Solar panels on flat roofs: what you need to know
Installing solar panels on flat roofs is a viable option, whether the installation is for a residential or commercial property. You’ll find that most domestic installations will be installed on sloping or pitched roofs as this is standard in UK housing. However, we still see some flat roofs, especially on extensions, garages and garden rooms.
Let’s delve into why it can still be beneficial to install solar on a flat roof, as well as any extra details you need before you make the decision.
Can you install solar panels on a flat roof?
The short answer is yes. While it’s only suitable for certain properties and locations, it’s possible to mount solar panels on a flat roof.
Why would you install solar on a flat roof?
While most will answer this with “because I don’t have a pitched roof”, some homes may only have a North facing roof, so flat panels on an outbuilding or the garden would be the best way to soak up the sunlight.
If the main roof on your home is sometimes shaded, or even partially shaded, it may be advised to fit some panels on a flat surface alongside the ones on top of the house.
Or in other cases, you may live in a flat. This could be slightly more complicated if you share the building with others, but otherwise, placing panels on the roof of a tower block and above smaller flat buildings will still generate energy.
So, how do you install solar panels on a flat roof?
There are many systems that can be used to attach solar panels to flat roofs. Let’s take a look at a few:
Mini rails
Mini rail systems are popular when fitting on box section metal roofs. These rails are screwed straight into the roof using self-tapping screws and usually hold one or two solar panels. For arrays with more panels, the rails are connected using a mid-fixing section.
The advantages of mini rails are they’re cheap, easy to fit and perfect for roofs with low load bearing. They do tend to place the panels at a shallower angle than A frames, causing the solar panels to be flat on occasion. When panels are flat, they don’t self-clean in the rain, so extra cost may be incurred for keeping the array clean.
A frames
A Frames are by far the most popular way of fitting panels on flat roofs or flat ground. They can be bolted to roofs, held down using ballast or tethered to ground anchors, and they allow the panels to be set at the perfect angle.
While they’re often more expensive than mini rails, they’re also much more adaptable, and since they angle the panels, rain will naturally clean them, so you don’t have to worry about the time and money spent on cleaning.
Tubs
If you imagine a domestic storage container cut in half diagonally, you’ll be somewhere near the look of a solar panel mounting tub. Usually, the tub is placed on a flat surface and then filled with sand to secure it. The panels are then secured to the tub at the correct angle, which will be the angle the tubs are made or cut to.
Tubs aren’t designed just for flat roof systems; they can be used for floor mounting too, especially on solar farms. They’re easy to manoeuvre, are lightweight, and are much simpler to get between, keeping cleaning nice and easy.
How much weight can a flat roof hold for solar panels?
The weight limit for flat roofs is usually 20-25 pounds per square foot, which is more than enough for solar panel installations. A standard array will weigh around 3-5 pounds per square foot, but you should always get your roof assessed prior to be sure.
Angled systems for east & west facing panels
One great advantage of a flat roof is you can lay the panels at any angle you like, and in any direction. You could point them in a Southward direction to get the most from that kind of set up, or you can go for an East/West array that can utilise many more panels to increase your yield without any shading. Plus, because the panels support each other, they’re less susceptible to air pressure and therefore can be more secure.
Best angle for solar panels on flat roofs
As mentioned, the fact you can choose the angle your panels sit on a flat roof is a huge benefit. The optimum angle for a solar panel is between 30 and 40 degrees, and roofs in the UK can be pitched between 20 and 50 degrees, with most falling between 40 and 50.
So, most roofs in the UK are ideal for solar panel installations but, if your home is in the lower or higher end of the scale, your panels are going to be less efficient.
Benefits of flat roof solar panels
We have looked at a few potential benefits to installing solar panels on a flat roof, but one of them remains constant no matter your choice of mounting: using solar energy. Let’s take a look at some other key advantages:
You won’t be limited to the types of solar panels you use as you'll likely have more room to play with, giving you more scope to use 2-way arrays without shading.
You can use less efficient panels, which are usually cheaper, to generate your electricity because you can fit a larger array in a smaller space.
Flat roofs do not dictate an angle so you can set your panels up to the perfect pitch for your property, which aids self-cleaning too.
They’re less visible from the ground if you’re concerned about presentation.
Flat roofs provide easy access and plenty of room for solar panel installation and maintenance.
What are the disadvantages of installing solar on a flat roof?
While you can still reap the rewards of solar energy with an installation on a flat roof, there are a few things to keep in mind before you make the investment.
Flat roofs are designed differently from pitched roofs and are not always the best for load bearing.
The weight of the panels and the mounting system need to be worked out perfectly to ensure the weakest parts of the roof will not be affected.
Panels on a pitched roof can butt up to each other, leaving no gaps. Whereas, panels on a flat roof need to be spaced out so they don’t shade each other. This means flat roof systems take up much more space.
Do solar panels on a flat roof cause leaks?
Very rarely. When properly mounted with flashing, panels shouldn’t cause leaks. Flat roof solar panels also use ballasted mounts that minimise the risk of penetrations, reducing leaks.
How long do flat roof solar mounting systems last?
Usually, they’ll last between 25-30 years, making them a solid investment for many homeowners.
Do you need planning permission for solar on a flat roof?
No, you generally won’t need to get planning permission to install panels on a flat roof, as long as you comply with all the building regulations already in place. For guidance, the panels themselves need to be at least a meter from the edge of the roof for wind loading reasons and must not stick up over a metre from the surface of the roof. However, if you move off the roof onto the floor, you’ll need to apply for permission for anything that covers an area over 9 metre square.
Also, if you live in a listed building, an Area of Outstanding Natural Beauty or in a home/area subject to any conservation protection, you’ll need to seek advice on planning. As for commercial installation on buildings like factories, you won’t usually need any planning permission.
How does the weather affect solar panels on flat roofs?
This’ll depend on the method of fitting, the location of the panels and the weather you’re likely to encounter in your location. Here’s a further insight:
Extreme heat
In the instance of extreme heat, flat roof panels can be an advantage. Generally fitted with a large space around them, panels on flat roofs will be slightly cooler than panels mounted close to the roof, as they’re on a traditional pitched roof system.
As these panels heat up, the resistance in the solar cells increases too, meaning the electricity finds it harder to ‘flow’ and the yield of the panel will drop. On the other hand, flat roof systems with angled panels use natural airflow that circulates around the panels and can reduce the temperature.
High winds
The winds in the UK aren’t usually strong enough to cause issues for solar panel installations. However, as climate change continues, extreme weather events will start to become more common, and we may start to see more ‘hurricane’ levels of wind.
In these circumstances, a flat roof solar system may be more at risk than its pitched roof counterparts due to the wind being able to get under the panels and lift them from the roof. Ballasted systems would be the versions most likely to fail in these circumstances, whereas those bolted to roofs would likely stand up against more robust winds.
Rain
The chances of rain harming any solar system is unlikely, especially in the UK. The only disadvantage a flat roof system may have in rainy weather, depending on the angles the panels are mounted at, would be the inability to naturally clean themselves.
Pitched roof solar systems rely on the rain to keep the panels clear of dust, dirt and grime, but the flatter the angle the panels are mounted, the less likely the rain will run off them, removing the dirt.
Snow
The flatter the angle, the more likely the build-up of snow. Snow doesn’t present much of a danger to a flat roof solar system, but these systems are more susceptible to getting covered in snow, which will prevent the light from getting to the cells. Although, it must be about 5cm thick to totally stop all production.
Flat roof solar panel maintenance checklist
To ensure you get the most from your flat roof solar panels, you’ll need to take steps to maintain them. These include:
Visual inspection: Regularly check your panel installation for cracks, chips or loose wiring. Do the same for your roof, but instead, check for signs of leaks, ponding water or membrane damage.
Mounting system: Ensure brackets, ballasts and all fasteners are secure and rust-free.
Cleanliness: Keep on top of cleaning your set-up, which a professional can do for you. You’ll want dirt, debris, leaves and bird droppings removed to maximise efficiency.
Electrical check: Ensure inverters, cables or connectors aren’t showing signs of wear or corrosion.
Professional servicing: Schedule an annual inspection for extra peace of mind.
Installing solar PV systems on flat-roofed buildings can be a good idea, especially if you’re concerned about your budget or are living in a flat, for instance. If you found this article helpful, learn about how solar panels are made.
