The short answer
A composite roof panel is a factory-made sandwich: two steel skins bonded to a core of insulation, supplied in long sheets that are fixed to the purlins of an industrial, retail or farm building. The core insulates and the top skin sheds the rain. Repairs are usually needed at the side laps, the ends and the fixings, which are the weak points.
How a composite panel is made
A composite panel, sometimes called an insulated sandwich panel, is built in layers. An outer skin of profiled, coated steel faces the weather. Beneath it is a core of insulation, bonded to both skins, and an inner skin of lighter steel faces the room below. The three layers work as one unit.
The core is usually a rigid foam or mineral material, chosen for its insulation value and its behaviour in fire. The thicker the core, the better the insulation, and the thicker and heavier the panel. The profile on the top skin gives stiffness and carries the water down the slope.
Panels arrive on site in long lengths, cut to suit the building, so that on many roofs a single panel runs from the ridge to the eaves. That means fewer cross joints and a quicker installation than fitting separate sheets, lining and insulation in layers.
They are used on warehouses, workshops, supermarkets, cold stores, agricultural buildings and extensions to industrial units. Where a building is new, panels are often chosen for speed, and where a roof is being renewed they can replace tired sheets with insulation included. Our answer on overcladding a metal roof covers the case where they go over an old roof.
Joints, laps and fixings
On a composite roof the joints do the waterproofing. Each panel overlaps its neighbour along the side, where the profile is shaped to lock together, and a bead of sealant or a tape sits in the lap. If the roof is made of more than one length per slope, the end laps are also sealed and fastened.
Fasteners go through the crown of the profile into the purlins, each with a sealing washer. Those washers, and the screws beneath them, are what age. Rubber perishes in sunlight, screws work loose with thermal movement, and a hole that sealed on day one can start to weep years later.
The ridge and eaves are closed with flashings and profile fillers. Rooflights are fitted in the same plane, their side laps and kerbs forming further junctions. Any penetration, such as a flue or a vent, needs a purpose-made flashing.
Because the core sits between the skins, water that enters at a lap can travel into it. A wet core loses insulation value and may rust the inner skin from within. That is why joints matter more than the middle of the panel, and why we check them first. Our guide to cut-edge corrosion shows a related fault at the cut ends of coated sheets.
What goes wrong and how it is repaired
The usual faults are failed sealant at laps, perished washers, loose fixings, damaged flashings and corrosion at cut edges. Impact damage from plant or falling objects can also dent or hole the outer skin.
Many of these can be repaired without replacing panels. Laps can be resealed, fixings tightened or replaced, flashings renewed and small holes patched with compatible materials. Cut edges that have started to rust can be cleaned and coated. A roof treated early may be repaired for a modest cost.
Where the core is wet, the answer changes. A panel with a waterlogged core cannot be dried in place, and repairing the surface over wet insulation traps the problem. The panel should be replaced, with the new one matched for profile, thickness and fixing pattern.
Old panels in poor condition across much of the roof may justify a larger scheme, either replacement or overcladding. A survey sorts patch from renewal. We reach the roof safely, avoid walking on the panels where we cannot be sure of the fixings, photograph the laps and washers, and show you what we found. Details of sheet replacement are in our answer on replacing roof sheets.
Buying and pricing panels
If you are choosing panels for a new roof or a replacement, a few questions help. How thick is the core, and what insulation value does it give? What is the fire performance of the core, since insurers often care? What coating protects the steel and for how long? How are the laps and fixings detailed, and are rooflights and flashings part of the system?
Check that the supplier's product is suitable for the roof pitch, as every profile has a minimum. And ask about handling: long panels need careful lifting, and a crane or telehandler may be part of the job.
The price depends on area, panel type and thickness, the structure underneath, access, lifting, flashings, rooflights and removal of the old roof. An old roof with asbestos cement sheets changes the method and cost, and the survey should settle that before anything is quoted.
We give one written total after surveying the building. Compare quotes on what they include, not only the headline, and check VAT and scaffold or edge protection. Panel fitting sits within the range described on our commercial roofing page.
