Solar panels on a flat roof: mounting, output and planning

Yes, you can put solar panels on a flat roof. They sit on low frames, held down by ballast or fixed to the roof; the frames we checked tilt panels at between 9° and 18°. At a 10° tilt facing south, a panel produces about 90% of what the same panel would make at 35° on a south-facing pitched roof, according to the European Commission's PVGIS model. In England, panels on a flat roof are normally permitted development if they rise no more than 0.6 m above the highest part of the roof, unless the house is listed, or is in a conservation area or other protected area, where the council must first approve the look.

How panels are mounted on a flat roof

The MCS installation standard treats any roof pitched at less than 10° as a flat roof. There are two main ways to hold panels on it.

A ballasted system avoids holes in the waterproofing but adds weight, calculated for each roof because the wind load depends on the building's location, height and shape and where each panel sits. The Renusol FS10-S datasheet, for example, gives about 9.3 kg per m² for frame and panels before ballast. Ask for the ballast plan and the engineer's confirmation.

On gravel or green roofs, the MCS standard says the sliding resistance must be tested, or the system restrained, for example with a kerb in front of the panels or tethers to a fixed point on the roof.

How much a flat roof produces at a low tilt

We ran the European Commission's free PVGIS model (version 5.3, SARAH3 data, 14% system losses) for 1 kWp of panels in three cities. The figures are long-term yearly averages; actual output varies from year to year.

Percentages compare each figure with 35° south in the same city; the east-west row averages an east- and a west-facing panel.

So each panel on a flat roof makes less than it would on a good pitched roof, by about 7% at 15° and 10% at 10°. You might expect a steeper tilt to be the answer, but steeper panels shade the row behind them, so the rows must be further apart: in one manufacturer's range, frames for similar panels need rows 1.49 m apart at 10° and 1.84 m apart at 18°. For how direction and pitch affect other roofs, see solar panel orientation and angle.

South-facing rows or east-west?

South-facing frames need gaps between rows so each row doesn't shade the next. East-west frames put panels back to back in a low tent shape, so they need smaller gaps.

Assumptions (illustrative). A flat rear-extension roof 8.0 m east to west and 6.0 m north to south, with nothing on it and no shade. Frames kept 0.6 m from every edge, the minimum in the Renusol datasheets, leaves 6.8 m by 4.8 m. Panels are 440 W, 1,762 × 1,134 mm (the JinkoSolar Tiger Neo from our solar panel sizes guide). Spacing comes from the Renusol FS10-S XL (south, 10°) and FS10-EW XL (east-west, 9°) datasheets; we used the 10° London PVGIS yields for both.

On this roof the east-west layout fits two more panels and produces about 22% more electricity a year (2,957 ÷ 2,430 = 1.22), even though each panel makes less. It also spreads output across the morning and evening. On another roof the answer may differ: a narrow roof, obstacles or a parapet can change the count. PVGIS doesn't model the rows shading each other in winter, so ask the installer for the shade allowance in their estimate. Solar panel shading explains how MCS installers calculate it.

Planning permission for flat-roof panels in each nation

Permitted development lets you install without applying for planning permission, within these limits. Listed buildings need listed building consent in every nation (in Northern Ireland, permitted development applies only once that consent has been granted), and a council can remove permitted development rights in an area (in England through an "Article 4 direction"), so check with your council if you're unsure.

England's 0.6 m limit also rules out steep frames. A 1,134 mm panel tilted at 10° rises about 0.20 m (1.134 × sin 10°); at 35° it rises about 0.65 m (1.134 × sin 35°), over the limit before the frame's own height is added. The full rules, including listed buildings and conservation areas, are in planning permission for solar panels.

Building regulations normally apply as well: the Planning Portal (England) and the Welsh Government both say the roof's ability to carry the load must be checked, and the electrical work is covered too.

Is your roof ready? Structure, waterproofing and warranties

Once panels are up, the roof under them is harder to reach, so have it checked first.

• Structure and waterproofing. The MCS standard says a system shouldn't harm the building's weather-tightness or structure for its whole life; for ballast, that means an engineer's check and a protection layer that suits the membrane (see the table above).
• Roof warranty. If the roof covering is under warranty, the installer should check with the warranty provider; if solar might invalidate it, an MCS installer must tell you in writing and get your explicit written agreement, not just rely on a clause in the standard terms.
• Re-covering first. If the membrane is near the end of its life, replace it before the panels go on rather than lifting them later. In England, replacing the membrane on more than half of a flat roof means the whole roof should be brought up to current insulation standards where technically, functionally and economically feasible (Approved Document L; a 15-year simple payback is the economic test).

Garage, extension and dormer roofs

Flat roofs on garages, extensions and dormers raise a few extra points.

• Planning. In England, the same Class A rules cover buildings within the grounds of a house, including the 0.6 m flat-roof limit. In Scotland, panels on an outbuilding may project up to 0.5 m and it must not be in front of the house. In a Welsh conservation area or World Heritage Site, panels on a building in the garden shouldn't be visible from the highway (Welsh Government guidance). In Northern Ireland, the house rules cover any building within its grounds.
• Asbestos. The HSE says asbestos may be in any building built or refurbished before 2000, including asbestos cement roofs. Don't drill, cut or stand on an old garage roof: asbestos cement sheets are hazardous if damaged and fragile underfoot. Ask for an asbestos survey before any work.
• Size and structure. After edge gaps, a small roof may hold only a few panels, so weigh the extra kWh against scaffolding, cabling and the structural check. A glazed or polycarbonate conservatory roof would need its structure checked and the panels would shade the room below: look at the main roof first.

What to ask for in a flat-roof quote

• The mounting system's name, tilt and layout drawing, with the number of panels and the gaps to the edges.
• The ballast plan (weight and position of every block) with the structural engineer's confirmation, or, for a fixed system, the pull-out test results.
• How the waterproofing is protected, and written confirmation about the roof warranty.
• An MCS performance estimate that states the tilt, direction and shade factor.
• How the design meets the planning limit in your nation, or confirmation that an application or prior approval has been made.

Line these up across two or three quotes; comparing solar quotes shows what else to check, and the MCS certificate explains the paperwork you should get at handover.

Frequently asked questions

Can you have solar panels on a flat roof?

Yes, on low tilted frames held by ballast or fixed to the roof, once the roof's structure has been checked. In England they must rise no more than 0.6 m above the highest part of the roof to be permitted development.

Do solar panels work lying flat?

Yes, but less well: in the PVGIS figures above, flat panels produce about 83% of what they would at 35° facing south, and a 10° tilt raises that to about 90%.

What is the best angle for solar panels on a flat roof?

There is no single best angle. Steeper panels produce more each, but need wider gaps between rows, and in England a steep frame can break the 0.6 m planning limit. The frames we checked tilt panels at 9° to 18°; compare the whole-roof output of the layouts you're offered.

Do you need planning permission for solar panels on a flat roof?

Usually not, if the panels stay within your nation's permitted development limits: 0.6 m above the roof in England, 1 m in Scotland and Wales (plus 1 m back from the edge in Wales), 1.5 m in Northern Ireland. Listed buildings need consent, and in England a flat roof in a conservation area, National Park, World Heritage Site or other protected area needs a prior approval check first.

How heavy are solar panels with ballast?

There is no standard figure: ballast is calculated for each roof, on top of about 9.3 kg per m² for frame and panels in one datasheet we checked, and a structural engineer should confirm the total.