends on the sheet profile, the fixings, the condition of the existing roof and the access, so it is fixed after the survey. The quote in writing sets out the overlay and the removal route as separate options, so you can compare them side by side.
The benefit of removal is simple: when it is finished, the problem is gone, the structure can be inspected and repaired, and the new roof can be insulated and ventilated properly. Rafters and purlins that were hidden under the old sheets can be checked, and any that have rotted or rusted can be repaired or replaced while the roof is open, which an overlay cannot offer. For a garage that may one day become a workshop or a room, that access is worth weighing.
Making the choice for your own garage
Only a handful of points decide it. We would think about overcladding when the building is a detached, unheated store, the sheets are sound with no cracking or movement, the supporting purlins pass inspection, the owner expects to keep the garage for years and understands the asbestos stays. We would lean towards removal when the sheets are damaged, when the garage is being converted or heated, when the property will be sold or extended soon, or when the structure underneath needs repair that cannot be done through a closed roof.
Protected buildings
An outbuilding in a listed house's grounds, or one in a conservation area, may carry extra constraints. The character of the existing roof can matter there, and a visible modern profile may not be acceptable, whatever its other merits. Our guide to outbuilding roofs in conservation areas covers how to check the status with the council first.
What we do on site
You get pictures of the roof from inside and from the eaves, plus a short written note of what we found. Quotes for either route follow, covering scaffold, preparation, materials, labour, disposal and making good. A 10-year workmanship guarantee goes with the finished work, in writing. Whatever you decide, do not let anyone clean, drill or pressure wash the old sheets before the position is clear.