Is Your Commercial Roof Suitable for Solar Panels? A Checklist

Before involving installers, you can usually answer the key questions yourself or with your facilities team:

• Roof covering: what is it made of? Metal sheeting, composite panels, membrane, tiles, slate or fibre cement each need a different approach.
• Age and condition: when was the roof last replaced, and are there leaks, corrosion or damaged sheets?
• Asbestos: was the building constructed or re-roofed before 2000, and does your asbestos register mention the roof?
• Orientation and pitch: which way do the slopes face, or is the roof flat?
• Obstructions: rooflights, plant, vents, parapets and nearby taller buildings or trees.
• Ownership: do you own the building, or will you need landlord consent?
• Drawings: are structural drawings or original construction details available?

None of these alone rules a roof out, apart from untreated asbestos cement. Together they tell an installer most of what they need for an initial design and help you avoid quotes based on assumptions.

• Trapezoidal metal sheeting: the most common commercial roof and usually the cheapest to work on. Rails fix to the sheet profile or through to the purlins with sealed fixings.
• Standing seam metal: panels attach with clamps to the raised seams, so the roof covering is not penetrated. Quick and low-risk where the seam type is compatible.
• Composite insulated panels: metal panels with an insulation core. Mounting is usually straightforward, but the core material matters for fire safety and insurance, as explained below.
• Flat membrane or felt roofs: usually take ballasted or mechanically fixed frames at a low tilt, often in an east-west layout. Ballast adds weight, so structural checks are essential.
• Tiles and slate: more common on older offices and mixed-use buildings. Installation takes longer, and in-roof systems or replacement tiles are sometimes used.
• Fibre cement: common on older industrial buildings and farm sheds. Often fragile, and sheets installed before 2000 may contain asbestos.

The roof type is the first thing an installer will ask about, because it drives the mounting system, the access arrangements and much of the labour cost.

A roof-mounted solar array typically adds around 12 to 25 kg per square metre of permanent load, including panels, rails and fixings. Ballasted systems on flat roofs can add considerably more, often 50 to 80 kg per square metre, because the ballast holds the frames down against wind.

Weight is only part of the picture. Wind uplift, which tries to pull panels off the roof, is often the critical design load, along with the effect of panels on snow drifting. The fixings must transfer these forces safely into the building's frame.

Many modern commercial buildings have enough structural capacity for a rack-mounted system without strengthening. Older buildings, lightweight portal frames and roofs designed to minimal loads may not, and some roofs suit a lighter mounting system rather than ballast.

For any commercial project, ask for a structural assessment by a qualified engineer, using the original drawings where available or a site survey where not. The assessment should confirm the roof can carry the dead load, wind uplift and snow load of the proposed design. If strengthening is needed, its cost should be included in the proposal from the start, not discovered after you have signed.

Many UK warehouses, factories and farm buildings built before 2000 have asbestos cement roof sheets. These need particular care, because work that disturbs asbestos is regulated under the Control of Asbestos Regulations 2012.

Solar panels should not be fixed into asbestos cement sheets. Drilling, cutting or walking on the sheets can release fibres, and older sheets are often brittle and fragile underfoot regardless of their asbestos content.

The usual routes are:

• Survey first: an asbestos survey confirms what the roof is made of. If your premises already have an asbestos register, check what it says about the roof.
• Over-cladding: in some cases a new metal roof is fitted over the existing sheets, using specialist methods that avoid disturbing them, and the panels are then fixed to the new roof.
• Strip and re-roof: licensed removal of the asbestos sheets, followed by a new roof designed with the solar array in mind.

Re-roofing adds substantially to the project cost, but it often makes sense on an old roof nearing the end of its life anyway. Solar savings can help offset the cost of a roof that would need replacing within a few years. Ask installers to price the roof work and solar as one combined project.

Solar panels typically last 25 to 30 years. If your roof needs replacing during that time, the panels must be removed, stored and refitted, which can cost a significant share of the original installation price, and the system earns nothing while it is off the roof.

That makes the remaining life of the roof one of the most important factors in the business case, and one that sales proposals often ignore. Before committing, ask:

• How old is the roof covering, and what was its expected life?
• Is there corrosion, cut-edge deterioration, failing fixings or leaks?
• Are rooflights brittle or discoloured?
• Is the roof under a manufacturer's or contractor's warranty, and would solar fixings affect it?

If the roof has less than around 10 to 15 years of life left, it is usually worth refurbishing, over-cladding or replacing it before installing solar. Combining the work avoids scaffolding twice and lets the new roof be designed around the array.

Also check the roof warranty. Some roofing manufacturers and contractors require approved fixings or their involvement for any penetrations, and fixing panels without that can void the warranty.

Insurers take a close interest in rooftop solar, particularly on large commercial buildings, because a fault in DC cabling or connectors can start a fire that is difficult to fight on a roof. Most UK insurers refer to the RISCAuthority RC62 recommendations on fire safety for solar PV installations when assessing commercial systems.

Points insurers commonly consider include:

• Roof construction: some insurers apply extra conditions where an array sits on composite panels with combustible insulation cores.
• Equipment quality: matched connectors from the same manufacturer, correctly rated cables and recognised panel and inverter brands.
• Layout: clear access routes and separation around the array, firebreaks on large roofs and appropriate spacing from rooflights and smoke vents.
• Isolation: clearly labelled DC and AC isolators, and safe routing of DC cables inside the building.
• Inspection: regular electrical testing and thermal imaging to find hot spots before they become faults.

Tell your insurer about the project before you sign, and ask whether it has specific requirements. An installer familiar with RC62 should be able to explain how the design addresses each point.

Orientation affects how much a roof generates, but most commercial roofs can produce worthwhile output.

• South-facing pitched roofs: the highest yield per panel.
• East and west-facing slopes: lower yield per panel, but generation is spread across the morning and afternoon, which can suit businesses with steady daytime demand.
• Flat roofs: frames at a low tilt, often in east-west rows, fit more panels into the space at a slightly lower yield per kWp.
• North-facing slopes: usually avoided unless the pitch is very shallow.

Shading can matter more than orientation. Parapets, plant rooms, ventilation equipment, lift overruns, neighbouring buildings and trees all cast shadows that move through the day and lengthen in winter. Installers usually leave space around rooflights and plant, and design layouts to keep shaded areas clear or use optimisers where some shading cannot be avoided.

Ask for a shading analysis that covers the whole year rather than a single site visit. Our guide to solar output in winter explains why low-sun shadows are often missed in summer surveys.

Working at height is the main safety risk in any rooftop installation, and the way access is managed affects both the cost and the programme.

• Fragile surfaces: rooflights and some roof sheets will not bear a person's weight. Installers need covers, netting or safe walkways, and this is a common source of extra cost on older buildings.
• Edge protection: scaffolding, guardrails or other edge protection is usually required for the duration of the work.
• Lifting: panels and rails need to be lifted to the roof, typically by telehandler, hoist or crane, which needs space and may affect site operations.
• Site operations: live loading bays, vehicle movements and public areas may need restrictions during installation.

On most commercial projects, the installer works under the Construction (Design and Management) Regulations 2015, so agree who will act as principal contractor and ask for a method statement and risk assessment.

Also plan for the future. Permanent safe access, such as walkways or anchor points, makes maintenance, cleaning and inspection safer and cheaper over the system's life, and some insurers expect it.

If you lease your premises, the roof usually belongs to the landlord, so you will need written consent before any installation. Most landlords will want to know:

• Who owns, insures and maintains the solar system
• Who is liable for any roof damage or leaks caused by the installation
• Whether the structural assessment and roof condition have been checked
• What happens to the system when your lease ends
• How the system affects the building's insurance and service charges

The remaining length of your lease also affects whether the investment makes sense. A system with a four to six-year payback suits a lease with at least ten years left. On shorter leases, options include asking the landlord to fund the system and sell you the electricity, or a power purchase agreement aligned to the lease.

Read more on our real estate and property portfolios page, and in our guide to PPAs versus buying outright.

Use this checklist when you instruct installers, so every proposal is based on the same, accurate information about your building:

1. Roof covering type, age and any known defects or leaks
2. Asbestos register entries or survey results for the roof
3. Structural drawings or construction details, if available
4. Remaining roof warranty and any conditions on fixings
5. Rooflights, plant, parapets and other obstructions
6. Access routes, lifting space and operational restrictions
7. Insurer requirements for solar installations
8. Landlord consent, if you lease the building

Then ask every shortlisted installer to confirm in their proposal that a structural assessment has been or will be carried out, how they will deal with fragile areas, and whether any roof repairs or strengthening are included in the price.

Our guides to commercial solar costs and comparing quotes explain how roof factors affect pricing. When you are ready, compare quotes from up to three MCS-certified commercial installers for your building.