The Roof Nobody Designed for Solar

Lightweight industrial roofs were typically designed for the loads they needed to carry: their own weight, maintenance foot traffic, wind and, in some places, snow. Additional dead load was often minimal. Solar arrays add weight, change how wind acts on the roof, and concentrate loads at mounting points. On many roofs this is fine. On some it is not, and the difference can only be known by checking the structure and connections.

In Australia, the 2022 National Construction Code required new commercial buildings to keep at least 20 per cent of the roof clear for future solar and to provide switchboard space for solar and batteries. For warehouses it went further: Part B required long-span warehouse roofs to allow for the future structural load of panels. The 2025 edition goes further again by requiring on-site solar on new commercial buildings. None of it reaches back to existing buildings, and most sheds built before 2022 were never designed for the load.

Metal roof sheeting has a finite life that depends on coating, environment and maintenance. Coastal and industrial atmospheres shorten it. A solar array installed on sheeting with ten years of life left will have to come off when the roof is replaced, at real cost. The sensible sequence is often roof first, panels second, which can move a roof replacement earlier than condition alone would justify.

Putting a twenty-five-year array on a roof with ten years left is a capital decision, whether or not anyone made it consciously.

The sequencing problem is a mismatch between two lives. Solar modules commonly carry performance warranties of around 25 years; many roofs under them have far less left.

The arithmetic favours doing the roof first where its remaining life is shorter than the array’s. Removing and reinstating an array to renew the roof beneath it adds cost, downtime and risk to a job that would have been simpler done in the right order.

Older sheds often have translucent roof sheets and skylights that become brittle with age and present a fall hazard. Solar installation means more people on the roof, which brings fall protection, safe access and walkways into scope. Some older industrial roofs in Britain and Australia still carry asbestos cement sheeting, which must not be walked on or drilled without specialist management.

Fire is the other consideration. Arrays add electrical equipment across the roof surface. Insulated panel roofs and walls with combustible cores, used in some buildings, change the fire risk. Insurers and fire authorities increasingly take an interest in how arrays are designed, isolated and accessed.

A structural assessment for rooftop solar on an existing shed typically covers five things. The additional dead load of panels and mounting against the design capacity of purlins, rafters and portal frames, including any corrosion loss. Wind effects, since arrays change uplift and can concentrate load at mounting points and roof edges. Connections, which are often the weakest link in light steel roofs. The condition and remaining life of the sheeting the mounts attach to. And the roof’s ability to take the foot traffic and access equipment that installation and maintenance bring.

The outcome is usually one of three: the roof can take the array as designed; it can with local strengthening or a lighter layout; or it should be re-sheeted first and the array designed with the new roof. Knowing which before procurement avoids the common failure of buying an array for a roof that cannot carry it.

The physics is the same in Dandenong, Doncaster and Dallas. The drivers differ. Australian owners of existing sheds face tenant demand, NABERS and the example set by the new-build mandate. British owners face minimum energy standards for letting and grid connection constraints. American owners face local performance standards in some cities and incentive regimes that change with federal and state policy. In all three, the roof is the physical gate, and the register should read it as one system: structure, sheeting, drainage, skylights, access, hazardous materials and the array itself.

Read the roof as one system: structure and connections, sheeting and its remaining life, penetrations and drainage, skylights and access, and any hazardous materials. That reading says whether the roof can host solar now, after strengthening, or only after replacement.

It also turns an energy decision into a sequenced capital plan, which is the right place for it. The roof is no longer just weatherproofing. It is part of the building’s energy system.