Solar on flat roofs
Flat roofs have their own permitted development rule, their own mounting method, and one counterintuitive advantage: you choose the angle. Since December 2023 the limit is 0.6 metres above the highest part of the roof, which is tighter than it sounds.
A flat roof has its own rule: 0.6 metres above the highest part of the roof, excluding chimneys — inserted on 21 December 2023 and still missing from a lot of guidance.
That is tighter than it sounds. A 1.7 metre module tilted at 20° rises about 0.58 metres before you add the frame beneath it.
The compensation is real though: on a flat roof you choose the orientation and the angle, which no pitched roof allows.
The permitted development position
Since S.I. 2023/1279 came into force on 21 December 2023, Class A treats flat roofs separately:
- the 0.2 metre protrusion limit applies to walls and, expressly, to pitched roofs — not to flat ones
- a flat roof installation must not result in the highest part being more than 0.6 metres higher than the highest part of the roof (excluding any chimney)
- on article 2(3) land — conservation areas, National Parks, areas of outstanding natural beauty, the Broads, World Heritage Sites — a flat roof installation additionally requires an application to the authority for a determination as to whether prior approval is required, with respect to the impact of the appearance on that land
That last one is a condition, not advice. Beginning development without applying means it is not permitted development at all.
What 0.6 metres allows
Work the geometry through, because it constrains the design more than people expect.
| Tilt of a 1.7 m module | Height gained by the module alone |
|---|---|
| 10° | about 0.30 m |
| 15° | about 0.44 m |
| 20° | about 0.58 m |
| 25° | about 0.72 m — over the limit before any frame |
So the practical ceiling is somewhere around 15–20°, once the mounting beneath the module is included. Steeper tilts need planning permission.
Why that is less of a loss than it looks
Two effects push in the same direction as the limit.
Inter-row shading. Tilted rows shade the row behind them, so a steeper array needs wider spacing. A shallower angle fits more capacity into the same roof — and total output per square metre of roof frequently beats output per panel.
Flat is orientation-blind. On our modelling a horizontal surface yields 83% of the south-facing 35° optimum, regardless of which way the building faces. That is worse than a good south pitch and better than an east or west one at 79%, and much better than north at 54%.
On a pitched roof, the building chose your orientation decades ago.
On a flat roof you choose — so a building whose only pitched planes face east and west can still have a south-facing array. Our orientation matrix shows what that is worth: south at 20° gives 96% of the optimum, against 79% for an east or west pitched plane.
For many buildings, the flat roof is the better surface, not the compromise.
Mounting and structure
Ballasted systems are the norm: the array is held down by weight rather than by fixings, so the waterproofing is not pierced. That avoids the main leak risk on a flat roof.
The ballast is a structural load. It is not incidental — it is often comparable to the array itself, and it concentrates at the points where the frames sit. The structural assessment must cover the whole assembly, and everything on structural surveys applies.
Wind uplift matters more here. A tilted array on a flat roof is an aerofoil, and roof edges and corners see the highest pressures. MIS 3002 requires evidence that a wind loading calculation has been completed for the specific mounting system — ask for it.
Penetrating systems exist and are sometimes necessary, particularly where ballast weight cannot be carried. Then the sealing detail and the effect on any roof warranty become the questions.
The covering’s remaining life decides the sequencing, as with any roof. A flat roof membrane near the end of its life should be replaced before an array goes on it — see roof types.
Access and maintenance
One genuine advantage: a flat roof is usually safe to walk, which makes inspection, cleaning and fault-finding far easier than on a pitched roof. Over twenty-five years that is worth something, even if it never appears in a quote.
Sources
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 14
- The Town and Country Planning (General Permitted Development etc.) (England) (Amendment) (No. 2) Order 2023 (S.I. 2023/1279)
- Photovoltaic Geographical Information System (PVGIS) 5.3
Contains public sector information licensed under the Open Government Licence v3.0.
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