Composite insulated roof panels: pros and cons

A composite panel, sometimes called a sandwich panel, is made in a factory. An outer profiled metal skin and an inner liner are laminated to an insulating core, and each finished unit is craned or hoisted into place and fixed to purlins. A built-up sheeted roof is put together on the building itself: liner sheet first, then spacers or rails, then insulation, then the weather sheet on top, each fitted in turn.

The difference shapes everything that follows. A panel gives you the finished build-up in one pass, with the insulation thickness fixed at the factory. A built-up roof lets you change the insulation, the rails and the outer sheet independently, which can suit a building with an awkward structure or an unusual specification. Single-skin sheets are different again, and our guide to metal roof sheets for commercial units covers them.

Speed is the first. With one component doing three jobs, there are fewer layers to fix, so a roof can be made weathertight in fewer stages. That matters on a unit where the business is still working underneath, because the time the roof is open is shorter.

The second is a predictable build-up. The thermal performance is a property of the product, and the manufacturer supplies it with the panel. We hold no verified thermal figure for non-domestic roofs, so we give no number on this page. Ask your supplier for the product data and put it to your building control body, because re-roofing a commercial building can bring Part L thermal requirements into play and the data sheet is the evidence they will look for.

Panels also look tidy, which matters on a unit facing customers or a main road. Long runs with consistent profiles and concealed or neatly capped fixings suit retail units, offices and light industrial sheds. Where the existing roof is a cold, uninsulated sheet that needs upgrading, panels can bring the whole roof up in one operation.

Panels are not the right answer everywhere. These are the points we raise at survey.

• Weight. A panel with a thick core can be heavier than a single sheet, and it may be heavier than what it replaces. A heavier covering is a question for the person who knows the frame. The Planning Portal's guidance leans on a structural engineer or surveyor for this, and Approved Document A says a check is due when the load on the roof rises significantly. Get the panel supplier's weight data and set it beside the covering coming off.
• Joints and fixings. Every side lap, end lap and fixing is a place that has to be sealed and stay sealed. Thermal movement can work them loose over time.
• Repair. A dented or punctured panel cannot be patched like a felt roof. The usual cure is replacing the panel, which means sourcing a matching profile.
• Cut edges. Metal skins cut on site can corrode from the edge if left untreated. Our guide to cut-edge corrosion on metal roofs explains the treatment.

Replacing a commercial roof covering often involves more than the covering itself. Your building control body can ask, and it may be the council's own service or a private registered building control approver. Either can raise questions about insulation, structure and fire performance. Stroud District's service is the Gloucestershire Building Control Partnership, and it is worth telling the team early what you plan to fit. Your insurer may also ask what the core is made of, so keep the data sheets and the delivery paperwork.

Details at the edges decide whether the roof keeps water out. Eaves, ridge, verge and gutter junctions, rooflight surrounds and penetrations need purpose-made flashings. Where rooflights are part of the plan, remember that in-plane units are a fragile area while the works go on. Panels laid at a low pitch need a clear route for water as well, and our answer on ponding on a commercial flat roof covers a drainage fault that can matter on a shallow-falling roof.

A church or community building may add its own rules about what can be fitted. See roofs on church halls and community buildings.

As a rough guide, panels tend to suit a building whose structure is sound and can carry the weight, whose covering is worn out, and where speed matters and one supplier's data sheet covers the whole build-up and so simplifies the paperwork. Built-up sheets can suit a structure that needs spacers, awkward geometry, or a gradual upgrade. If the existing covering is still sound, an overlay or a repair may be all it needs, and the survey will tell you that instead of steering you to a re-roof.

To start, send us photographs and the dimensions of the roof. We come out to look at it, photograph the purlins and the underside, and then send a written quote covering every item of the work. If the roof is replaced, you receive our 10-year workmanship guarantee in writing, and the old sheets leave site with us once the work is done. The wider service is on our commercial roofing page.