The short answer
Composite roof panels are factory-made sandwiches of two metal skins bonded to an insulating core, fixed to purlins in a single pass. They are quick to fit and well insulated, and with a good coating last 25 to 40 years. Leaking side laps and fixings can usually be sealed; panels with failed coatings, damaged skins or wet cores are replaced like for like, and insurers will want to know what the core is made of.
On this page
Walk round almost any distribution park built in the last thirty years and most of the roofs will be composite panels. For a facilities manager inheriting one, the questions are usually practical: what is it made of, how long has it got, what is going wrong with it and what will it cost to put right. This note answers those in order.
What a composite panel is
A composite (or insulated) roof panel is a factory-made sandwich: a profiled steel outer skin, an insulating core, and a flatter steel inner skin, all bonded together. Panels are typically about a metre wide and run from ridge to eaves in long single lengths, fixed through to the purlins. Side laps overlap the neighbouring panel; end laps, where two lengths meet on a long roof, are sealed and stitched.
Compared with a built-up roof, where a liner, insulation and outer sheet are assembled separately on site, composite panels go on in one pass. That is why they became the default for new warehouses: faster to weathertight and fewer site trades.
Thickness, insulation and pitch
Panels come in a range of depths. Thin panels around 40 mm deep (with a U-value in the region of 0.46 W/m²K on one maker’s data sheet) are used on stores and outbuildings. Panels for heated buildings are much deeper, because meeting the 0.18 W/m²K standard for a renovated roof takes a lot more insulation. Trapezoidal roof panels are generally designed for pitches of about 4 degrees and above; lower pitches need different products or details.
Core types and why insurers care
Not all cores are the same, and this is the question insurers ask first. Older panels may have polystyrene or polyurethane cores, which insurers regard as a higher fire risk. Later panels mostly use PIR (polyisocyanurate), and some use mineral wool, which is non-combustible.
A risk management paper from a large broker in January 2024 sets out the insurer’s view plainly: without evidence, insurers assume a panel is the lowest-rated type. Panels tested and approved to the Loss Prevention Certification Board standard LPS 1181, or equivalent approvals such as FM Global, are treated more favourably. The panel manufacturers’ trade body, EPIC, describes LPS 1181 Part 1 as a large-scale fire test of the system as installed, graded EXT A or EXT B, and valid only for the build-up tested. Its practical advice to owners:
- identify every panel type on site and mark them on a plan;
- keep evidence of the core type and any approvals;
- repair damaged panels promptly and fire-stop any openings;
- control hot work and electrical installations near panels;
- replace damaged panels with non-combustible or insurer-approved panels.
If you are about to replace panels, ask your insurer what they want before the order is placed, not after.
Common defects
Leaking side laps and end laps
Sealant in the laps ages, fixings loosen with thermal movement, and water gets in at the overlap. This is the most common leak on panel roofs.
Cut edge corrosion
Like single-skin sheets, panels are cut to length after coating, leaving bare steel at the ends. Rust starts there and works back under the coating. See cut edge corrosion treatment.
Failed fixings and washers
Perished washers on through-fixings let water track down the fixing into the panel.
Rooflights
Rooflights in panel roofs discolour, become brittle and leak at their joints with the panels. They are also a major fall hazard: HSE treats rooflights as fragile until shown otherwise.
Damaged or delaminated panels
Impact from plant, foot traffic or storm debris dents the outer skin. Once the skin and core separate, the panel loses strength and insulation and needs replacing.
Repair or replace?
| Problem | Usual fix |
|---|---|
| A few leaking laps or fixings | Reseal and refix |
| Leaking laps across the roof, coating sound | A lap and fixing coating system |
| Cut edge rust, steel still sound | Clean back, prime, seal and coat |
| Failed outer coating across the roof | Full recoat after preparation, or replacement |
| Dented, delaminated or wet panels | Replace those panels like for like |
| Most of the roof failing | Replace the roof in new panels |
Unlike single-skin sheet roofs, composite panel roofs are not usually overclad in the conventional way, because the panel itself is already insulated and the system was not designed to carry a second roof. Coating systems and targeted panel replacement are the normal answers until the roof reaches the end of its life.
A useful example of the decision: a 2025 case study from a national roofing distributor describes a leaking asbestos single-skin roof where overcladding was rejected because the old roof was in poor condition, and a built-up system was rejected for its labour and programme. The owner chose new composite panels with matching rooflights, completed in phases over a few months.
How long they last
With a good coating and maintenance, composite panel roofs typically last 25 to 40 years. The coating sets the pace: steel makers’ coating guarantees run as long as 40 years on some pre-finished steel, on condition that the roof is maintained. The weak points are the cut edges, laps and rooflights, which is where inspection effort should go.
What it costs
Panel replacement is priced per job because it depends on the panel depth, length, core, access and how many panels are involved. Coatings and lap treatment are also priced by the job. A roof survey that records the panel type, core, coating condition and each defect is the starting point for a sensible quotation.
Working on a panel roof safely
Panel roofs often feel solid underfoot, but rooflights within them are not, and older panels with corroded skins can fail. Work should be planned with edge protection and rooflight covers or netting. Do not let staff go up to inspect a leak.
If your panel roof is leaking or rusting at the laps, the form below gets it surveyed and priced by a contractor near you. More on the options is on the Commercial Roofing UK home page.