Installing in roof solar panels on existing roofs

Marley Solar panels being installed onsite
Director Roof Systems
Stuart has been working in the construction industry for over 30 years and has spent the last 14 years at Marley.

Considerations for installing in-roof solar panels on existing roofs

When installing in-roof solar panels – also known as roof-integrated solar panels – onto an existing roof, there are several factors that contractors must consider to ensure the project is completed successfully, safely and to a high standard.

The benefits of in-roof solar panels

Solar panels are a common feature of many new-build homes. They are an environmentally friendly choice for families who want to reduce their carbon footprints, and they can also help cut energy costs, providing free electricity for lighting, heating, operating household appliances and even charging electric vehicles.

Stuart Nicholson, Director of Roof Systems here at Marley explains that in-roof solar panels are also an ideal choice for roofing contractors whose clients want to add the benefits of solar PV to their homes. “In-roof solar panels are easy to install and look good on both new and existing homes,” he says.

“Marley’s SolarTile is a roof-integrated solar panel that is quick to install and requires no specialist tools. The panels are lightweight for safe and easy movement on site, and in many cases, installation of a solar array can be completed in just a day,” he adds.

When is the best time to install in-roof solar panels?

The best time to install in-roof solar panels in an existing roof is in conjunction with a full re-roof. This enables the whole roof to be checked and new roofing products installed throughout. In many cases, this can be the most cost-effective option and will help to ensure long-term performance.

In-roof solar panels can be installed without a complete re-roof, but the project's full potential value may not be realised.

Installing in-roof solar panels: assessing the existing roof’s condition

Before installing in-roof solar panels on an existing roof, it is important to assess the condition of the roof coverings. This includes checking the roof tiles for:

  • build-up of moss and lichen
  • broken, missing or cracked tiles/ridge tiles
  • missing or loose flashings

The approximate age of the roof covering should be established, and an assessment made as to how many years it will be before it needs to be replaced. The rafters and the roofing battens should also be checked to ensure they are of adequate size and condition for the specified fixings, and the roof underlay should be checked to ensure it is free from damage and has not degraded, especially where exposed at the gutters.

If the roof condition is deemed suitable and structurally sound, and any necessary remedial works have been carried out, then areas of tiles can be stripped out and the in-roof solar panels installed.

Many in-roof systems rely on the rafters being reasonably level for a successful installation. Rafters that are not level, will require packing timbers to level them up. Very old roofs which have obvious undulations may not be suitable for some in-roof systems.

Counter battens and in-roof solar panels

The Marley SolarTile has been designed to fit to tiling battens and the rafter without the requirement of extra timbers or counter battens. Some in-roof systems were originally designed to be installed over counter battens, and this should be carefully considered when fixing to existing roofs.

Stuart explains: “Whilst the counter battening of roofs is common across Europe, in most parts of the UK, we do not normally install counter battens due to the disruption caused by the level of the roof being lifted, especially at the junction between adjoining properties. As such, the fixings that are supplied with the system could puncture the underlay where there is insufficient drape, or the underlay is tauter on either side of the rafters.”

He advises: “Installers should reduce the length of the fixings accordingly but check with the manufacturer to ensure that wind loadings are not compromised and to understand how this may affect their own liabilities. You can contact the Marley technical team to discuss your project requirements in more detail.”

Dealing with sources of shade when installing in-roof solar panels

Tall trees and nearby buildings that cast large shadows onto the roof can be an issue when installing any solar PV on an existing property. It is advisable to assess the risk of shading and, where possible, remove or minimise that risk before designing and installing the system.

In some cases, the person responsible for designing and commissioning the system (normally the MCS installer) may specify devices such as optimisers or micro-inverters, which can limit the problems caused by shading, this could significantly increase the cost of the array.

Weight considerations of in-roof solar panels

Most in-roof solar panels weigh much less than the clay or concrete roof tiles they replace. This means that if one side of the roof is substantially covered in solar panels, then the weight could be significantly less on one side of the roof than on the other.

Installing solar on both sides of the roof would obviously balance out the weight, this is ideal for east/west facing roofs as you will maximise the solar generation throughout the day, but not feasible with north/south facing roofs as the north elevation will generate minimal amounts of solar. On-roof systems add additional weight to the roofs, this can be problematic as it is likely to change the roof's weight by as much as 15%, so a structural survey may be required. It’s best to consult a structural engineer if there are any concerns.

Electrical considerations for in-roof solar panels

While any roofing contractor can install in-roof solar panels, the electrical connection must be made by a qualified electrician. To ensure the electrical connection goes smoothly, here are some things to remember:

  • String diagrams: A string diagram should be provided by the electrician/MCS installer. This will show how many panels there are per ‘string’, how the cables are to be routed and where the final connections should be fed back into the roof space.
  • Recording serial numbers: Assuming the PV system will be connected to the national grid, the serial number of each in-roof solar panel should be recorded for MCS compliance. Most panels have a sticker on the underside or the sides, which can be photographed as part of the record-keeping process.
  • Testing before installation: It is good practice to test each solar panel individually with a multimeter before installing it and, where possible, test the full ‘string’ before inserting the final connections into the roof for the electrical installation.

Choosing compatible products for an in-roof solar panel system

Stuart points out that compatibility among products is another important consideration: “Products that are designed to work together will make specification, installation and commissioning of a solar array that much easier.”

He adds: “As part of the Marley Roof System, the Marley SolarTile is completely compatible with our entire range of roofing products, including roof tiles, battens, underlays and roofing accessories – and the system comes with a 15-year warranty.”

Marley also offers a variety of compatible solar accessories, such as inverters, battery storage systems, EV charging systems and fire protection. Please visit the renewables section of our website to find out more.

If you have any questions about installing in-roof solar panels on an existing roof, please get in touch – a member of our technical team will be happy to help.

At Marley, we are able to offer roofers a 1-day training course on the installation of our roof integrated Marley SolarTile® at our Head office in Burton-upon-Trent. If you are interested, you can find out more and book your place on our website.

Category: Solar