The short answer
Metal roof sheets can be replaced individually where damage is local, and the roof can be overclad or stripped where corrosion, leaks or failed fixings are widespread. The new sheets must match the profile and gauge, lap correctly and be fixed with the right screws and seals. Rooflights and the supporting purlins are checked at the same time, and the old sheets are tested for asbestos first.
Replacing sheets one at a time
Profiled metal sheeting is made in lengths that run from the ridge to the eaves, laid in overlapping courses side by side. Because each sheet is a separate unit, a damaged one can often be lifted out and replaced without disturbing the rest.
That suits local damage. A sheet dented by a fallen branch, holed by corrosion at one end or split by a fixing that was over-tightened can be removed and a new sheet fitted in its place. The roofer unscrews the fixings along the sheet, releases it from the laps on each side, slides it out and fits the new one with fresh seals.
The new sheet has to match. The profile, depth of corrugation, width of cover and thickness all need to correspond, or the laps will not seal and the sheet will not sit flat. The coating matters too, since a new sheet beside weathered ones will look different at first.
Single-sheet replacement has limits. If the neighbouring sheets are corroded at their edges, the fixings in them may not hold a new sheet properly, and a patchwork of repairs becomes expensive. A survey tells you whether repair is sensible or a sign that the roof as a whole has run its course.
Corrosion at cut edges is one common cause of decay.
Overclad or strip
Where decay is widespread, the options are a new roof fitted over the old, or stripping the old one off first.
Overcladding places new sheets, with insulation if wanted, over the existing roof. It avoids removing the old sheets, which keeps the building in use, reduces waste and, where the old sheets contain asbestos, leaves them undisturbed. It needs the old roof to be sound enough to carry the new fixings and the structure to take the extra weight, and it reduces the clear height slightly.
Stripping removes the old sheets, so the new roof is built on the frame. It allows the purlins to be checked and repaired, the insulation to be renewed and the falls and details to be corrected. It costs more, and it disturbs the building more, but it gives a renewed roof with no hidden defects.
The choice follows the state of the frame, the type of old sheeting and the plans for the building. If the sheets are asbestos cement, the choice is governed by the asbestos rules, and our guide to an overclad asbestos roof explains why overcladding is often preferred.
The survey gives the evidence for the decision, with photographs of the roof and the frame, and a price for each option.
Fixings, laps and details
Leaks on a sheet roof usually come from details, not from the sheets. Fixings, laps, flashings and sealants decide whether a new roof stays dry.
Fixings are the first. Screws are driven through the crown or the trough of the profile, depending on the system, and each carries a sealing washer. They must be the right length and material, driven to the correct tightness, since too loose lets water in and too tight crushes the washer and dents the sheet. Fixing spacing is set by the manufacturer for wind loads.
Laps are the second. Side laps join sheets across their width, and end laps join them up the slope. The overlap distance depends on the pitch, shallower roofs needing more, and a lap sealant forms the weather barrier. Poorly made laps let water creep in by capillary action.
Flashings seal the roof at the ridge, the eaves, the verge and the junctions with walls and plant. Each needs its own detail and seals, and each is a place where leaks start.
Gutters and downpipes must be able to carry the flow. Check their falls and outlets for capacity.
Ask a roofer what fixings and laps will be used. A good answer is specific.
Rooflights, purlins and checks first
When sheets are replaced, the rooflights usually come into the plan. They are cut into the same profile and lap with the sheets around them, so a roof recovered with new sheets and old lights is a poor combination. Lights are often renewed at the same time, and our guide to industrial roof light replacement explains the access controls that apply.
Purlins, the horizontal members that carry the sheets, need inspection when the old roof is stripped. Corroded or bent purlins are repaired or replaced, because new sheets are only as secure as the frame under them.
Before anything starts, the survey checks for asbestos in older sheets, since HSE advises presuming asbestos in older buildings, those built or refurbished before 2000, until shown otherwise. It also records the roof's condition, access and the points where water gets in.
The work itself is planned at height. Metal sheets can be slippery when damp, edges are sharp and wind can catch a loose sheet, so the plan covers edge protection, safe routes and lifting. Many old sheet roofs are also fragile, and the controls in our guide to fragile roof safety apply.
The pictures and a written price for your chosen option come to you, with access included. The commercial roofing hub lists the other work we do.
