Solar Panel Installer Runcorn, an independent guide › Flat roofs
Flat-roofed extensions, garages and dormers
Plenty of Runcorn houses with pitched roofs also have a flat roof somewhere: a garage, a single-storey extension, a box dormer. On their own these are small spaces, but they can rescue a system on a house whose main roof faces the wrong way.
Checked against its sources on 3 November 2026. Published by SEO Dons; we do not supply or install solar.
What small flat roofs take
| Roof | Best layout | Panels | kW | kWh a year |
|---|---|---|---|---|
| Single garage roof, 3 m by 6 m (long side north-south) | south rows | 3 | 1.32 | 1,129 |
| Rear extension, 4 m wide by 3.5 m deep | south rows | 2 | 0.88 | 753 |
| Double garage roof, 6 m by 6 m | south rows | 8 | 3.52 | 3,012 |
| Box dormer roof, 5 m by 3 m | south rows | 3 | 1.32 | 1,129 |
Our layouts with the flat-roof planner's rules and the example 440 W panel; PVGIS 5.3 output for Runcorn. South rows spaced to stay clear from late February at midday.
Why bother with a small roof
- It faces the sky, not a direction. A flat roof can carry panels facing whichever way works best, so a house whose main roof faces north can still have solar.
- It adds to a main-roof system. A few panels on a garage or extension, wired to the same inverter or a second input, can lift a small main-roof system usefully.
- It is easier to reach. A single-storey roof is quicker to scaffold or reach with a tower, and easier to clean and inspect.
What to check first
- The covering. Felt and asphalt roofs have a limited life. If yours is near the end of it, renew it before panels go on; lifting the array later means paying twice.
- The deck and joists. Ballasted frames rely on weight to resist wind, and that weight sits on the roof; a fixed frame goes through the covering. Either way someone should check the structure.
- Drainage. Frames must not block outlets or ponding areas.
- The height limit. Steep frames on a small roof can break the 0.6 m limit quickly; low tilts and landscape panels keep the array down.
- Shade from the house. A garage or extension on the north side of the house sits in the house's shadow for much of the winter. The main roof, or a south-side extension, will do better.
Connecting a second roof
Panels on a garage or extension have to reach the inverter. If it is in the house, the cable runs from the outbuilding or extension roof and needs its own route and isolator; a detached garage may need the cable buried or fixed along a wall. Because the flat roof's panels face a different way or sit at a different tilt from the main roof's, they should go to their own input on the inverter, so that each set works at its best. Ask the installer to show the cable route and the inverter inputs on the quote.
Planning in brief
On a house, an array on a flat roof is permitted development if no part stands more than 0.6 m above the roof surface, it is sited to limit its effect on the building's appearance, and the house is not listed. In a conservation area, a flat-roof array needs the council's prior approval first. A detached garage within the curtilage of the house is covered by the same national rules for buildings within a house's curtilage. See planning rules.
Keep reading
Where the figures come from
Questions about small flat roofs
Can I put solar panels on my garage roof?
Often. A flat garage roof can take a small tilted array as permitted development if it stays within 0.6 m of the roof and the garage is within the curtilage of the house; in a conservation area, prior approval is needed. The roof deck and covering must be sound enough to carry the frames.
Is a flat extension roof worth using?
It can be, especially if the main roof faces east or west or is shaded. A 4 m by 3.5 m extension roof takes a handful of panels; on its own that is a small system, but added to a main-roof array it can lift the total usefully.
Can panels go on a box dormer?
Yes, if the dormer roof is sound and can be reached safely for maintenance. The panels must stay within 0.6 m of the dormer roof and below the highest part of the main roof.
A flat roof, a quote, or a question about your Runcorn home?
Tell us about the roof or the quote, with your Runcorn postcode. Your message goes to SEO-DONS LTD, the company behind this independent guide, and the reply comes by email. Asking is free and commits you to nothing.
What happens next
- Fill in the form; your postcode tells us which part of Runcorn you are in.
- We look at what you sent and reply by email with the figures and sources that fit.
- Whether you go ahead, and with which firm, is up to you.