Can You Overclad an Asbestos Cement Roof?

Why overcladding is allowed and when it makes sense

Asbestos cement sheets are most dangerous when they are broken, drilled or abraded, because that releases fibres into the air. Left whole and sealed, they are a low risk. That is the reasoning behind overcladding: it avoids the disturbance of stripping, gives you a new roof, and keeps the cost and disruption of removal down. The HSE task guidance on small AC structures lists overlaying as an option, and its removal advice only applies where the sheets can be taken off carefully and are not broken up. Dropping a roof in pieces is the case that turns the work into notifiable non-licensed work, so the wish to avoid that is another good reason to leave the sheets alone.

It makes most sense on a large industrial or agricultural roof with a long span, where removal would mean weeks of controlled work, a skip lorry and a closed building. If you are weighing the general approach for a business premises, our guide to overcladding a commercial roof covers the principle for every substrate.

What the old roof has to be able to do

An overclad roof is not a cosmetic job. The old sheets and the purlins beneath them must carry the new layer, because a lightweight sheet or insulated panel system still adds load. An engineer or an experienced surveyor looks at the purlins, the fixings and the sheets themselves. If the sheets have cracked across the profile, if corners have crumbled, or if the roof has moved, overcladding is the wrong choice and the sensible route is planned removal by the right people.

Fragility matters as well. Old AC sheets cannot take a person's weight, and falls through roofs of this kind cause serious injuries. Work at height rules apply to everyone on site, and safe access means crawling boards, edge protection and netting beneath. Read our page on fragile roofs and rooflights before any access is arranged. A roof with rooflights that have faded or yellowed is especially risky because the lights can look like the rest of the roof.

How the new roof is fixed above it

Plan the order of work before the first fixing goes in. The survey should map every sheet, every rooflight and every gutter, because the gutters on an AC roof are often asbestos cement as well, and they need a decision of their own. Scaffold, edge protection and netting go up first, and nothing is dragged across the old sheets. Waste from the work, such as offcuts of the new roof, is bagged and removed so loose debris never sits on the old surface.

The usual system uses new purlins or spacer brackets fixed over the old roof, with a lightweight sheet or insulated panel on top. The skill is in the fixings. Pushing screws through the old sheets would create dust, so the work is planned to avoid drilling the asbestos wherever it can, and where fixings must pass through it a risk assessment sets out the controls, the trained operatives and the clean-up. A good scheme also solves what the old roof did badly: insulation, condensation and the rooflight layout. Because the new roof adds a gap, a vapour and ventilation plan is part of the design.

Building control will normally be interested, because a new roof covering on a non-domestic building is renovation of a thermal element, and the insulation standard has to be considered. Your building control body can confirm what it needs to see.

What stays your responsibility afterwards

Overcladding hides the asbestos, it does not remove it. The duty to manage asbestos in non-domestic premises carries on, so the roof should stay on your asbestos register, with a note that the AC sheets are sealed beneath the new roof. Anyone working on the new roof later, and anyone demolishing the building, has to be told. Removal is only postponed. It is still the likely end point of the building's life, and the next owner should know about it. Our page on asbestos cement roofs on commercial buildings sets out the duty to manage and the options.

We start with a free survey and photograph the roof, but asbestos identification and sampling are for a specialist surveyor, and we say so. Where overcladding is the sensible route, you get a fixed written quote. Where the sheets are past it, we say that too. See our commercial roofing in Basingstoke service for the wider picture.