The short answer
A commercial roof can take solar panels when its structure can carry the added load and its covering will outlast the installation. Panels add weight, wind uplift and foot traffic, so a structural check comes first. A tired membrane or corroded sheet should be renewed beforehand, because lifting panels later to repair what lies beneath costs far more.
Loading: what the structure has to carry
Panels are not heavy by themselves, but a solar array brings more than its own weight. The mounting frames add load, and on a flat roof ballast, usually concrete blocks, may be used to hold the array down without drilling the membrane. Wind pushes and pulls on a tilted panel as it would on a sail, so the fixings resist uplift as well as downward load. Snow and the weight of people maintaining the array come on top.
A structural engineer should assess the roof before the design is finalised. On a steel portal frame the question is whether the purlins and rafters have spare capacity. On an older timber or concrete roof the question may be about decay, past alterations or loads that nobody recorded. The assessment tells the installer where panels can go, where they cannot and whether the frames need strengthening.
Re-covering a roof with a heavier material can bring Building Regulations approval into play. Per the Planning Portal, a replacement covering that makes the roof 15 per cent heavier or more brings approval into play. Panels are a separate installation, so ask your building control body whether your scheme needs notifying rather than assuming it does not.
Roof condition comes before the panels
Solar equipment is built to run for many years. A roof covering underneath it may not. Typical lifespans vary with the product and the care it gets: single-ply membranes such as PVC and EPDM are commonly quoted at 20 to 30 years or more, built-up felt at about 10 to 20 years, and profiled metal sheet at over 40 years overall, with the coating needing attention long before that. Those figures come from trade and manufacturer sources, and exposure and the standard of the installation move them either way.
The practical point is simple. If the covering has ten years left and the panels have twenty, the roof will need opening up while the array sits on top, which means taking panels off and refitting them. Renewing a worn membrane or recoating a corroded metal roof before installation costs less than that. Our page on commercial roof systems sets out the main options.
Before any panels go on, we look for ponding water, blown or blistered membrane, open laps, corroded fixings and soft insulation. Wet insulation under a membrane adds weight and loses its value, and fixing through it hides the problem. An array installed over a failing roof also makes leaks harder to find, since the leak path is often several metres from the damp patch inside.
Fixing methods and keeping the weathering intact
How panels are attached depends on the roof. On standing seam and some profiled metal roofs, clamps grip the seams or ribs so that no holes are made in the sheet. On trapezoidal sheets, brackets are fixed through the sheet into the purlin, each with a seal that needs to stay sound. On a flat membrane roof, a ballasted frame sits on protective mats, or brackets are stuck down and flashed into the membrane.
Every penetration is a possible leak. A flashing is the metal or membrane strip that closes the gap round anything passing through the roof, and the more fixings, the more flashings to look after. Ask the installer how each fixing will be sealed and who is responsible for the weathertightness afterwards. If the installer and the roofer are different firms, the responsibility can fall between them. We prefer to be told early what will be fixed where, and to leave clear working room around each plant item.
Access matters as well. Roofs with fragile rooflights need protection before panels go near them, and the array should leave safe routes to rooflights, plant and gutters. A fall arrest system, a fixed anchor line that workers clip into, can be added at the same time. See our page on the roof fall arrest system.
Asbestos, listed buildings and the lease
Some roofs cannot simply have panels added. A roof of asbestos-cement sheeting should not be drilled or loaded without a plan, as our answer on asbestos on an industrial roof explains. Listed buildings and conservation areas bring planning questions of their own, and in Bath the council to ask is B&NES, before you commit to any change that shows from the street.
If you rent your unit, the lease decides who may alter the roof. Many commercial leases reserve the roof to the landlord, and panels usually need written consent. The cost of any roof repair that the panels make necessary can also become a lease question, as we set out in our answer on who pays for roof repairs on a commercial lease. Where a managed building has several leaseholders, large works may trigger a formal consultation, covered in our page on Section 20 roof works.
Settle these points with the landlord before spending anything on design. Our commercial roofing page lists the other jobs we take on for business premises.
Steps to take before you order panels
A sensible order of work avoids expensive rework.
- Survey the roof. We inspect the covering, drainage, fixings and edge details, backed by pictures from your own roof.
- Get the structure checked. A structural engineer reports on capacity and on any strengthening.
- Decide on the roof works. Repair, overlay or renewal, done before the array goes on.
- Agree the fixing method. The installer and the roofer settle penetrations, flashings and access together.
- Check consents. Lease, landlord, planning and building control as they apply to your building.
Some roofs are ready for panels as they stand, and if yours is, that is what the survey will tell you. If it is not, the report lists the roof works needed first, and we put a single fixed figure for them on paper after the visit. Our roofing work carries a 10-year written workmanship guarantee, and it is fair to expect the panel supplier to put their terms on paper too.
