Many garages built in the middle of the last century still wear their original corrugated cement sheets, and many of those sheets contain asbestos. When the roof starts to leak or crack, an owner reaches for the obvious saving: leave the old sheets where they are and fix a new covering over the top. This is called an overlay, or over-sheeting. It can be a sensible answer on the right roof, and a poor one on the wrong roof. It is not a loophole, and it does not make the asbestos go away. It sets the problem aside for a later owner or a later year. This guide explains when over-sheeting is on the table, what condition the old sheets and the frame need to be in, how the fixings are handled, and the drawbacks you should weigh against full removal. It cannot replace a survey, since the answer depends on the roof you have.
What an overlay is, and what it is not
An overlay places a new roof covering on top of the existing one, leaving the old material in place. On a garage with corrugated asbestos cement sheets, the new covering is usually profiled metal, often a box profile or a composite panel, carried on its own supports above the old roof. The old sheets stay as a hidden layer.
It is different from encapsulation, which is a coating or sealant painted onto the old sheets to bind the surface. It is also different from removal, in which the old sheets are taken off whole and sent for disposal. An overlay changes the weather surface without touching the old sheets, apart from the fixings, and that detail turns out to be where most of the risk sits.
The term "allowed" needs care too. We are not aware of a rule that forbids an overlay as such, and we do not give legal advice. What exists is a set of practical conditions, and a framework around work on asbestos. Under HSE guidance, lifting asbestos cement sheet off carefully and intact counts as non-licensed work. Work that breaks the sheets into pieces and makes significant dust and debris is notifiable non-licensed work. An overlay that breaks up or badly damages the old sheets can drift towards the second category, which is why the method matters.
Your choice turns on how the roof stands today, how long you expect to own the garage, and how you feel about leaving the material in place. The rest of this guide takes each of those in turn. If you are not sure the sheets are asbestos at all, start with a test, because the answer changes every option.
When over-sheeting can be a reasonable option
An overlay becomes a realistic choice when several things line up.
- The old sheets are sound. They are not cracked through, not crumbling at the edges, and not so weathered that the surface is flaking. A roof that has only a slow drip at a lap is a better candidate than one with holes.
- The frame is healthy. The purlins and rafters are straight and dry, not rotted or rusted, so they can take the weight of a second roof.
- A method exists that avoids damage. The new covering can be carried on supports and fixings that do not break up the old material.
- The walls can carry the extra load. Two roofs weigh more than one, even if the new one is light steel.
- The garage will stay in the family for a while. The saving is real, but it is partly borrowed from the future.
Typical cases are a long-term detached garage where the cement sheets are in reasonable shape and the aim is to stop a leak at low cost, or a row of lock-ups where taking the whole roof off would close access for weeks. In those, over-sheeting with a lightweight metal covering can work. It is also considered when removal is hard, for example on a roof with a poor eaves that cannot be reached safely.
It is rarely right for a roof that is already broken. Sheets that have cracked or slipped, a roof with large holes, or one that has been walked on and pitted, is in a state where any fixing risks making more damage. In those cases, the choice usually moves to controlled removal and replacement.
Only a survey can place your roof on one side or the other. We look at the sheets from below and above where it is safe, check the frame, and tell you whether an overlay is a fair option or whether it would be a poor use of money.
How the new covering is fixed
The fixing method is the heart of the matter. A new metal covering has to be held down in a way that resists wind uplift, and wind lifts a light roof hard. HSE task sheet a14, which covers asbestos cement sheets, says the new material should be attached to the existing purlins and that drilling through the asbestos cement should be avoided. In practice that means rails or brackets that take their hold from the purlins themselves, often clamped to the purlin edges, with spacers that raise the new sheets clear of the old ribs. Each hole in a cement sheet is a disturbance of the material and a point at which dust can be made, so the aim is to make none.
Where a hole truly cannot be avoided, the same task sheet points to its companion sheet a9 on drilling asbestos cement, which sets out how to keep the dust down. Whatever the method, it has to be planned for the specific frame, and none of it works on a roof where the purlins cannot be reached.
Wind uplift is the other test. The frame under the old sheets was designed for the old covering. A new roof of light metal sheets resists the wind only if its fixings reach sound timber or steel, and those fixings are the ones that tend to be pulled out when a storm gets under a loosely held sheet.
If a method calls for the old sheets to be broken, cut or removed in part, the job moves into the area of notifiable work, and the question becomes whether to remove the whole roof instead. We lay that out clearly in a survey, and we do not recommend a method that could create significant dust and debris on a roof we have seen to be fragile. The same questions arise on sheds, workshops and car ports, and our guide to outbuilding, workshop and car port roofs deals with those buildings.
The drawbacks you accept
An overlay buys time and savings, and in return you take on several limits.
The asbestos stays
The cement sheets remain part of the building. Removing them later still costs money, and by then the new covering sits above them and must be taken off first. Disposal does not get cheaper with time. The overlay can end up adding the cost of a second roof to the cost of removal rather than replacing it.
You cannot see what is happening below
With the old sheets covered, you lose sight of cracks, damp patches and rusting fixings. A leak in the new layer can run along the old one and show up well away from its source.
Condensation can form in the void
Two layers with a gap between them create a place for damp air to meet a cold surface. Without ventilation the void collects moisture, which can rot the timber and corrode the fixings. The way condensation is managed on a metal roof has its own section in the box profile sheets guide.
It reduces your options later
A garage roof with a layer of asbestos beneath a layer of metal is harder to convert, extend or insulate. A buyer or a surveyor may question it, and you should be ready to explain what is on the roof and what was done.
It does not add structure
An overlay does not repair a rotten purlin or a cracked wall plate. If those are failing, the work covers up the problem.
Overlay or removal: what each costs and what each leaves
Cost is the reason most people ask about an overlay, and the numbers are worth setting out carefully. MyBuilder's asbestos removal guide (2026) reports removal averaging about £2,000, with general removal at £50 per m², as typical UK figures. MyBuilder's garage roof guide (2026) gives £2,500 to £4,500 for a pitched garage roof, £700 to £1,000 for a single flat and £900 to £1,400 for a double flat. These are national ranges, so they show the scale, not your quote. An overlay avoids the removal line today. It does not avoid the new covering, the frame work or the fixings, and it may add spacers, extra framing and a second layer to dispose of later.
Removal does the opposite. It costs more now and the roof is clean afterwards. The old sheets are taken off whole where possible, wrapped and sent to an authorised waste site, and the new roof goes onto the frame directly. You can see the purlins, replace the ones that have gone, and fit proper ventilation.
A fair way to compare is to count the lifetime cost. If you expect to keep the garage for many years, an overlay now followed by a removal later may total more than a single removal today. If you plan to sell soon and the roof is in decent shape, the overlay may be cheaper in the short term but may raise questions from a buyer.
Our written quote lists both routes where both are possible, so you can see where each pound goes. The lower figure is not always the better one, and the fixed price comes once we have been on the roof.
Checks before you choose an overlay
Before committing, work through these questions.
- Is it asbestos? Do not assume. Corrugated cement sheets from some periods are asbestos cement and from others are not. A sample test or an inspection settles it.
- What condition are the sheets in? Look from inside the garage for light coming through, cracks along the ridges, and sagging between purlins.
- What is the frame like? Rotted or rusted purlins make an overlay pointless.
- How will the fixings pass the old sheets? Ask for the method in writing. A roofer who cannot explain it should not do it.
- Who deals with the waste? Any offcuts or damaged pieces of asbestos must be handled and disposed of correctly.
- What do planning and Building Regulations say? A change of covering can be a re-cover, but the weight rule applies and a listed building or conservation area brings extra checks. The wider picture sits under garage roof replacement. Our guide to garage roof planning covers the permissions side.
- What does your insurer need to know? Tell them what is on the roof and what you are doing.
The HSE sets out the position on non-licensed work with asbestos cement in its guidance on non-licensed work, and you should read it before you let anyone drill into old sheets. A roofer who dismisses the issue is a warning in itself.
If the answers point to removal, we price that. If they point to an overlay, we price that, and we say what you are leaving behind. The workmanship is covered by our 10-year written guarantee, and we begin with a survey that records your own roof.