Can My Commercial Roof Take Solar Panels?

Solar panels add weight, and on a flat or low-pitched commercial roof they also add wind load, because an array catches the breeze like a sail. Whether the roof can take that is a structural question, and nobody should answer it by eye from the yard. A structural engineer looks at the steel or timber frame, the purlins (the horizontal rails the sheets are fixed to) and the fixings, then states what load the roof can carry and how the array should be held down. On a flat roof that may be ballast; on a metal roof it is often clamps to the seams or fixings through to the frame.

Ask the solar installer and the engineer to speak to each other before anything is ordered, and ask for their calculations in writing. If you rent the building, your lease may require the landlord's consent for work on the roof, so read it early. Planning and building control rules can apply as well, and they depend on the building and where it stands, so check with the council's planning team and your building control body rather than assuming either way. Our commercial roofing page lists the wider work we do on industrial and retail buildings.

An array sits on the roof for decades, so the covering underneath has to outlast it or be renewed. Lifting panels off to repair a membrane later costs far more than doing the roof properly once. On a single-ply or felt roof we look for blistered or split laps, ponding where water stands after rain, brittle edges and old patches that have started to lift. On profiled sheets we look at rusting fixings, cut-edge corrosion and cracked sealant at the laps.

A survey also shows whether the drainage copes. Panels and their cable routes can block outlets or channel rain into one spot, so gutters, outlets and falls matter as much as the surface. Our guide to a commercial roof condition survey lists what is checked. The written result tells you plainly whether the covering can carry on beneath an array or should be replaced first, and you can then give the solar company a firm answer instead of a guess about remaining life.

If the covering is nearly worn out, renew it before the array goes up. Panels hide the membrane, make leaks harder to trace and have to be disconnected, lifted and refitted whenever the roof below needs work. Many owners choose to re-roof and install in one programme, so the new waterproofing is laid, the array fixings are detailed into it and the two trades do not damage each other's work.

Fixings that pass through the covering are the weak point. Each one is a hole that needs a proper upstand, flashing or sealed bracket, and the people who made the roof watertight should agree how it is done. Ask the solar company to share the fixing layout with your roofer before the installation date, and keep a record of where each one sits. A decision between a coating and full replacement is easier when it is made with the panel layout on the table, because a coating that cannot be walked on safely, or that suffers from the point loads under ballast blocks, may not suit an array at all. Ask for the maintenance plan too: someone will need to reach the membrane for inspection, cleaning and repair, so walkways and gaps between rows are worth settling at design stage.

Many older industrial roofs include rooflights and sheets that cannot take a person's weight, and installers carrying panels across them face a serious fall risk. Fibre-cement sheeting from before 2000 should be treated as asbestos-containing until it has been tested, and a roof made of asbestos cement can crack under a footstep without warning. Panels must never be laid over damaged sheets, and any overcladding or removal has its own rules, so the type of sheet has to be known before an installer sets foot on the roof. Our pages on fragile roof safety and overcladding an asbestos roof explain the options.

Edge protection, safe routes for cabling and a place to land materials all need planning, and the construction rules on who coordinates site safety apply once more than one contractor is on the project. Send us the engineer's report and the proposed array layout, and we will inspect the roof, share photographs of its condition and set out the roof work needed before, or alongside, the installation as a single written total, so the solar project and the roof project can be planned together. Check whether any price you are given covers VAT and the access equipment.