Flat or pitched: which roof makes more?

Per kW of panels

PVGIS 5.3 for Runcorn, 14% losses, read 3 November 2026.

Per roof

On a pitched roof the number of panels is set by the slope's size and shape. On a flat roof it is set by the tilt you choose, because steeper rows need wider gaps between them to avoid shading each other. That is why low tilts usually win on a flat roof: each panel makes a little less, but many more fit. The flat-roof planner shows the trade-off for your own roof.

Practical differences

• Fixing. Pitched roofs take brackets screwed to the rafters. Flat roofs take frames held by ballast or fixed through the covering, which must then be sealed and guaranteed.
• Height limit. A flat-roof array on a house must stay within 0.6 m of the roof to be permitted development. Pitched-roof panels must stand no more than 0.2 m off the slope.
• Upkeep. Low-tilt panels need cleaning a little more often; flat roofs are usually easier to reach.
• The covering. Flat-roof coverings tend to need renewing more often than tiles or slates; time the panels to suit.

Summer and winter

Tilt changes when a panel makes its electricity as well as how much. In PVGIS's monthly figures for Runcorn, a south-facing panel at 10 degrees slightly out-produces one at 35 degrees in June, about 120.6 kWh per kW against 116.2, because the summer sun is high. In December it makes little more than half as much, 16.3 against 28.2, because the winter sun is low and strikes the shallow panel at a glancing angle. A flat-roof array therefore leans towards summer. That suits a household that uses more power in summer, and it is a point against buying a large battery for winter.

Which to choose when there is a choice

• The main roof faces between south-east and south-west. Start there: it gives the most per panel, needs no frames and takes the usual brackets.
• The main roof faces east or west. Panels on both slopes still work well; a south-facing flat roof can add the midday output the slopes miss.
• The main roof faces north. A flat roof is the way in, at a low tilt facing south or as east-west pairs.
• Either roof is tired. Whichever covering is closer to the end of its life should be renewed before anything is fixed to it.

Using both

Many houses have both: a pitched main roof and a flat-roofed extension, garage or dormer. A main roof facing east or west plus a south-tilted array on the flat roof can make a well-balanced system, with the inverter set up to handle the two parts separately.

Keep reading

• Flat-roof planner
• Extensions and garages
• New Town homes
• Is it worth it?

Where the figures come from

1. EU JRC PVGIS 5.3: solar output by location, tilt and direction
2. Legislation.gov.uk: GPDO 2015 Schedule 2 Part 14 (solar on homes)
3. Legislation.gov.uk: SI 2026/896, solar permitted development changes (27 August 2026)
4. MCS: The Solar PV Standard (Installation), MIS 3002 v6.0