The short answer
Roof overcladding means fixing a new covering over the existing roof instead of stripping it. On an old industrial or farm building, new profiled or insulated sheets are carried on support rails above the old sheets. It avoids removing asbestos cement where that is present, but the old roof must be sound, the structure must carry the extra load, and the old layer stays on the building.
It is a common answer to an old fibre-cement roof. Whether it fits depends on the frame, the old sheets and your plans.
How overcladding is built
The old sheets stay where they are. New support rails or brackets are fixed down through them into the steel or timber purlins that carry the roof. A new layer, whether a profiled sheet, a built-up system with insulation or an insulated panel, is then fixed to those rails. The old roof becomes a hidden layer under the new one.
Why is it so often used on asbestos cement?
Because it leaves the asbestos cement undisturbed. HSE sorts careful removal of whole sheets into the non-licensed category and substantial breaking-up into notifiable non-licensed work, so leaving sound sheets alone avoids both the removal and the waste. The result is a new weathertight roof above the old one, usually quicker than a strip.
The fixings are the risk point. Drilling through old sheets is work on asbestos cement, so the method has to be chosen with that in mind and agreed with the person who surveyed the roof. A good survey records which sheets are intact, which are cracked and where the purlins sit, so the rail positions can be planned before anyone climbs up. It also notes the condition of the gutters and the eaves, because new sheets finish at those edges and any weakness there shows.
When it makes sense
Overcladding suits a roof whose old sheets are intact, whose purlins are sound and whose building can take the added weight. It suits an owner who wants to carry on trading during the work, because the building is not opened up. It also suits a building whose roof is a fragile surface, as HSE says asbestos cement roofs always are, since the crew works from above with the interior left sealed.
What does the structure need to carry?
The new layers add load. Building Regulations approval is a question once the new covering adds 15 per cent or more to the load the roof carries, and the Planning Portal says a structural engineer or a surveyor ought to be brought in before the work starts. In Taunton the building control body is Somerset Building Control, and ringing them early avoids a surprise later in the programme.
Where rust, sagging or broken sheets have already damaged the old roof, the support rails sit on weak ground and overcladding becomes a poor choice. It covers a problem rather than curing it, and the problem keeps working underneath.
Careful removal and the HSE categories
The alternative is to take the old roof off. HSE treats sheets of asbestos cement that are unscrewed carefully and stay in one piece as non-licensed work, even where the sheets are weathered, provided they are not substantially damaged. Work that will break the sheets up becomes notifiable non-licensed work, and HSE's own example is dropping a roof.
Is removal ever the better answer?
Yes. If the structure cannot carry another layer, if the old sheets are cracked and fragile, if you plan to alter the building, or if a later owner will want a clean roof, removal ends the problem for good. It leaves the building open for longer and needs a waste route, since the sheets are wrapped twice, labelled and carried under a consignment note.
Taking out asbestos insulating board, such as soffits, is often licensable work, so a survey checks it first. Where a unit has both sheets and board, the two can fall into different categories, and the plan has to treat them separately, with the person who does each task named in the method statement before anyone starts. Our page on sheet disposal covers the waste side in more detail, and the two routes are best compared once you know what the waste leg involves: the number of sheets, how they are wrapped, how they are carried and where they end up.
Questions to settle before you choose
Start with a survey that identifies what the old sheets are. HSE says a building from before 2000 should be presumed to hold asbestos, so the sensible default for an older unit or barn is to test before anyone drills. Our page on fibre-cement roof sheets explains how that works.
What else changes the decision?
Rooflights, ridge details and eaves all need new flashings and closures over the old profile. Insulation requirements may dictate the depth of the new layer. A roof that is already near the end of its useful life is cheaper to overclad than to strip, but you carry the old layer into the next stage of the building's life.
Our survey ends with pictures of the building and one written price for whichever route suits the roof, overclad or strip, with the reasons given in plain words. For a comparison with a one-pass system, see insulated roof panels, and for the whole service see commercial roofing. To begin, send photos of the roof and the age of the building.
Overcladding questions
Does overcladding need Building Regulations approval?
It can. The Planning Portal lists two triggers: a covering that raises the weight by 15 per cent or more, and one that would perform significantly differently if fire broke out. Somerset Building Control can say if your own job is notifiable.
What happens to the old roof later?
It stays beneath the new layer. A future owner or contractor who removes the new roof will find the old sheets underneath, so keep the survey report and the roof record with the building's papers, and pass them on when the property changes hands or is let.
The full picture: Commercial roofing in Taunton ยท More short answers