Drive round any trading estate in the Stroud valleys you will see profiled metal sheet on a great many of the roofs. It is the covering industry has settled on for large, low-pitch buildings: light enough for a modest steel frame, quick to lay across big areas and easy to combine with rooflights. Because the product looks alike from the road, it is easy to assume the sheets are interchangeable. They are not. Two roofs of the same profile can differ in base metal, coating, thickness and fixings, and those differences decide whether the roof is trouble-free or a source of leaks and rust streaks.
This guide covers the product: what the sheets are made of, how the coating system protects them, how they are fixed and lapped, and how to choose between steel and aluminium. The commonest fault has its own page, cut-edge corrosion on metal roofs. For the other systems and how they compare, see the main commercial roofing page.
Steel or aluminium: the base metal
Profiled roof sheet for industrial work starts as one of two metals, and each has a character.
Coated steel
Steel is strong, stiff and relatively inexpensive, which is why it is the default for industrial and agricultural buildings. Bare steel rusts, so the sheet is first coated with a metallic layer, either zinc (galvanised) or a zinc-aluminium alloy (often sold under the name Galvalume). The metallic layer is a sacrificial barrier: where it is scratched, it gives itself up first, which protects the steel nearby for a time. On top goes an organic coating, which carries the colour and adds a second layer of protection.
Aluminium
Aluminium weighs less than steel for the same sheet, does not rust and forms its own thin oxide skin. It is softer and dents more readily, it expands and contracts more with temperature, and it is dearer per sheet. It makes sense where weight matters, where the building is in a damp or aggressive setting, or where long, light spans are wanted. Aluminium should not sit wet against some other metals, because electrochemical corrosion can follow, so fixings, washers and flashings are matched to the sheet.
| Quality | Coated steel | Aluminium |
|---|---|---|
| Strength and stiffness | High | Lower, so thicker sheet or closer supports may be needed |
| Weight | Heavier | Lighter |
| Corrosion behaviour | Depends on the coating system; edges are the weak point | Forms its own protective oxide |
| Denting | Resists | Marks more readily |
Neither is right for every building. A survey of the existing sheets, the frame and the setting decides which base metal is worth specifying.
Coatings: what is on the sheet matters as much as the sheet
The organic coating is the part of a metal roof you see, and the part that does most of the protecting. Several systems are common, and a data sheet from the maker should state what is on the product you are buying.
Polyester
A thin polyester paint is the economy option, used on agricultural and light industrial buildings where appearance matters less. It weathers faster than thicker systems, chalking and fading in time.
Plastisol
Plastisol is a thicker PVC-based coating, often embossed. It resists scratches and handling damage on site, which is why it has been used widely on industrial roofs. It can lose flexibility and fade in strong sun, and it has limits on service temperature.
PVDF and other high-performance finishes
PVDF coatings hold colour and gloss longer and resist chalking, and are used where a roof must keep its appearance. They suit buildings that are visible from the road or sit in a setting where the roof is part of the design.
Backing coat
The underside of a sheet carries a lighter backing coat. On a single-skin roof, condensation forming on the underside meets this coat, and a thin backing coat gives less protection to the metal. Specifying a roof liner or anti-condensation layer protects the sheet from underneath as well.
Coating choice also affects repair. Maintenance coatings and edge treatments have to be compatible with what is already there, which is why we identify the existing system before suggesting a treatment.
Profiles, spans and the frame underneath
The shape of the sheet gives it stiffness. A deeper rib means a longer span between supports. Profiled sheet is rolled in a range of rib heights and pitches, and each manufacturer publishes a load and span table showing how far a given sheet at a given thickness can run between purlins under wind and snow loads. The span decides how many purlins the building needs, and the purlins need to be straight, level and sound.
Why the frame matters
A good sheet fixed to a poor frame will not stay weathertight. Purlins that have rotted, rusted or moved leave the sheets unsupported, and fixings lose their bite. When we survey an old unit, we look at the purlins and rafters under the sheets as well as at the sheets themselves, and the quote lists any frame repair as its own item.
Pitch and laps
Profiled sheet sheds water best on a suitable pitch, and the lap between sheets is where water can be driven in on shallow slopes. Manufacturers give a minimum pitch for each profile and recommend sealant in the end laps on shallow roofs. The side lap is usually a single rib. Check the data sheet for the product in use rather than trusting a general rule, because the figure differs between profiles.
Sheet length
Longer sheets mean fewer end laps, which means fewer places for water to get in, but they are harder to lift and fix and expand more with temperature. On a big unit the sheets are often laid in full-length runs from eaves to ridge, with the thermal movement allowed for in the fixings.
Fixings, washers and sealants
A metal roof is held down by hundreds of small fixings, and on a leaking sheet roof, we go to the fixings early.
Crown and trough
Screws can be driven through the crown (the high rib) or the trough (the low flat between ribs). Crown fixing leaves the screw head above the waterline but puts more stress on the rib. Trough fixing has the sheet held firmly to the purlin but places the screw in the flow of water, so the sealing washer has to do more work. The maker's instructions say which is permitted for the profile, and the choice is part of the specification, not the fitter's preference.
Washers and screw heads
Each screw has a sealing washer, usually a metal washer faced with rubber (EPDM). The washer is compressed just enough to seal, and over-tightening crushes it, while under-tightening leaves a path. Over years, rubber ages and metal expands and contracts around the screw, so fixings that were tight on day one can loosen. Checking and renewing fixings is routine maintenance on an older metal roof.
Sealants and fillers
Profile fillers close the gap at the eaves and ridge, and lap sealant closes the laps. They are consumables and need replacing over the life of the roof. Flashings and ridge closers go over the top. The materials in contact should be compatible, because certain metals set up corrosion where they touch in damp conditions.
When fixings have failed or sheets have been over-fixed with extra screws as a quick repair, the pattern of holes tells its own story, and we record it.
Insulated build-ups and condensation
Single-skin sheet on its own is a weather barrier, not insulation. Where a building needs to hold heat, the sheet is part of a build-up: an inner liner, a layer of insulation and an outer sheet, usually spaced with brackets. The layers have to be assembled so that warm, moist air from inside the building cannot reach the cold underside of the outer sheet and condense.
Condensation and its signs
Condensation is a common complaint under metal roofs. Water drips from the underside of sheets on cold mornings, or stains appear along the purlins and at the laps. The cause may be the use of the building (wet processes, unflued heaters, washing), poor ventilation, a missing vapour barrier, or insulation that has been compressed or has gaps. It is easy to mistake condensation for a leak, and the opposite mistake happens too, so we look at the pattern of wet before deciding which it is.
Rooflights
Rooflights in profiled sheet roofs are often made of glass-reinforced plastic or polycarbonate in the same profile as the metal. They are a weak point for leaks at the laps and, as they age, they can become brittle, which makes them fragile to walk on. Our guide to fragile roofs and safe access sets out the precautions, and nobody should walk on a rooflight or an old sheet without proper crawl boards and edge protection.
Treat every rooflight on an older roof as fragile until a roofer has confirmed otherwise, and never step on one to take a look.
Old sheets: overcladding, recovering and asbestos cement
Many industrial units have a roof that is simply tired rather than broken. There are three routes, and each depends on what is there.
Repair and refix
If the sheets and frame are sound, new fixings, lap sealant and a treated edge can bring the roof back. This is the lowest-cost route, and it suits roofs where the coating is intact over most of the sheet.
Overcladding
A new sheet system can be fixed over the old, using the old sheets as a deck. This avoids stripping and the disruption and waste that follow, which suits a unit that has to go on trading. It adds weight, so the frame must be checked. If the original sheets are a flat roof covering, then built-up felt may be a competing answer.
Strip and replace
When sheets, fixings and purlins are all failing, the best answer is usually a full strip and a new roof, with new insulation.
If the old sheets are asbestos cement
Roofs built in earlier decades may carry asbestos cement sheets, and they look much like other profiled sheet from below. HSE guidance puts asbestos cement sheets lifted off whole in the non-licensed category, and sheets that are badly broken in the notifiable one. Sampling confirms what a sheet is. Overcladding is sometimes chosen because it leaves intact sheets undisturbed. See HSE guidance at hse.gov.uk.
Whichever route applies, the quote is written after a survey, with photographs of your roof, and the site is left clear. The workmanship guarantee runs for 10 years, in writing. For a first look, send the address, some photographs and any sheet markings you can read.