Solar panels on a flat roof: can you, and at what angle?

Can you put solar panels on a flat roof?

Yes, on a mounting frame, once the roof has been checked. Flat roof mounted solar panels are either ballasted or fixed, and MCS treats a roof as flat below 10 degrees and sets out checks for the two ways of holding a frame down: for ballast, things its certified installers shall do; for fixings, things they should do [1][2]:

Either way, the system should not harm the roof's weather tightness or structure for its whole life [1]. If the roof covering is under warranty, the warranty provider should be consulted; where the work may invalidate it, the installer is to tell you in writing and get your written agreement first, under the current scheme's standard (where a clause in their standard terms does not count) [1] and the redeveloped scheme's Customer Commitment [7]. In England and Wales, building regulations normally apply too: the roof's ability to carry the load must be checked and proven [5][6].

What angle should solar panels be on a flat roof?

Per panel, steeper is better, up to the optimum of about 39 to 42 degrees. Tilted south at 10 degrees a panel makes 89 to 91 per cent of the optimum-angle output, at 15 degrees 92 to 94 per cent and at 30 degrees 98 to 99 per cent, while a panel laid flat makes 81 to 84 per cent, by our calculation from PVGIS [3]. Rain also helps: the Energy Saving Trust says rain cleans panels tilted at 15 degrees or more [4].

Per square metre of roof, the order reverses. Tilted rows cast shadows, so each needs a gap behind it, and the steeper the tilt the wider the gap; east-west pairs can sit back to back. In our illustration, with rows spaced so that no row shades the next at noon on the shortest day, a low tilt fits more panels and produces more from the same roof. East-west pairs need little or no gap, so they fit more still, though each panel makes 80 to 83 per cent of the optimum [3]:

The choice depends on the complete layout. Per panel, a steeper south tilt makes more. Per square metre of roof, in this simplified comparison, lower-tilt south-facing rows produce more than steeper rows when each is given the stated spacing, by our calculation. East-west pairs can fit more panels, but we have not compared their output per square metre, because a fair comparison has to count shading between the pairs, access routes and roof-edge clearances. Ask the installer to compare complete layouts for your roof rather than choosing on output per panel alone.

Is it worth putting solar panels on a flat roof?

On output, it can be, on a sound roof with a clear view of the sky: tilted to 10 or 15 degrees, a panel loses 6 to 11 per cent against the optimum angle in our five places, by our calculation [3]. The extra flat-roof costs are not counted in that, and the government's cost data do not price them separately [15]. Whether panels pay back at all is covered in are solar panels worth it.

When it is not worth it yet. If the roof covering is near the end of its life, panels on it now would have to come off when it is replaced; that cost is covered in solar panel removal and reinstallation. Replace the covering first. For ballast, MCS requires a qualified structural engineer to be consulted on whether the roof can take the load of the system and the ballast; if it cannot, and the frame cannot be fixed through, ask about alternatives before going further [1].

Do I need planning permission to put solar panels on a flat roof?

Not if a permitted development right applies and the panels stay within its limits. In Wales and Northern Ireland the rights are for houses, not flats; listed buildings are excluded or restricted; in England, protected land needs a prior approval application; Scotland excludes World Heritage Sites [10][12][8][11]. Where a right applies, the main limits are heights, so they also cap how steeply a panel can be tilted. England measures from the highest part of the roof; Wales and Northern Ireland from the plane of the roof; Scotland from the roof plane [8][10][12][11].

Garages and extensions have the same height limits as the house in England, Wales and Northern Ireland, where the rights cover buildings in the grounds as well [8][10][12]; in Scotland a garage or other outbuilding has the lower 0.5 m limit [11]. The full rules, including listed buildings and conservation areas, are in do you need planning permission for solar panels.

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

The government's cost data for MCS-certified installations in Great Britain are split by system size, region and type of building, not by roof type, so they give no flat-roof price [15]. Prices by system size are on solar panel costs. On a flat roof the work can include the frame and either ballast with a slip layer and a structural engineer's check, or fixings with a condition survey and, where needed, pull-out tests [1], so ask the installer to list those in the quote.

What to ask the installer

1. Ballasted or fixed, and why for this roof?
2. The evidence: the structural engineer's view of the ballast load, or the pull-out test results, and the wind-load calculation.
3. The roof covering: what protects the membrane, whether the roof warranty provider was consulted, and how water will drain around the frame.
4. The layout: tilt, direction, gaps between rows and how many panels that fits.
5. The planning route: which limit the design stays within, or what application it needs.