Can you install solar panels on a flat roof? What you need to know

Solar panels are soaring in popularity, thanks to their ability to slash energy bills and produce renewable electricity from sunlight. They generate most energy when they’re angled correctly towards the sun, so it does help to install them on a pitched roof, but solar PV panels can be just as efficient and cost-effective on a flat roof.

When solar panels are installed on a flat roof, the installer usually uses a mounting frame to pitch the panels at an angle of between 20 and 40 degrees. This puts them in the best possible position to capture maximum sunlight for converting into electricity to use in your home.

The additional hardware and time required to install solar panels on a flat roof can add to the cost of installation, so that’s something to bear in mind. You may also need to get planning permission. Despite these extra challenges, installing solar panels is still worth doing if you have a flat roof. We estimate that the average three-bedroom house will save £608/yr with a 3.5 kWp solar PV system.

Read on to find out more about how solar panels are installed on flat roofs, and what you need to know if you’re considering investing in solar for your flat-roofed home.

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How are solar panels installed on flat roofs?

To install solar panels at the most efficient angle on flat roofs, installers use a device such as an A-frame to mount panels at an angle, ideally around 35 degrees from horizontal. This ensures that they capture as much sunlight as possible throughout the day, and also helps to keep the panels clean with the help of the rain.

Flat roofs don’t need special types of solar panels, but certain features can be especially useful for capturing maximum sunlight. Thin film solar cells are particularly light and flexible, and can be turned into automatic panels that move with the sun throughout the day, capturing as much light as possible as the day goes by.

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What angle is best for solar panels?

As we explain in our article on the best direction for solar panels, solar panels work at their maximum potential when they’re pitched at 35 to 40 degrees. If you lived on the equator, your solar PV panels would get most light at an angle of 0 degrees from horizontal – that is, lying completely flat on your roof, or on the ground. But in the UK and across much of the northern hemisphere, a 35 degree angle captures maximum sunlight.

However, the direction your solar panels face is more important than the angle they’re pitched at. If your panels face south, they’ll capture the full maximum (100%) of solar energy yield at a 35 degree angle, and 90% of maximum yield if they’re 10 degrees from horizontal. North-facing panels capture much less sunlight, whatever angle they’re pitched at.

A qualified installer will work out the best angle and position for your solar panels, depending on the type of roof you’ve got and the amount of light it gets.

Can I lie solar panels flat on my flat roof?

You can, and it may be cheaper to do this than to have them installed on a frame to pitch them at an angle. But the big problem with this approach is that it can invalidate your warranty. Only a few manufacturers give warranties on panels installed at an angle of less than 2 degrees from horizontal.

One of the reasons manufacturers are reluctant to guarantee horizontal panels is that they’re more vulnerable to damage from water and dirt. When solar panels are pitched, they’re kept largely clean by the rain (any angle more than 10 degrees is reckoned to be ‘self cleaning’), but flat panels tend to collect dust and debris, which can reduce electricity generation by up to 10%.

What are the disadvantages of installing solar panels on a flat roof?

Even when solar panels are pitched on a frame, a flat roof can present problems such as dirt and water pooling. Pitched roofs direct water to eaves and gutters, after giving your panels a nice wash, but flat roofs allow dirt to accumulate in the mounting system. Flat roofs also tend to collect (‘pool’) water in certain areas. If pooling has ever caused leaks of water damage to your home, get it fixed before you consider installing solar panels.

High winds can pose a problem to panels installed on a frame on top of a flat roof. Gusts can whip through the frame and make the panels vulnerable. The higher up and more exposed your flat roof is, the more problematic the wind can be.

If you live in a particularly windy area, you could opt to have your panels fixed horizontally to your roof, or add weight to them to keep them secure. But both these solutions have their downsides. Horizontal panels are less efficient, as we’ve seen, and adding weight can risk damage to your roof. Ask a structural engineer to check that the roof can hold extra weight before you’re tempted to go that route.

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What are the advantages of flat roofs for solar panels?

The main advantage is ease and safety of access to your solar PV system. Your installer will be able to work more easily than on a pitched roof, and you’ll be able to get up there to clean, check and maintain your solar panels.

Panels on flat roofs also tend to be less visible from the ground, which may be an advantage if you or your neighbours aren’t keen on the appearance of a solar PV array.

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Do I need planning permission to install solar panels on my flat roof?

The need for planning permission is another challenge presented by flat roofs. With a pitched roof, solar panels are designated as a ‘permitted development’, which means you don’t need planning permission. But in many cases, flat roofs are exempt from this designation.

You will need to apply for planning permission to install solar panels on…

• A flat roof, using a frame to mount the panels at an angle. You don’t need planning permission for completely flat panels, but you do for angled solar panels that protrude more than 0.2 metres above your flat roof.
• A listed building, whatever the pitch of your roof. First you will need listed building consent, and then planning permission. You’ll have much more chance of getting permission to install solar panels within the grounds of a listed building, so if your home is listed you may be better off asking to install solar panels in your garden, on a frame to pitch them at an angle.
• Any building in a conservation area.

The government’s planning permission guide has more information about planning applications across the whole of the UK. To apply to your local authority in England and Wales, use the Planning Portal website.

How much does it cost to install solar panels on a flat roof?

We have seen one or two installers claim that a flat roof installation costs no more than a standard pitched roof installation, but this is rarely the case. Generally, installing solar panels on a flat roof will cost from around 5% to 20% more, depending on your roof and the mounting technique you choose.

In 2023, the average cost of a standard 3.5 kWp solar panel system is £7,860. This size is generally the right choice for a three-bedroom household. To find out more about pricing up your solar plans, see our article Are solar panels worth it?