Solar panels on metal and profiled sheet roofs

Why metal roofs are often a good fit

Large, shallow-pitched, unshaded metal roofs are common on warehouses and factories, and they usually sit over buildings with steady daytime electricity use. Panels on rails or short brackets typically add around 10 to 15 kg per square metre, far less than a ballasted flat roof system, so many portal frame buildings can take an array without strengthening. That still needs confirming by a structural survey, because purlin spacing, original design loads and later additions such as hung services vary a great deal.

Trapezoidal and standing seam profiles

Trapezoidal sheet

Trapezoidal (sometimes called box profile) sheeting has regular raised ribs. Mounting brackets fix to the crown of the ribs, either into the purlins below or directly into the sheet with fixings designed for thin metal. Fixings into the crown sit above the water line, which is why they are favoured over fixings in the troughs. Each one uses a sealing washer or gasket, and the installer has to follow the bracket manufacturer's guidance for the sheet gauge and profile on your roof.

Standing seam

Standing seam roofs have raised interlocking seams and concealed fixings. Many systems allow clamps that grip the seam itself, so no holes are made in the roof sheet. That avoids a whole category of leak risk, but the clamp has to be compatible with the specific seam profile, and the roof system has to be able to take the extra load through its clips, which are designed to let the sheets move with temperature. The survey identifies the seam type so the installer can choose a tested clamp.

Composite insulated panels

Many modern buildings use composite panels: two metal skins bonded to an insulating core. Panels can usually be mounted on them using brackets fixed through the outer skin or into the purlins, but the core material matters.

• Insurers increasingly ask what the core is made of, because some foam cores behave differently in a fire from mineral wool. RC62, the fire safety guidance for rooftop PV from RISCAuthority and the Fire Protection Association, is widely referred to by insurers on this point.
• Fixings must not crush the panel or create a cold bridge or a route for moisture into the core.

The surveyor records the panel type and, where possible, the manufacturer, so the design can address both structure and fire risk. Our fire safety and insurance guide covers the insurer conversation in more detail.

Clamps, brackets and rail-less mounting

There are three common mounting approaches on metal roofs:

Rail-less systems are popular on trapezoidal roofs, but they depend on the sheet being sound and thick enough to hold the fixings. On older or thin sheets, fixing back to the purlins may be needed.

Whichever system is chosen, the installer should provide calculations showing that the fixings and sheets can resist the wind uplift for your building's height, location and exposure. Edge and corner zones of the roof usually need more fixings or are left clear.

Rooflights and fragility

Rooflights let daylight into warehouses and sheds, but they are almost always fragile. A fall through a rooflight is one of the most serious risks on any industrial roof. The survey maps every rooflight so the design can:

• keep panels off and clear of rooflights,
• plan safe working with netting, covers or edge protection during installation,
• leave safe walkways for future cleaning and maintenance.

Separation from rooflights also matters for fire safety, as RC62 and many insurers expect gaps around rooflights and other openings. Some owners take the opportunity to replace old, discoloured rooflights at the same time, while the roof is already being accessed.

Fixings and corrosion

Metal roofs fail at their fixings and laps long before the main sheet surface wears out. The roof survey looks for:

• corroded or loose fixings and perished sealing washers,
• cut edge corrosion, where the coating peels back from the exposed edges of sheets at laps and eaves,
• bimetallic contact risks, for example aluminium rails in direct contact with steel sheet without isolation,
• any previous over-sheeting or coating that may hide problems underneath.

Installers normally specify stainless steel or coated fixings and isolating pads to avoid galvanic corrosion between different metals. These details are small but they decide whether the roof stays watertight for the life of the array.

Roof sheet age and remaining life

Solar panels commonly carry performance warranties of 25 to 30 years. If the roof sheets have significantly less life left than that, you may face lifting the array to re-roof part way through. The survey estimates the age of the sheets and coating, records their condition, and helps you compare three options: install now, repair and install, or re-roof and install. Asbestos cement sheets are a separate case covered on our asbestos roofs page.

Pitch, orientation and shading

Most profiled metal roofs are shallow, so panels usually follow the roof slope rather than sitting on tilted frames. On a duo-pitch portal frame, that often means one slope faces roughly east and the other west, or one south and one north. East and west slopes both work well. North-facing slopes at shallow pitches can still be worthwhile in some cases, but they produce less, and the feasibility work will show whether they earn their place. The surveyor also maps shading from adjacent taller buildings, extraction flues, roof-mounted plant and parapet walls at gable ends.

Getting a survey

A roof survey is part of every free assessment we arrange. If your metal roof looks suitable from the details you give us, we match you with one MCS certified installer who visits or carries out a virtual survey, with no obligation to proceed.