Guide for roofers and installers · 14 min read

Roof Overcladding or Replacement Before Solar: A Guide for Roofers and Installers

Short answer: many commercial buildings that suit solar, such as warehouses, factories and farm buildings, sit under ageing asbestos cement or profiled metal roofs. Solar panels are expected to stay up for decades, so the roof under them has to last as long, and a business weighing up solar is also weighing up its roof. Overclad or replace first, then fit the array. That means scoping both together: asbestos, structure, fire classification, Part L, sequencing and how the cost is presented. For roofers it is a reason to work with solar installers; for solar installers, a reason to survey the roof properly or bring a roofer in.

Why roof work and solar are being priced together

A tired roof used to be a reason to say no to solar. Increasingly it is part of the same conversation. On our own commercial solar calls we hear business owners who have been putting off roof work ask whether it makes sense to sort the roof at the same time as the panels, because the electricity the array saves and the way the spending is treated for tax can help them justify a job they knew was coming anyway. We do not have figures on how common that is; it is simply a question that comes up.

The timing logic is simple. HMRC treats plant that can reasonably be expected to have a useful economic life of at least 25 years as a long-life asset, and its manual notes that most solar panels were in that class before they were made special rate expenditure in April 2012. An array is expected to stay on the roof for that kind of span. If the roof under it needs replacing half way through, the array has to come off, be stored and go back on, with access equipment twice and the system off in between. Doing the roof first, once, avoids that.

The roofs this applies to

• Asbestos cement (fibre cement) sheets. HSE’s advice is that if a building was built or refurbished before 2000, you should assume it contains asbestos. Asbestos cement roofs are fragile: HSE warns not to walk on them.
• Older profiled metal roofs. Corroded sheets, failed fixings, leaking laps and discoloured or brittle rooflights. HSE says all roofs should be treated as fragile until a competent person has confirmed they are not, and fragile rooflights are a particular hazard because some are hard to see or painted over.
• Any roof that will not outlast the array. RC62, the insurer backed fire safety code for commercial rooftop PV, points out that older roofs may have deteriorated with age or be in a poor state of repair, and that a competent person should survey the load-bearing capacity of the roof before installation.

For the asbestos detail, including registers, which survey to ask for, licensing and drilling versus overcladding, read the Solar Surveys guide to solar panels on an asbestos roof. This guide does not repeat it.

Overcladding vs full replacement (vs leaving it)

There are three ways to handle the roof. Which suits depends on the condition of the sheets and of the frame under them, and on how much disruption the business can take.

On asbestos cement, HSE says most work should be done by trained non-licensed workers, but work likely to cause significant break-up of the material, such as dropping an asbestos cement roof, has to be notified, and badly damaged material may need a risk assessment to decide whether a licensed contractor is required. Overcladding avoids removing the sheets; it does not remove the asbestos, which stays part of how the building is managed under the duty to manage asbestos in non-domestic premises.

How to scope a combined roof and solar job

Work through these before anyone prices anything:

1. Asbestos. Ask the dutyholder (the owner, landlord or whoever is responsible for maintaining the building) for the asbestos register. HSE says a refurbishment/demolition survey is required before any work that may disturb the fabric of the building, and that it is likely to be intrusive and destructive. Build it into the programme.
2. Structure. Planning Portal guidance on rooftop solar says the existing roof’s ability to carry the panels “will need to be checked and proven” and that some strengthening may be needed. With overcladding the frame carries the old sheets, the new build-up and the array, so the engineer needs all three. RC62 adds that ballasted systems can significantly increase roof loading and can increase water pooling as the roof flexes.
3. Remaining life. Compare the roof’s remaining life with how long the array is meant to stay up. Where the roof will not last, the roof work is part of the solar job.
4. Fire classification. RC62 says PV should be installed on non-combustible roofs meeting Class A1/A2-s1,d0 to BS EN 13501-1, and that new rooftop PV, including its fixings, shall not lower the fire performance or classification of the roof. That makes the overclad or new roof build-up, and the insulation in it, a decision for both trades. The Solar Surveys guide to RC62 and solar fire safety covers what insurers expect.
5. Rooflights and access. Decide what happens to every rooflight and how the array will be laid out around them. HSE’s guidance is to protect fragile rooflights with secured, labelled barriers or covers and to use a platform beneath the roof where possible. RC62 also asks for safe, clear access routes for maintenance and firefighting, and warns against high-density coverage that stops firefighters venting the building.
6. Building control. Overcladding and re-roofing can both bring in Part L (next section). Confirm the route early.
7. Grid and certification. Start the DNO application in parallel: it should not wait for the roof. MCS covers solar PV up to 50kW, so larger commercial arrays sit outside it.

Part L: the insulation upgrade that comes with the roof

This is the part many solar quotes miss. Regulation 23 of the Building Regulations 2010 (England and Wales) says that where the renovation of a thermal element covers more than 50% of its surface area, or is a major renovation, the whole element must be brought up to the energy efficiency requirement, as far as that is technically, functionally and economically feasible. Replacing more than half of an element brings in the same requirement.

In England, Approved Document L Volume 2 (2021 edition incorporating 2023 amendments) defines renovating a thermal element to include providing a new layer by cladding its external surface, which is what overcladding is. A major renovation is when more than 25% of the external envelope is renovated. The renovated element should reach the improved U-value in its Table 4.2:

If the improved value is not technically or functionally feasible, or would not pay back within 15 years, the element should be upgraded to the best value that is feasible and pays back within 15 years, and generally to no worse than 0.7 W/(m²·K). A 2026 edition of Approved Document L Volume 2 takes effect in England on 24 March 2027, so check which edition applies to the job. Wales, Scotland and Northern Ireland publish their own guidance; ask building control. For the business, the upshot is a better-insulated building as well as a new roof and an array, and for both trades it is a line that has to be in the price.

Sequencing and who does what

Agree in writing, before work starts, how the array will be fixed to the new roof and who signs off each penetration and flashing. Ask the roof system’s manufacturer what its warranty allows for mounting brackets, and make sure the roofer’s warranty, the cladding system’s warranty and the solar installer’s workmanship warranty do not leave a gap between them.

With two contractors on one project, CDM 2015 applies: on a project involving more than one contractor, the client appoints a principal designer and a principal contractor to coordinate the work. Decide which of you takes that role, or whether the client appoints a main contractor above you both.

Presenting the combined cost to the business

Keep the roof and the solar as separate lines, because they are treated differently. The business should take its own advice from its accountant; what follows is what GOV.UK and HMRC say.

• The solar array is plant and machinery. Solar panels have been special rate expenditure since April 2012. A company can claim a 50% first-year allowance on new and unused special rate plant bought from 1 April 2023 (full expensing, at 100%, is for main rate plant). HMRC’s policy paper of 22 November 2023 made both permanent by removing the 1 April 2026 end date. The Annual Investment Allowance gives 100% relief on most plant and machinery up to £1 million, and is open to sole traders and to partnerships whose members are all individuals as well as companies.
• The roof is not plant. GOV.UK lists buildings among the things you cannot claim capital allowances on.
• Repair or improvement. HMRC’s manual gives a new roof that simply returns the roof to its original condition as a repair, which can be deducted as a business cost. If the work alters or improves the asset, it is not a repair and is capital.
• Structures and Buildings Allowance. Capital spending on construction, including renovation, repair and conversion, of a building used for a qualifying, non-residential activity may qualify for the SBA at 3% a year (from 1 April 2020 for companies), for contracts signed on or after 29 October 2018. It cannot be claimed on costs that qualify for plant and machinery allowances.

Then show the solar savings from the business’s own figures: its half-hourly data or 12 months of bills, the array’s modelled generation and how much of it the site will use. Do not borrow savings figures from another site. The guide to selling commercial solar on Solar Surveys covers building the proposal.

Roofers and solar installers working together

For commercial roofers:

• Know how your overclad and re-roof systems take PV mounting, and what each manufacturer’s warranty allows.
• Have an answer on fire classification and Part L for each build-up you offer.
• Talk to the commercial solar installers in your area: they are the ones being asked about the roof.
• Offer a joint survey slot, so a roof question does not stall a solar proposal for weeks.

For solar installers:

• Record roof type, age, condition and rooflights on every commercial survey, with photos.
• Ask for the asbestos register before the survey, not after the quote.
• Have a roofer you trust to price overcladding or replacement quickly, or quote the roof yourself if you have the competence.
• Decide in advance who leads the project and holds the CDM roles when both trades are on site.

Where TNW Marketing fits

TNW Marketing books commercial solar appointments for UK solar installers, £250 to £1,000+VAT per booked appointment, with no retainer. Some of the commercial solar appointments we book are on sites that also need roof work first, a roof replacement or an overcladding quote, before panels can go on. We do not sell roofing leads. If you are a roofer, the way into that work is through the solar installers who take those appointments (more on our page for commercial roofers). If you are a solar installer, see how pay per appointment commercial solar leads work, or read how to qualify commercial solar leads.