The short answer
A composite roof panel is a prefabricated sheet made from two metal skins, usually coated steel, bonded to an insulating core of PIR foam or mineral wool. It is fixed in a single operation to purlins and gives a weathertight, insulated roof without separate liner, insulation and outer sheet. Panels are common on warehouses, factories, retail sheds and farm buildings. The core material affects fire performance and cost, and the joints and fixings are where problems begin.
How a composite panel is built
Picture a sandwich. The top skin is a profiled sheet of coated steel or aluminium, which carries the weather. The bottom skin is a flatter or lightly profiled sheet, which becomes the visible ceiling inside the building. Between them is an insulating core, bonded to both skins so the whole panel works as a single stiff unit. This bond gives the panel its strength: the panel spans between purlins without the extra support a thin single sheet would need.
The alternative is a built-up system, where an inner liner sheet, spacers, insulation and an outer sheet are assembled on the roof in layers. A composite panel replaces all that with one product, which is why it is faster to fit, and why it is so common on industrial roofs. Our guide to industrial roof sheeting explains how panels sit alongside other profiles.
PIR or mineral wool
The core decides how the panel behaves. PIR, short for polyisocyanurate, is a rigid foam with good thermal performance for its thickness, so panels with a PIR core are slimmer and lighter for a given insulation value. Mineral wool cores are denser and heavier, resist fire better and absorb sound, so they are chosen where fire performance or noise matters, such as in food production, in buildings close to boundaries, or where an insurer asks for them. Your insurer and building control body will tell you what the building needs. Ask for the manufacturer's data, and check it against your own requirements, and weigh more than the price.
Panel thickness and the profile of the outer skin also vary. Deeper profiles are stiffer and span further, and panels with concealed fixings give a clean look and avoid exposed screw heads, which are leak points.
Why owners choose panels
Speed is the first reason. A crane lifts a long panel onto the purlins, and a small crew can cover a large area in a short time, so the building is weathertight sooner. Consistency is the second: a factory-made panel has a known insulation value and a known finish, where a site-built sandwich depends on the care of the installer. Appearance is the third, since the clean lines suit modern industrial units. Against that, panels are not easy to alter once fitted, and a damaged panel cannot be patched as neatly as a single sheet. The long lengths also need careful handling and storage on site, because a panel left flat in the wet can trap water between sheets.
Fixing, joints and rooflights
Panels are lifted onto purlins and fixed through the top skin with long screws, or in concealed-fix designs with clips under the joint. Each panel overlaps its neighbour at side joints, which are sealed with tape or mastic, and the end laps are sealed as well. Flashings close the ridge, eaves and verges. Rooflights are made to match the panel profile, and sit within the same system. Our page on replacing rooflights on industrial roofs explains how those are matched and sealed.
The joints are the vulnerable part. Sealants dry out, fixings loosen as the building moves, and the thin outer skin can be scratched or dented by foot traffic. Where cut edges are exposed, such as at a rooflight opening or a trimmed panel, rust can start, as set out in our guide to cut edge corrosion. Damp can then wet the core, which loses insulation value and can work its way along the panel.
Looking after a panel roof
Panels need little, but not nothing. Clear the gutters, remove moss and debris from the laps, and look at the sealants and fixings once or twice a year. Keep foot traffic to marked walkways, because the outer skin is thin, and ask anyone fitting plant to avoid drilling the panels without sealing the holes. A small amount of care keeps the coating intact, and the coating is what keeps the steel from rusting.
Repair, overclad or replace
A panel roof with local damage can be repaired: re-sealing laps, replacing fixings and patching dents. If a roof has widespread corrosion or wet cores, it is usually better to overclad or replace the panels. A new sheeted layer over a sound frame is a common choice, covered in our guide to overcladding a commercial roof. Old panels on older buildings can contain asbestos in the form of fibre-cement outer sheets, so an asbestos survey comes before any work on a roof of that age.
Before we quote, we walk the roof and the underside, and photograph the panel joints and rooflights so you can see what we see. Our fixed written quote reflects the repair that works and does not push a replacement you do not need. Other business roof types are grouped under commercial roofing in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
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