Can an old commercial roof take solar panels?

Putting panels on a worn roof means that a later repair has to lift them, and that costs more than doing the roof first. So we begin with the covering. Published lifespans vary by system and are never promises: built-up felt is quoted at 10 to 20 years, EPDM at around 20 to 30 years or more, and GRP at 25 to 40 years, all depending on workmanship and upkeep. Metal sheets and fibre cement sheets have their own wear patterns.

A roof that is already leaking, ponding, corroding at the laps or showing split seams is not a candidate for fixings through it. If the covering needs to be replaced within a few years, we would usually renew it before the panels are fitted, or at the same time. Our answer on what a commercial roof survey covers sets out how we record condition, drainage and fragile areas, which is where a solar assessment begins.

Solar panels add weight, and on a flat roof so do the ballast, trays or frames that hold them down. The roof structure, whether steel portal frames, timber trusses or a concrete deck, must be able to take that on top of its existing loads. The Planning Portal steers owners towards a structural engineer or surveyor when the weight on a roof changes materially. Approved Document A defines a significant increase as more than 15%, with strengthening needed where the structure cannot sustain it, while the Planning Portal's re-roofing page words the trigger as 15% or more. Because the two differ slightly, ask your building control body which applies to your scheme.

In practice we do not guess. An engineer calculates the existing capacity and the added load, and tells you whether the roof can take it, needs strengthening or is the wrong place for the array. Ponding or a dipped deck counts against a roof, and our guide to ponding on a commercial flat roof explains why.

The fixing method depends on the covering, and each has its own risk to the waterproofing.

Metal sheets and standing seams

Panels are often clamped to the seams or fixed to the purlins through the sheet. Clamps avoid new holes, while through-fixings need sealing and good flashing details.

Fibre cement and older sheets

These are brittle, and an asbestos cement roof raises a safety question before anything is fixed. Drilling is not something to do without advice. See the HSE position described in our answer on asbestos cement roofs.

Flat roofs

Frames may be ballasted with weights, fixed through the membrane or adhered to it. Each changes the load or the waterproofing details, and each needs the membrane manufacturer's approval for the detail to be sound.

The installer of the panels and the roofer should agree who does which part, so that every penetration is detailed once and responsibility is not left between two trades.

Fitting panels means many people walking on a roof and carrying heavy items across it. That is hard on a membrane and risky on a fragile surface. Rooflights, corroded sheets and weathered fibre cement can fail under a person's weight, so walkways, edge protection and rooflight covers are planned before anyone goes up.

A solar scheme also changes how a roof is maintained. Gutters and outlets still need clearing, and the roof still needs inspecting, but there is now equipment in the way. We leave clear routes and note where the panels sit on the roof plan, so later repairs know what to lift. A leak found after installation can be hard to trace, and our answer on what to do when a commercial roof leaks covers the first steps.

Shop roofs in the town centre add another layer; our answer on roof repairs above shops in Stroud covers the conservation area position for those buildings.

Ask your building control body whether the installation needs notification, and speak to Stroud District Council's planning team if the building is listed or lies in a conservation area. Check your lease or insurer too if you do not own the roof. Our answer on who repairs the roof on a leased unit explains why this matters.

The sensible order is a survey, a structural check, repairs or renewal of the covering, and then the panels. Doing it that way means the roof is in a known condition on the day the array goes up, and anything found later can be traced back to a recorded starting point. We can do the first three, and work with whoever supplies the solar installation so the details agree. Photographs of the roof are taken before and after our work, the written quote covers the whole job, and our own work carries a 10-year workmanship guarantee in writing.

To begin, send us photographs of the roof with its approximate size, and its age if you know it. Tell us too what the solar installer has proposed, such as the number of panels, where they would sit and how they would be fixed, so that the survey can check those details. For the wider service, see commercial roofing in Stroud.