What is a composite panel roof?

Think of a sandwich. Two thin metal skins, usually steel and sometimes aluminium, are glued to a core of insulation in a factory. The result is a stiff panel that spans between the purlins, carries its own insulation and sheds water without a separate liner or sheet.

That is the difference from a built-up system, where a liner sheet, insulation and an outer sheet are fitted in separate layers on site. A panel arrives as one unit, so there are fewer layers to go wrong, but the joint between panels carries more of the weathering.

Each panel has a profile on its outer face and an edge detail that locks to its neighbour. The panels run from eaves to ridge or are joined end to end with an end lap, and the fixings pass through the panel into the purlin below. Our commercial roofing page covers the other coverings we work on.

The face of a panel rarely leaks. Water gets in at the joints and fixings. Side joints rely on a sealant strip that can dry out, end laps need the right amount of overlap and sealant, and fixings with failed washers let water follow the screw into the core.

Penetrations are next. Flues, vents, rooflights and plant brackets all break through the panel and need flashings that suit the profile. Cut edges are the third. Where a panel has been cut on site, the steel is exposed and the coating no longer protects it, so rust starts there. Our guide to cut-edge corrosion explains the process.

When the core gets wet it loses insulation value and can hold moisture against the lower skin, so a leak that looks small from outside can be a bigger problem inside the panel. That is why a stain on the inside deserves a proper look rather than a dab of sealant.

Cores come in different materials, such as mineral wool, polyisocyanurate foam and polystyrene. Each behaves differently in a fire and in use, and which one is on your roof matters to insurers, to building control and to anyone planning to alter the building.

You may not know what you have. Panels carry a manufacturer mark, and the original specification or a data sheet should state the core and its performance. If neither exists, a sample taken at a cut edge or a panel end can identify it. We record what we find and photograph it.

Fire performance is a design matter settled when the roof is specified, not when a panel is repaired. Ask your insurer and your building control body what they need to know, and we can supply photographs and details of the roof as it stands. Our page on commercial roof insurance covers how to approach that conversation.

A single damaged panel can be replaced if a matching one is available and the purlins beneath are sound. The method follows that of any sheet, with the extra step of ensuring the side joint and end lap are sealed to the maker's detail. Small punctures can sometimes be repaired with a patch and sealant, but a patch over a wet core is a short-term measure and should be treated as one.

When many joints leak, or the outer skin has corroded across a wide area, the better answer can be to recover the roof. A new weathering layer or a refitted sheet system is fixed over the old panels, which keeps the building working while the work goes on. Our guide to over-roofing sets out when that is sensible.

The verdict reaches you with photographs: repair, recover or replace, and the reason for each. The price covers the least work that does the job.

We start inside the building, looking for staining on the underside of the panels, daylight at joints and any sag between purlins. Outside, we check the sealant at the side joints, the end laps, the fixings and washers, every penetration and every cut edge. A moisture meter can show wet areas beneath an intact surface, which is how a hidden problem is found before it spreads.

Delamination is a particular worry. If the skin has lifted from the core, the panel has lost much of its strength, and walking on it can be unsafe. Treat such panels as fragile until proven otherwise, and read our guide to fragile roof safety before anyone goes up.

Photographs and a written quote follow the survey, so the decision can be taken at your own pace. Underneath, the purlins are checked for rust wherever a leak has run down them.