Metal roof sheeting for commercial buildings

Metal roofs for commercial buildings fall into three groups, and telling them apart is the first step in any discussion about repair or replacement.

Single-skin profiled sheets are a single layer of corrugated or trapezoidal steel or aluminium, fixed directly to the purlins. They suit unheated buildings such as stores, covered yards and agricultural sheds. There is no insulation, so the underside is exposed to the inside air, and condensation can form on it in cold weather.

Built-up (double-skin) roofs have a liner sheet on the underside of the purlins, spacer bars or brackets, a layer of mineral wool or similar insulation, and an outer profiled sheet on top. The liner carries the load, the insulation sits in between, and the outer sheet keeps the weather out. These are common on heated industrial buildings.

Insulated composite panels are factory-made sandwiches of an outer metal skin, an insulating core and an inner skin, delivered as a single panel. They go up faster than a built-up system, and the factory-made joints are more consistent. Our answer on composite roof panels covers them in detail.

Where does standing seam fit?

Standing seam is a fourth category that overlaps the others. Long panels, usually of aluminium, zinc, copper or coated steel, are joined by raised seams that are folded or clipped together, with the fixings hidden underneath. It is widely used on shops, offices, schools and public buildings where appearance matters, and on low-pitched roofs.

The shape of a sheet gives it stiffness. Deep trapezoidal profiles span further between purlins than shallow corrugations, which is why they dominate on large buildings. The gauge, or thickness, of the steel also matters: a thicker sheet resists denting and spans further, but weighs more and costs more.

Steel sheets are galvanised for rust resistance and then coated with a paint-type finish. The finishes vary in toughness and in how they cope with sunlight, chemicals and exposed coastal or industrial atmospheres, and the manufacturer's data sheet states what each is rated for. Aluminium needs no galvanising and does not rust, though it dents more easily and has a higher initial cost.

Why does the coating matter so much?

Because the coating is the sheet's protection. Once it is scratched or cut through, the steel underneath begins to corrode, and the damage spreads sideways under the paint. Cut edges at the ends of sheets, drilled holes and places where fixings have been overtightened are the usual starting points.

Before choosing a replacement, a roofer should ask how the building is used. A chemical store, a food factory or an animal shed puts very different demands on a roof from a dry storage unit, and a cheaper finish in an aggressive atmosphere can fail well before its time. The survey notes how the building is used and the state of the old coating.

Metal sheets rarely leak in the middle of a sheet. Water gets in at the joints and at the fixings, so these are the details to inspect.

At the side laps, where one sheet overlaps the next along its length, the join is usually sealed with a strip of mastic. At the end laps, where sheets overlap up the slope, the overlap needed changes with the slope, and shallow slopes need more overlap and a continuous seal. The manufacturer's data sheet gives the minimum pitch and overlap for each profile, and it is worth checking before any new sheet is ordered.

Sheets are fixed to the purlins with screws or bolts. On exposed-fix systems the fasteners go through the face of the sheet, each with a washer to seal the hole. Over time the washers perish, the screws work loose as the metal flexes with the weather, and a roof that was watertight leaks at dozens of tiny points. Concealed-fix systems, including standing seam, avoid holes in the weathering surface altogether.

Can leaking fixings be fixed without re-roofing?

Often yes. Replacing fixings with correct fasteners and new washers, resealing laps and renewing cracked flashings at ridges and walls can restore a roof that is otherwise sound. It makes sense where the sheets are in good condition. Where the sheets themselves are corroded, a repair of the fixings simply moves the problem on, and our guide to cut edge corrosion helps decide which applies.

Metal moves. A long sheet exposed to sun in the afternoon and frost overnight expands and contracts every day, and the roof is designed so that this movement does not tear the fixings or buckle the sheets. Elongated holes, sliding clips and expansion joints in long runs all serve this purpose. A roof where those provisions were skipped usually shows it with tearing at the screws and a ticking sound as it cools.

Noise is the other effect that building users notice. Rain on a single-skin metal roof can be loud, and a built-up roof with a mineral wool layer is much quieter. Where rain noise is a problem on an existing roof, adding insulation, an acoustic layer or an over-cladding system can help. Our answer on why a metal roof is noisy explains the causes.

Why does it drip when it has not rained?

That is condensation. Warm, humid air inside the building reaches the cold underside of a single-skin sheet, cools below its dew point and turns to water. Anti-condensation felts or fleeces fixed to the underside of the sheet absorb the moisture and release it slowly, and insulation stops the surface getting cold enough in the first place. A drip from a metal roof in cold, dry weather is more likely to be condensation than a leak.

A metal sheet roof is only as watertight as its details. Rooflights, ridge flashings, verge trims, eaves gutters and pipe penetrations all interrupt the sheeting, and each must be formed to suit the profile.

Rooflights set into profiled sheets are usually translucent panels with the same profile as the metal. They should lap in the same way as the sheets. They are also the weakest part of the roof underfoot, which is why safety planning matters; see our guide to fragile roofs and safety. When a roof is renewed, rooflights are normally replaced with it, because old ones will not match the life of the new sheets. The answer on replacing rooflights on an industrial unit explains what to consider.

What about valley gutters and parapets?

Many industrial buildings have valley gutters between two sheeted slopes, or parapet gutters at the walls. These carry large volumes of water, and when they fail they leak into the building directly. Linings, new falls and enlarged outlets are the usual fixes. Flashings at the ridge and where a sheet roof meets a masonry wall need close attention at survey, because water finds the smallest gap.

Pipes, flues and fans that pass through the roof need purpose-made collars or flashings that fit the profile. Mastic smeared round a pipe is a temporary measure at best, and a collar properly sealed to the sheet outlasts it many times over.

Three routes are open for an ageing metal roof, and the choice turns on how the sheets and the frame have held up.

Repair suits roofs whose sheets are sound but whose fixings, laps or flashings have failed. It is the smallest job, and where it solves the fault, it is what we recommend.

Over-cladding puts a new weathering sheet over the old roof, which stays in place as a liner. It avoids the cost and disruption of stripping, keeps the building usable and can add insulation. It needs the old sheets and purlins to be sound enough to carry a second layer. Our guide to roof over-cladding for industrial units covers how it works.

Replacement strips the old sheets and fits new ones. It is the right route where sheets have corroded through, the frame needs repair or the old covering is a hazard in itself.

What if the old sheets are cement-based?

Many older sheds have asbestos cement sheets, and HSE tells owners of any building from before 2000, whether newly built or refurbished then, to assume asbestos could be there. Lifting intact cement sheets off carefully is not licensed work, but once sheets are substantially broken the job has to be notified. A metal roof is much lighter than cement sheeting, which can help an old frame, although the structure should still be checked. Our answer on asbestos cement roof sheets sets out the steps.

For a metal roof, the survey looks at much more than the sheets. We check the type and condition of the sheets, the coating, the fixings and washers, every lap and flashing, the rooflights, the gutters and outlets, and the purlins and frame where they can be seen from inside. Photographs of your own roof go with the quote.

The quote is written and gives a total price. When you compare it with others, check that each names the sheet type and finish, the thickness of the metal, how the sheets are fixed, what happens to the rooflights and whether VAT and scaffolding or access equipment are included. Ask how the old sheets will be removed and disposed of, and whether asbestos has been considered.

How does this compare with other systems?

Metal is only one family of roof system, and on some buildings a membrane is the better choice. Our guide to single-ply roofing describes the alternative for low-pitched and complex roofs. The wider range of work is on our commercial roofing page.

Workmanship on a roof we replace is guaranteed in writing for 10 years. If new fixings and laps cure the fault, that is what the quote will cover, not a full replacement. When we finish, the yard is left tidy and the old sheets are carted off.