What are composite roof panels?

Think of a thick, rigid sandwich. The outer skin is a profiled steel sheet, usually with a coloured coating, which sheds the rain. The inner skin is a lighter, flatter steel liner that faces the room. Between them sits the core: rigid foam such as PIR or PUR, or mineral wool.

The panel is made as a single unit, so the insulation cannot slip or sag, and fitting is quicker than building up a roof from separate liner, insulation and outer sheet. The core also stiffens the panel, so it can span between purlins without extra support.

You will see panels on warehouses, distribution centres, farm buildings, sports halls and retail units. They are also used on newer barn replacements and extensions, where a single layer saves time and gives a tidy finish. Our page on barn roof replacement covers one common use.

Panels are lifted onto the purlins and fixed with long self-drilling screws that pass through the ribs of the profile. Each fixing has a washer and a sealing cap. Along the side of every panel there is a joint where one rib laps the next, usually with a sealing strip trapped inside it, and at the end of a run there is an end lap or a flashing.

Because the panels are long, the number of end laps is small, which reduces leak points compared with short sheets. The details at the ridge, eaves, rooflights and gutters become more important, though, since they are where different components meet.

Good fixing is a matter of the right fixing at the right spacing, with the right torque. Over-tightened screws crush the profile and let water in around the head; loose ones allow the panel to move and open the seal.

Most trouble starts at the joints and fixings, not the face of the panel. Side seals dry out and shrink. Screw caps crack in sun and frost. Sealant at flashings and rooflights splits as the roof expands and contracts through the day.

Cut edges are another weak point. Where a panel has been cut on site, at a rooflight opening or a penetration, the steel edge and the core are exposed unless they were sealed properly. Rust can start there and spread under the coating. Our guide to cut edge corrosion on metal roofs explains how it develops.

Damage is a third cause. A dropped tool, a foot on a ridge, or a bad fix by a previous contractor can dent the outer skin and break the coating. Panels also carry water further than ordinary sheets: it can run inside the core and appear a long way from the leak.

If the fault is a failed seal, a loose fixing or a split flashing, a repair is usually the right answer. Re-sealing laps, replacing damaged caps and fitting new flashings are all targeted jobs that leave the panels in place.

If the outer skin has corroded or the core is wet, the choice widens. A wet core loses insulation value and holds moisture against the steel, so the panel may need replacing. Where the structure is sound and the panels are only weathered, an over-cladding scheme can add a new weatherproof layer on top.

Which is right depends on what the survey finds. Our page on how long a commercial re-roof takes explains what the programme looks like for a full replacement. Whichever route suits, the aim is the lightest job that cures the fault.

The core is not the same on every panel, and different cores behave differently in a fire. If you are buying a building or taking a lease, find out which core your roof has. Building control, your landlord, your insurer or your lender may have views on it, and it is easier to establish before work than after.

The panel supplier can tell you the type and rating if you have the product name from the manufacturer's label on the underside. Old paperwork from the original build, such as an O&M manual, is worth looking for.

If you are replacing panels, ask the supplier for the data on the new product, including its fire classification, and check with your building control body that it suits the use of the building. Questions about cover belong with your insurer, and we do not advise on claims.

A panel roof does not need much, but it does need looking at. Clear gutters and valleys, check sealant and caps at the ridge, eaves and rooflights, and look at the underside for stains or rust. A dated photograph each time makes slow changes easier to see.

Do not walk on a panel roof without proper crawl boards. The outer skin is thin steel over a soft core, and it can dent or buckle under a person's weight. Treat any rooflights set into it as fragile.

A planned programme of checks is cheaper than emergency visits, and our page on commercial roof maintenance plans explains how this works. Each survey ends with dated photographs of the panels and a written quote, and any repair we make is covered by a 10-year written guarantee on the workmanship. More about the wider service is on our commercial roofing page.