An industrial roof that has reached the end of its life presents an awkward choice. Strip it and you pay for removal, disposal and a building left exposed while the new roof goes on. Leave it and you keep paying for leaks. Overcladding offers a middle route: a new, weathertight roof is built over the top of the old one, which stays in place as a temporary shelter and, in some cases, as part of the structure. Done on the right building it saves time and disruption. Done on the wrong one it hides a problem that returns. This guide walks through how overcladding works, the checks that decide whether it is suitable, and the alternatives.
What overcladding is, and how it goes on
Overcladding means fixing a new roof system over an existing roof covering without removing the covering. On a typical sheeted unit the old roof is a layer of profiled metal sheets, or fibre cement sheets, fixed to steel purlins. These are the horizontal beams that run along the roof and carry the sheets. The overcladding contractor does not take those sheets off. Instead, the work starts by fixing a support layer over them.
That support layer is usually a spacer system. Brackets or rails are fixed down through the old sheets and into the purlins below, forming a new grid standing proud of the old roof. Insulation is laid between and over the rails, and new profiled sheets or composite panels (two metal skins with an insulating core) are fixed to the top. The old roof ends up as a lining, hidden above the ceiling, and the new outer skin sheds the water.
The spacer system does two things. It lifts the new roof clear of the old one so that dirt, old fixings and uneven profiles do not interfere with it, and it creates room for thicker insulation than the original roof ever had. That second point is why many overclad roofs end up warmer than the roofs they replace. Our overview of industrial roof sheeting and profiles describes the sheet types you can choose between.
Where overcladding beats stripping
The strongest case is a building that cannot stop. A distribution unit with racking full of stock, a factory with a production line or a workshop with machinery that must stay dry cannot easily be emptied for a roof strip. When the old roof is removed, the interior is open to the weather until the new one is on, and every bay of sheets lifted off must be covered. With overcladding the old sheets stay in place until the new skin has been fixed over them, so the building remains weathertight throughout. Our page on roof work while a business is trading goes into how that is planned.
The second strong case is a roof covered in asbestos cement sheeting that is in sound condition. Taking asbestos cement sheets off a roof means handling a regulated material, and it needs a risk assessment, controls and trained workers every time. If the sheets are broken up during removal, the job becomes notifiable non-licensed work. Overlaying is sometimes an alternative to removal, which is why overcladding is common on older industrial estates. It needs care, because the fixings must pass through the old sheets, and the condition of the sheets decides whether that can be done safely. Our page on overcladding an asbestos cement roof covers the checks in more detail.
A third case is simple economics. A strip-out has costs that an overcladding job does not: skips, haulage, labour for removal and the temporary protection needed in between. Where the structure is sound and the old roof is simply tired, leaving it in place can bring the whole job down in cost and time. We do not quote a general figure, because the saving depends on the roof. Your fixed quote follows the survey.
The checks that decide whether it can be done
Before we recommend overcladding, we look at the things that would make it a bad idea. The first is the structure. A new roof adds weight, because there are new sheets, insulation and a spacer system on top of the old sheets. The purlins and the frame must be able to carry that. If the steelwork is corroded, deflecting or poorly connected, no amount of new cladding fixes it. Where the load is a real question, a structural engineer should confirm the frame can take it, and Approved Document A treats a covering at least 15% heavier than the one it replaces as needing approval, with possible strengthening.
The second check is the condition of the old purlins at the points where the new fixings will land. The spacer brackets must bite into sound steel or timber. If the purlins are rusted through at the fixing positions, the new roof is held on by nothing. We open up a handful of positions during the survey to look.
The third check is moisture. Wet insulation or a waterlogged deck under the old covering will not dry out once a new roof is sealed on top. In a fibre cement or metal sheet roof the old covering is usually only the outer skin, but if there is a lining below with failed insulation, trapped water can corrode steel and rot timber. We look for staining and rust on the underside, and for signs of long-term condensation.
The fourth is the shape of the old roof. A heavily distorted roof, one with large holes, or one whose profile varies a great deal across the building will make the spacer system difficult to set out. Rooflights, ridge details, valleys and the gutters at the eaves all have to be reworked, and the new gutters must be big enough for the new roof area. If the old roof is so far gone that it cannot be walked on safely, the first job is protection and access, not cladding.
Insulation, condensation and Building Regulations
The roof is a thermal element of the building, which means energy rules apply when you change it. Approved Document L, the part of the Building Regulations covering energy efficiency, applies to the renovation of thermal elements in non-domestic buildings. The Planning Portal sets a size limit below which re-covering needs no building control application, and above it approval is needed and the insulation of the whole roof may have to be upgraded. Overcladding a whole roof is normally above that limit, so we assume it is notifiable and raise it at the start. Our page on building regulations for commercial roofs goes through Part L and Part B.
Insulation is also where condensation risk comes in. If warm, moist air from inside the building reaches a cold surface, it condenses. In a new layered roof the aim is to keep the moist air out with an effective vapour control layer, and to keep the layers where condensation could form dry. When an old roof is left in place, the gap between old and new must be designed so that it either stays warm or is vented, and the design should not trap moisture between two impermeable layers. Poorly designed overcladding can create exactly the damp problem it was meant to end. We set out the build-up for approval, and the building control body decides what applies.
Fire performance is another point to check. The panel type, insulation core and any rooflights have to suit the use of the building. We choose products to match the use, and we leave fire strategy and any insurer requirements for you to confirm with your own advisers.
Rooflights, gutters and the details that cause trouble
The details decide whether an overclad roof stays dry. Old rooflights must be dealt with: they can be removed and replaced with new units in the new roof plane, or fixed over and covered. Both choices need thought, because a rooflight in the new skin must line up with an opening in the old roof, or the building loses daylight. Our guide to industrial rooflight replacement sets out the options. Fragile rooflights are a danger throughout the work, so they are boarded over or covered by a safe system of access before anyone steps near them.
Gutters are the second pressure point. Many industrial units have valley or parapet gutters built into the roof. A deeper roof build-up changes the level of the sheets against the gutter, and the gutter may need to be lined, raised or replaced. If water backs up behind a gutter, it will run under the new sheets. We check the capacity of the existing outlets against the new roof area, and we adjust where we need to.
The eaves and gables also change. A thicker roof needs deeper flashings, new closers at the eaves to stop birds and wind-driven rain entering the gap, and often new barge details at the gable. Done properly these are invisible. Done badly they are the first places the roof leaks.
| Situation | Overcladding | Stripping |
|---|---|---|
| Building must stay in use | Usually suits it, old roof stays as shelter | Needs full weather protection during the strip |
| Sound asbestos cement sheets | Often considered, subject to fixing checks | Regulated removal with risk assessment and controls |
| Corroded or weak purlins | Not suitable until the steel is repaired | Allows steelwork repair as part of the job |
| Wet insulation or deck | Hides the problem, risk of trapped moisture | Removes and replaces the damp material |
| Wish to raise insulation | Room for thicker build-up in the spacer zone | Possible, with a larger scope of works |
When stripping is the better answer
There are roofs that should not be overclad. If the structure is weak, the new roof will rest on a failing frame. If the old covering holds a lot of trapped moisture, a new skin will seal that in. If the existing roof is itself made up of several layers of earlier patching, the fixing points will be unreliable. On a flat roof with failed insulation and a rotten deck, overcladding is not a sensible option at all, because the problem lies below the covering. These roofs need the deck exposed, repaired and covered again, which is what our page on repair, overlay or replace deals with.
A strip also gives you the chance to put right what has been hidden for years. You can inspect the purlins, treat the steel, change the insulation, alter the pitch and add new rooflights in better positions. For a building you plan to keep for decades, that may be worth the extra cost and disruption, and we say so when the survey points that way.
What we do at the survey and in the quote
We start with a free survey and photograph the roof, the underside where it can be reached, the gutters and the details. From that we tell you whether the roof is a candidate for overcladding, what needs to be checked by an engineer, and what the alternatives are. Where repair will hold, we say so. If the sheets are asbestos cement we explain how that affects the method and what must be in place before work begins.
The quote is fixed and written, with the build-up set out layer by layer so you can see what you are paying for. Where the work is notifiable we take care of the Building Regulations paperwork, we clear the site and take waste away, and our workmanship is guaranteed for ten years, set down on paper. The other business roofing work we take on sits under commercial roofing in Basingstoke. Ring 01256 213956 or send a WhatsApp when you want the survey booked.
