Take a tired fibre-cement or steel roof on a workshop, a distribution unit or a farm shed. The sheets leak at the laps, the fixings have rusted, and the whole thing is cold in winter and noisy in rain. Stripping it means scaffolding, waste, an open building and a long list of things to protect. Over-cladding is the alternative: a new set of sheets, usually with insulation, fixed above the old surface so that the building stays covered the entire time.
It is a sensible route when the structure beneath is sound and the old roof is not going anywhere. It is the wrong route when the structure is weak, the old sheets are rotten, or the building cannot take extra weight. This guide covers how a system is built up, what the survey has to prove, and how the cost compares with a full strip. If you are still weighing the options, our guide to commercial roof systems sets this one beside the others, and our commercial roofing page lists everything else we do for industrial and agricultural buildings.
How a new roof sits on top of an old one
The old roof is not hidden and forgotten. It becomes the deck. A set of support brackets or spacer bars is fixed through the old sheets into the purlins beneath, creating a new frame at the right height. Insulation, usually a mineral wool or rigid board, is fitted between or over those spacers. A vapour control layer or an airtight membrane sits on the warm side, and the new outer sheets are fixed above them with the usual side and end laps, sealed and capped at the ridge and edges.
The result is a new weathering layer with a new thermal layer inside it, and the old sheets are retained below as a secondary skin. Because everything is fixed back to the original purlins, the quality of those purlins decides the quality of the job. If they are bent or corroded, the new frame carries those faults forward.
What goes at the eaves and gables?
The new sheets overhang the old gutter line, so gutters usually need to be moved or replaced, and the verge and ridge details are rebuilt in new flashings and closers. This is where over-cladding often reveals hidden trouble, because the old edges have to be opened up to form the junctions.
Why owners choose it
The strongest argument is disruption. A building under over-cladding stays watertight from the first day to the last. No sheeting is stripped, so no stock is exposed, no production line is covered in tarpaulins and no rain gets in on a day when the forecast was wrong. For a unit that cannot close, that is often worth more than any saving on the roof itself.
Waste is the second argument. Stripping an old roof produces a large volume of sheeting, fixings and insulation that has to be sorted, carried off and disposed of by an appropriate carrier. Over-cladding leaves most of that material in place. Where the old sheets are suspected of containing asbestos, leaving them undisturbed can be a significant advantage, though the fixing method matters and is covered below.
The third is thermal. A new insulated layer fixed above an uninsulated roof transforms how the building behaves, cutting heat loss and condensation drips. Many owners come to us because the unit costs a fortune to heat, not because it leaks.
What the survey has to prove first
Over-cladding is not quoted from the ground. The survey looks at five things, and each one can rule the method out.
- Purlins and rafters: straight, sound and properly fixed, with no rot, serious rust or sag.
- Existing sheets: fully in place, free from large cracks or collapse, and stable enough to take the new fixings.
- Loading: the additional dead weight of insulation, new sheets and brackets, set against what the structure was designed to carry.
- Fall and drainage: whether the roof drains properly, because over-cladding preserves the shape of the old roof, including its faults.
- Moisture: evidence of condensation, wet insulation or leaks that will need resolving rather than sealing in.
Weight deserves a plain explanation. A heavier covering is a structural question under the Building Regulations. The Planning Portal says that a new covering which increases the weight of the roof by 15 per cent or more needs approval, and Approved Document A describes a rise of more than 15% in loading as significant. Over-cladding nearly always puts extra weight on top of what is there already, so a structural opinion belongs in the process, and we say early if one is needed.
Does asbestos cement change the plan?
HSE advice is to treat any building built or refurbished before 2000 as though it contains asbestos. Drilling, cutting or fixing through such sheets is work with asbestos, so the method has to be agreed before anybody climbs up. See our answer on asbestos on commercial roofs for the main points.
Where over-cladding does not work
Some roofs should be stripped. Telling you that, even when it means a bigger job for us, is part of giving advice you can rely on. The usual disqualifiers are structural. Rusted or twisted purlins, a roof that has already sagged between frames, or timber members damaged by years of leaks will not support a new layer and cannot be inspected once it is covered.
Moisture is the other common one. If the old roof has been dripping condensation onto wet insulation for years, covering it merely seals the problem in. The timber or steel rots from the inside and nobody sees it until the new roof starts to move. A wet roof is repaired or stripped, not over-clad.
Shape matters too. Complex roofs with many valleys, parapets, rooflights and plant penetrations offer limited scope for a neat over-clad, because every junction has to be rebuilt at a new height and the amount of flashing work rises quickly. On those roofs a recover with a single-ply membrane or a full replacement often costs about the same and leaves a cleaner result.
If the purlins cannot be checked and the roof cannot be kept dry while you wait, over-cladding is the wrong tool.
Insulation, condensation and building control
A new insulated layer needs thought about where moisture goes. Warm, moist air from inside a building that reaches a cold surface condenses. In an over-clad roof the cold surface is the new sheet, and the job of the vapour control layer and its taped joints is to keep that air from getting there. A layer of insulation with gaps, torn membrane or missed penetrations around rooflights and pipes is the common cause of drips in a newly over-clad building.
Approved Document L sets energy performance standards for replaced and renovated elements, and commercial buildings follow a different volume from houses. In practice, over-cladding prompts a conversation with your building control body about whether the work counts as renovating the thermal element, and that answer fixes the insulation standard the roof must reach. We raise it at survey stage, deal with the notification where the work is notifiable, and put the outcome to you in writing.
Fire performance is the third check. The new covering and insulation must not behave significantly worse in a fire than the roof they cover, which is one reason the build-up is chosen from a tested system rather than assembled from whatever is on the shelf.
Does over-cladding need planning permission?
It can, because it changes the look and sometimes the height of the building. Our answer on planning for commercial roofs explains the checks, and we recommend asking the planning team at your local council before you sign anything.
What drives the cost
The figure on your quote is one total, fixed after the survey, and we price fairly. What moves that total is easy to list: the roof area, the access needed to reach the eaves and ridge, the depth and type of insulation, the profile of the new sheets, the number of rooflights, penetrations and edge details, and any repairs that must be made to the old roof before the new one goes on.
Compared with a strip and replace, over-cladding generally saves on removal, waste carriage and temporary protection, while spending more on brackets, spacer systems and the insulation build-up. Whether the balance tips in its favour depends on the building. For the wider picture on how commercial roof costs are built, our guide to commercial roof cost sets out the usual drivers, and a barn or farm building follows similar logic, covered in barn roof replacement.
One thing to check on any quote, ours or someone else's, is whether VAT and scaffolding are included. Over-clads on tall buildings can need substantial access equipment, and that is the line most often missing from a first price.
What about a coating instead?
Where the sheets are sound and only the surface has failed, a coating may do the job for less. Our guide to commercial roof coatings explains when that works and when it only delays the problem.
Sequence on site and what you will see
A typical over-clad starts with access: scaffold or edge protection, and boards or a platform if any part of the old roof is fragile. The old gutters and edge trims are removed, and the roof is cleaned of loose debris, moss and any sharp projections that could puncture the membrane.
Next come the brackets or spacers, fixed through the old sheets into the purlins at a set pattern. The vapour control layer and insulation follow, laid progressively so that no area is left open overnight. The new sheets are then fixed from eaves to ridge, with rooflights and flashings fitted as the sheets reach them. Ridge, verge and gutter details finish the job, followed by a clean-up of fixings and offcuts.
You will notice less noise and less mess than a strip would cause, though the work is still a roofing job and the site needs to be cleared of vehicles and stock around the building line. We tell you the order in which areas will be worked, so your team knows which doors and yards to avoid.
When the last sheet is fixed we walk the roof with you or your site manager, check each detail against the plan, and clear the area. Waste goes with us. A 10-year workmanship guarantee comes in writing.
Timing depends on the roof area, the access and the weather. Wind is the main brake, because large sheets are hard to control at height, and wet weather slows the insulation and membrane stage. We give you a realistic programme at quote stage and keep you informed if conditions change it, rather than promising dates the sky may not honour.