Solar Panels on Flat Roofs

Flat roofs are common across Wiltshire and Somerset — on 1960s bungalows, modern extensions, flat-roofed garages, commercial properties, and many residential properties built in the 1970s through to the 1990s. They are also found on agricultural and barn buildings. A widespread misconception is that flat roofs are unsuitable for solar. In practice, the opposite is often true: flat roofs give far greater design flexibility than pitched roofs because tilt frames allow you to mount panels at any angle and in any orientation.

On a pitched roof, you are constrained by the roof slope: if your pitched roof faces east-west, you have to accept that orientation or mount on a different roof section. On a flat roof, a south-facing tilt frame can be installed regardless of which direction the building faces, placing panels at the ideal 30–35 degree angle for maximum UK generation. This design freedom is one of flat roof solar's most underappreciated advantages.

The two primary mounting approaches for flat roof solar are ballasted systems and mechanically fixed systems. The choice between them depends on your roof type, structural capacity, wind exposure zone, and local planning requirements.

Ballasted mounting systems hold tilt frames in position using the weight of concrete blocks or dense rubber/EPDM ballast pads placed on the roof surface. The frames are not physically fixed to the roof structure — they rely entirely on gravity and friction. Ballasted systems are the most common approach for flat roof residential solar because they do not penetrate the roof membrane. This preserves waterproofing integrity and typically avoids the need for roof manufacturer sign-off on the installation method.

The limitation of ballasted systems is weight. Concrete ballast adds substantial loading to the roof structure — typically 15–25 kg/m² depending on wind exposure — and the roof must be structurally capable of carrying this load. We carry out a structural assessment of every flat roof before installation, and in some cases will recommend a lightweight aerodynamic framing system that uses shaped panels and minimal ballast to reduce structural loading.

Mechanically fixed systems are anchored directly to the roof structure using penetrating fixings through the roof covering. This approach is inherently stronger and better suited to high wind exposure zones or properties where structural loading from ballast would exceed capacity. Penetrating fixings require careful waterproofing at each penetration point — standard practice is a lead or EPDM flashing boot over each fixing. We always confirm that penetrating fixings are compatible with the roof warranty and, where required, work with the roofing installer to obtain sign-off.

For UK installations at latitudes between 50° and 52°N (which covers all of Wiltshire and Somerset), the optimal fixed tilt angle for maximum annual generation is approximately 30–35 degrees. South-facing orientation is ideal, with east-west orientations reducing generation by 15–25% compared to due south.

On a flat roof, tilt frames are typically available in standard angles of 5°, 10°, 15°, 20°, and 30°. The choice of angle is not purely about maximising generation: higher angles also affect inter-row shading (panels in front casting shade on panels behind), the wind loading on the framing structure, and the visual impact from street level. We model your specific flat roof layout using shading analysis software to determine the optimal tilt angle, row spacing, and panel count for your roof area.

For roofs with east-west constraints, east-west facing installations using low-tilt (10–15 degree) frames can be highly effective. Lower tilt east-west installations reduce inter-row shading losses, allow denser panel coverage of the available roof area, and generate a flatter daily generation profile (generating from morning to evening rather than a sharp midday peak). This profile is particularly well suited to battery storage systems that benefit from a longer, slower charge cycle.

Different flat roof systems have different implications for solar mounting. The main types we encounter across Wiltshire and Somerset:

EPDM rubber membranes are highly compatible with both ballasted and mechanically fixed solar. EPDM is durable, flexible at low temperatures, and provides a clean, puncture-resistant base for ballasted frames. For penetrating systems, EPDM can be sealed reliably at penetration points. This is our preferred flat roof surface for solar installation.

GRP fibreglass is rigid and strong — an excellent base for ballasted solar. Penetrating fixings require careful drilling and sealing but are entirely feasible. The main consideration is that GRP can be susceptible to point loading, so spreader plates are used at all frame feet to distribute ballast loads.

Felt and built-up roofing (BUR) is the most common older flat roof system and presents the most variable conditions. Older felt roofs may have limited remaining life and it is worth considering whether to re-roof before installing solar, which avoids the disruption of removing and reinstalling panels later. We can advise on the condition of felt roofs and, where needed, work with a roofing contractor to sequence re-roofing and solar installation together.

Green/living roofs are less common but increasingly present on modern extensions and extensions to listed buildings. Ballasted solar on a green roof requires careful assessment of the additional loading and drainage implications, but is feasible in appropriate circumstances.

See our full roof types guide for detail on pitched roof mounting systems including slate, clay tile, and Cotswolds stone.

Flat roof solar installations carry a modest cost premium over pitched roof equivalents, reflecting the additional design work, tilt framing, and structural assessment involved. Typical costs for residential flat roof solar in Wiltshire and Somerset:

• 3kW flat roof system: £6,000–£7,500 installed (vs. £5,500–£7,000 for pitched)
• 4kW flat roof system: £7,500–£9,500 installed (vs. £6,500–£8,000 for pitched)
• 5kW flat roof system: £9,500–£11,500 installed (vs. £8,500–£10,500 for pitched)

All prices include 0% VAT (until March 2027), ballasted tilt framing, MCS certification, DNO notification, and Smart Export Guarantee registration. A structural survey is included for all flat roof projects.

Generation returns for an optimally oriented flat roof installation at 30–35 degrees south-facing are equivalent to or better than a well-oriented pitched roof installation. The key advantage is consistency: even if your building does not face south, the tilt framing corrects for this, and a flat roof south-facing installation will typically outperform an east- or west-facing pitched roof by 15–25%.

For full cost detail and payback calculations, see our 2026 cost guide. For detail on the Smart Export Guarantee income, see our SEG guide.