Solar Panels on Metal Sheet Roofs

Every method that drills through the sheet carries some risk. What we do is keep the risk as low as possible: fixing on the ribs, not the valleys; using screws with built-in EPDM rubber seals; adding a layer of sealant on top; and always fixing into the steel purlins. If your roof is a standing-seam type, we use seam clamps, which need no drilling at all.

In that case the roof has to be repaired or replaced first. Drilling more holes in sheets that are already corroded will leak no matter how well it is sealed, because the steel around the holes has no strength left to hold a screw. We tell you at the site survey that this is a cost to plan for before installation, so the roof under the panels starts out sound.

The sheet isn't what carries the weight — the steel purlins underneath do. That's why we locate the purlins and measure their spacing before deciding on the fixing points. If the purlins are further apart than the mounting system needs, extra purlins have to be added or the rail layout changed. Sheet thickness matters because thin sheets dent when walked on, so we use load-spreading boards while working on the roof.

The structure has to be checked every time. Older wide-span buildings were usually designed to carry only the original roof and wind loads, with no allowance for panels. Panels plus mounting weigh around 12 to 15 kg per square meter, which most buildings can take, but we don't judge it by eye. An engineer has to calculate and certify it, because that certificate is also needed for the permit application.

It's a real effect that has to be designed for. Long metal sheets can expand and contract by several millimeters with temperature. If the rails are locked tight at every point with no allowance for movement, the stress builds up at the fixings until they loosen or the screw holes widen. So we leave clearances and use fixings that allow movement as the rail manufacturer specifies, rather than tightening every single one solid.

There is some effect. Solar panels produce less power as their temperature rises, and metal sheet roofs hold more heat than tiles. But the real impact is smaller than many people fear, because we leave an air gap under the panels so the breeze can carry the heat away. That gap is part of any correct installation anyway — panels should never be laid flat against the roof sheet.

Our installation work carries a 2-year warranty, which covers leaks from the fixing points we make. It doesn't cover leaks from the roof's existing screw holes or from sheets that were already corroded. That's why we photograph the condition of the roof before every job starts, so both sides have the same record of what was there beforehand.