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Installing solar panels on a flat roof should normally be no different than installing solar panels on a sloping roof; in some circumstances, it may be cheaper, as the ease of access reduces labour costs.
However, additional costs might be incurred by the addition of a tilted mounting system, which is required for maximum efficiency, or in circumstances in which the roof might have to be strengthened if weighted slabs are used to stabilise the mounting brackets.
In most circumstances, solar panels do not require planning permission, but on a flat roof, this might change, as the solar panels will protrude above the roof plane and contravene permitted development regulations.
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This question is asked frequently, but the installation of solar panels on flat roofs is now very common in Britain. This is because solar panels are suitable for most types of roofs, including flat roofs. Although these systems are more commonly found on the roofs of commercial businesses, they’re also suitable for flat-roofed residential properties.
However, solar panels on flat roofs require a particular type of mounting equipment to maximise generation efficiency, as keeping the solar panels in a horizontal position will restrict the amount of power they generate. It will also prevent rainwater from running off the panels as it would do on an angled solar power system, and this might risk damage to the panels.
There are three types of mounting equipment – brackets, slabs used as ballast and the Bauder system.
It’s important to consider the angle at which your solar panels are mounted. This may depend on your location and the season. During the summer, the sun will be higher in the sky than in winter, which means, ideally, the angle would need to be altered from one season to the next.
There’s special equipment for this process, but it’s currently very expensive, so the optimal angle for fixed solar panels is between 30 and 45 degrees, with the best angle in the UK being 35 degrees.
It’s also important to consider the best orientation. In the UK, this is due south. This means solar panels facing due south and angled at 35 degrees will capture 95 per cent of the light energy that the sun provides during the course of the year.
Even if the solar panels are not facing due south, they’ll still generate significant amounts of solar power. If they face southeast or southwest and are angled at 35 degrees, they’ll capture 80 per cent of the sun’s light energy. If they face northeast at 35 degrees, they’ll still capture over 60 per cent, with the loss being compensated for by the installation of extra panels. This is because light from the sun in the UK is often diffused by clouds and buildings, and solar panels can capture that diffused light in addition to direct sunlight.
Installing solar panels on flat roofs delivers several key advantages. A flat roof is more accessible than a sloping roof, so the solar panel system is easier to install and maintain. This helps cut installation and maintenance costs.
A second advantage is that the solar panels are less visible to passers-by on the street. This arrangement has aesthetic benefits; some people regard solar panels as unattractive, but flat-roof solar panels don’t alter the overall visible impression of the building on which the solar panels are mounted.
Solar panels, whether on flat roofs or other types of roofs or locations, can radically cut your electricity bill by up to 70 per cent. They can also help cut your carbon footprint.
Flat roofs are more vulnerable to the accumulation of standing water, which could damage your solar panel system. However, this can be remedied by mounting the panels on a tilted mounting system, enabling the rain to easily fall away from the surface of the solar panels rather than collecting on a flat surface if the panels are positioned horizontally.
The lack of a mounting system also means the solar panels won’t be able to capture as much solar energy directly from the sun as it moves across the sky. This means a tilted mounting system is essential for efficient solar power generation.
If the solar panels are positioned horizontally, without a tilted mounting system, this can make some warranties null and void because of potential damage to the solar panels.
The addition of a tilted mounting system, which is essential when a sloping roof automatically tilts the panels without the need for such a mounting system, incurs installation costs, though this is normally no more than about 3 per cent of the system’s total cost.
To minimise damage from strong winds, the mounting system will also have to be firmly fixed to the roof (which may mean drilling into the roof) or be weighed down by the brackets being slotted into weighted slabs, providing ballast for greater stability.
Installing solar panels on a flat roof is cheaper, in many circumstances, than installing panels on a sloping roof, despite the extra cost for a tilted mounting system. The cost is usually between £750 and £900 per kilowatt (kW), compared to £1,250 per kW for solar panels on a sloping roof. One of the reasons for this is that installing solar panels on a flat roof is considerably easier, which means reduced labour costs.
However, this may change if the mounting system requires expensive ballasted slabs for stability or if the roof needs reinforcement, in which case a structural engineer may be required. In these circumstances, installing solar panels on a flat roof may be more expensive than installing panels on a sloping roof.
A solar panel array on a roof is normally considered as a “permitted development” in the UK, meaning that it will not require planning permission.
Solar panels on flat roofs must not project more than 60cm above the roof’s ridgeline (previously 20cm) in order to comply with permitted development rights. Otherwise a full planning application will need to be submitted.
Solar panels can be installed on listed buildings, but planning permission may be required if the buildings are located in a conservation area and are of historical importance.
In most cases, a flat roof will be strong enough to support a solar panel system. However, if the panel mounting system requires weighted ballast for stability, the roof might need to be strengthened, in which case a structural engineer should be consulted.
Solar thermal panel systems can be mounted on a flat roof in the same way a solar photovoltaic panel system is mounted and will operate in the same way.
A solar panel system installed on a flat roof will not normally invalidate your warranty. The only exception is roofs where the panels are installed horizontally, rather than on a tilt. In these circumstances, water may collect on the surface of the panels, rather than running off, and this may invalidate the warranty. This is a strong argument for mounting the panels on a tilted mounting system, rather than laying them directly on the flat surface of the roof.
