Walk around the industrial estates on the edge of Basingstoke and almost every unit has the same kind of roof: long metal sheets, ribbed along their length, running from the ridge to the gutter. Industrial roof sheeting looks simple, but it is a family of systems with very different build-ups, lifespans and repair needs. A single sheet over bare purlins, a double-skin roof with insulation sandwiched inside and a factory-made composite panel all look similar from the car park. This guide explains each one, the finishes that protect them and the fixings that hold them down, so you can read your own roof.
What industrial sheeting actually is
Industrial sheeting is profiled metal or fibre cement cladding laid over a steel or timber frame. The frame is usually a steel portal frame or timber trusses carrying purlins, the horizontal rails that the sheets are fixed to. Profiling, the pattern of ridges and troughs pressed into each sheet, gives a thin material enough stiffness to span the gap between purlins and to shed water along the troughs.
Two things set industrial sheeting apart from a domestic roof. First, the pitch is shallow, often only a few degrees, so the sheets depend on generous laps and sealants to keep water out. Second, spans are long and loads are high, because wind uplift acts hard on large, light roofs. Both facts shape the way sheets are profiled, lapped and fixed.
Three families cover most buildings around here. Single-skin sheeting is one layer of metal or fibre cement. Double-skin built-up systems add a liner, insulation and an outer sheet. Composite panels, sometimes called sandwich panels, are made in a factory with the insulation bonded between two metal faces. Each is described below.
Single-skin profiles
A single-skin roof is the oldest and simplest form. It is one sheet of profiled steel, aluminium or fibre cement, fixed straight to the purlins with no insulation. It is common on agricultural buildings, storage sheds, workshops where heat does not matter and older industrial units.
The most familiar profile is the trapezoidal or box profile, with flat-topped ribs and wide flat troughs. Corrugated sheet, with its rounded wave, is the older pattern. Both channel water well, but neither insulates. In winter, warm air from inside meets the cold underside of a bare metal sheet and condensation forms, dripping back onto stock and equipment. Some single-skin roofs have an anti-condensation felt bonded under the sheet to soak up and release that moisture, and those can hold water at their edges for years.
Fibre cement sheeting looks similar from a distance, and it was used on a very large part of the post-war industrial stock. Older fibre cement sheets can contain asbestos, so they should never be drilled, scrubbed or broken up without the right assessment. Our guide on asbestos cement roofs on commercial buildings explains the duty to manage and the options for dealing with them.
Double-skin built-up systems
A double-skin roof is assembled on site in layers, and its parts are worth naming. At the bottom is a liner panel, a lightly profiled steel sheet that gives the building a clean ceiling and supports the insulation. Above it sit support brackets or spacer bars, which hold the outer sheet clear of the insulation and carry its load back to the purlins. Between the liner and the outer sheet goes mineral wool or similar insulation, fitted in a thick layer with the joints taped or sealed. A vapour control layer or sealed liner joints stop warm, moist air from the building reaching the cold outer skin. The top layer is the weathering sheet, usually a profiled steel sheet in a trapezoidal pattern.
Built-up systems insulate far better than a bare sheet and let the designer choose each layer for the building. The price is more labour and more joints. A gap in the vapour control layer lets moisture reach the insulation, which loses its value when it gets wet, and a fixing that has pulled out of a spacer bar can leave the outer sheet free to move in the wind.
Repairs to double-skin roofs often hide a larger problem. A stain on the underside of the liner may be a leak at a lap above, or it may be condensation from a failed vapour layer, and the two need different cures. That is why we trace the cause before quoting. Our guide on finding a leak in a warehouse roof sets out how that tracing works.
Composite panels
A composite panel is made in a factory, with an insulating core bonded between an outer and an inner metal face. The core is usually rigid foam, though mineral wool cores are also made for buildings with stricter fire requirements. Because the layers are bonded, the panel is stiff for its weight, quick to lay and consistent, and it needs no separate liner. A crane or hoist lifts each long panel into place, and the side joints are made watertight by a profile that interlocks.
The strength of composite panels is speed and insulation performance. The weakness is the joints, which carry all the weather-tightness. The sealant in the side and end laps ages, the panel ends may have been cut on site, and any penetration for a rooflight, vent or services is a hole in a sealed skin. Older panels can also develop faults at the core, such as delamination or damp getting in at a damaged edge. We cover the panels themselves in more depth in our answer on what a composite roof panel is.
Coatings, finishes and corrosion
Bare steel would rust within a few seasons, so industrial sheets arrive with a protective layer. The base is a zinc coating, applied to the steel by galvanising. Over that goes an organic coating: a polyester paint or a thicker plastisol (a PVC-based finish) that gives the sheet its colour and gives the zinc some protection from the weather. Different coatings suit different settings. A building near a chemical process, a farm with ammonia from livestock, or one that gets a lot of salt air needs a more durable finish than a dry storage unit.
Coatings fail first at the places where the protective layers are interrupted. The cut edge of a sheet exposes bare metal, and the zinc is gradually used up in protecting it. The edge rusts and creeps in under the paint, lifting it in a thin line along the lap. The same thing happens around fixings that were over-driven, scratches and scrapes made during installation, and the point where a sheet is drilled for a vent. Our page on cut edge corrosion explains how to catch it early, when treatment is cheap, rather than late, when the sheet has to be replaced.
Fixings, laps and wind
If the sheets are the roof's skin, the fixings are its stitches. Most profiled sheets are fixed with self-drilling or self-tapping screws that pass through the sheet into the purlin below. Each carries a sealing washer, a metal washer bonded to a rubber seal, which closes the hole the screw makes. In built-up systems, the screws go through the outer sheet into the spacer system, and then the spacer system is fixed back to the purlin.
Sheets are overlapped along their sides and at their ends. Side laps are usually sealed with a mastic tape or bead and held tight with stitching screws, small fasteners that clamp one sheet to the next between purlins. End laps, where one sheet meets the next up the slope, need a sealed overlap long enough to resist water being driven uphill by the wind. A shallower pitch needs a longer overlap and more care with sealant.
Roofs of this size are lightweight, and wind pulls at them as much as it presses down. Uplift is strongest at eaves, ridges and corners, which is why fixing patterns are tighter there. Over years, the fixings can loosen, corrode or pull out of the purlin, and thermal movement gradually works the holes oversize as the metal expands and shrinks with the temperature. When we survey, we look for backed-out screws, perished washers, split sealant, stains running from a fixing and sheets that rattle or lift. Those are all signs of fixings that need attention before the next gale.
| System | What it is | Where it usually leaks |
|---|---|---|
| Single-skin steel or aluminium | One profiled sheet fixed to purlins | Laps, fixing holes and corroded cut edges |
| Single-skin fibre cement | Brittle cement sheet, possibly asbestos-bearing | Cracks, ridge flashings and worn laps |
| Double-skin built-up | Liner, insulation, spacers and an outer sheet | Outer laps, vapour layer gaps and loose spacers |
| Composite panel | Factory-bonded insulated panel | Side and end joints, penetrations and damaged edges |
Repair, overclad or replace
When a sheeted roof has problems, there are three honest routes. A repair suits a roof whose sheets and structure are sound but which has local failures: a split lap, a run of failed fixings or a cracked rooflight. Overcladding means fixing a new weathering layer over the existing roof, which suits a roof that is leaking widely but is structurally sound and can carry the extra weight. Full replacement suits a roof whose sheets are corroded, whose insulation is wet or whose structure is not sound.
Which is right depends on the evidence. Overcladding can keep a business trading and avoids the disposal of old sheets, which matters when they contain asbestos, and our guide to overcladding a commercial roof covers the trade-offs. Re-roofing work that affects a large part of a non-domestic roof can also bring the energy rules into play, since a roof is a thermal element. The Planning Portal sets out where building control approval starts, and where it applies we take care of the paperwork with your building control body.
New metal sheeting and any substantial work above a working building also need a safe way of reaching the roof, because the old surface may be fragile. We set out the principles in our guide to fragile roofs and rooflights.
How we survey a sheeted roof
A free survey is where we start. We inspect the roof from safe access, and we photograph the laps, fixings, rooflights, flashings, gutters and any corrosion we find, so that you see your own roof. Inside, we look at the underside for staining and daylight. We identify the system, whether it is single-skin, built-up or composite, because the repair depends on it.
Our report says what is wrong, what is the cause and what it would take to put right. We repair where repair works, and we tell you when a roof has life left in it. If the sheeting is at the end of its useful life, we explain the options, with a fixed written quote for the route you choose. The work is covered by a 10-year workmanship guarantee in writing. You will find the rest of our business-premises work on our commercial roofing in Basingstoke page, and a survey can be arranged on 01256 213956 or through WhatsApp.
