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Measuring a flat roof, from the ground
The flat-roof planner needs the roof's width from east to west and its depth from north to south. You can get both, and the other things worth noting, without setting foot on the roof.
Checked against its sources on 3 November 2026. Published by SEO Dons; we do not supply or install solar.
1. Find north
Stand by the building with a phone compass, or look at an online map, and note which wall faces south. The planner's "width" runs east to west and its "depth" north to south. If the building sits at an angle, use the side nearest to east-west as the width; the planner's result will be close enough to compare quotes.
2. Width and depth
- Tape along the walls. For a garage or single-storey extension, measure the outside walls at ground level.
- A floor plan. The plan from your purchase, or the building regulations drawings for an extension, gives exact sizes.
- A map. Online maps can measure a roof outline from above to within a few tens of centimetres.
If the roof has a parapet wall, the usable area is inside it; subtract the wall thickness on each side.
3. What breaks up the roof
Note rooflights and their kerbs, soil-vent pipes, extractor outlets, aerials and the drainage outlets. Each needs space round it. A box dormer's roof may be crossed by the main roof's ridge or by a chimney.
4. What shades it
A flat roof on the north side of a house sits in the house's shadow for much of the winter. Note anything taller to the south, east or west: the house itself, a neighbour's extension, trees.
5. The covering
From an upstairs window, look for ponding water, splits, blisters or moss. A covering that is near the end of its life should be renewed before panels go on; see roof coverings before solar.
A worked example
Say a garage roof measures 6.5 m east to west and 4 m north to south, with no rooflight and nothing taller to the south. Keeping 0.4 m clear at every edge leaves 5.7 m by 3.2 m to work with. The planner fits 6 example panels in south rows at 10 degrees, 2.64 kW, making about 2,259 kWh a year in Runcorn, against 4 as east-west pairs. Moving the depth by half a metre in either direction can add or remove a whole row, which is why it pays to measure carefully and then let the installer confirm on site.
Common slips
- Inside or outside. Outside wall measurements include the walls' thickness; a roof with a parapet is smaller inside it.
- Overhangs. Fascias and gutters can make the roof look bigger from the ground than the deck the frames sit on.
- Width and depth swapped. Width runs east to west, depth north to south; swapping them changes the layout completely.
- The edge nearest the house. Where an extension meets the house wall, the wall is a shade source and an edge, so leave the margin there too.
Using the result
Take the planner's panel count, tilt and frame height to each firm you ask to quote. If a firm's layout fits fewer panels, ask what it found on the roof that the measurements missed; if it fits more, ask how it keeps the edges clear and the rows unshaded. Either answer tells you something about how carefully the firm has looked.
Safety
Do not walk on a flat roof to measure it: many are not built to carry a person's weight between the joists, and edges are unprotected. Everything above can be done from the ground, a window or a plan.
Keep reading
Where the figures come from
Questions about measuring
How do I measure a flat roof for solar without going up?
Measure the walls below it: the width and depth of the extension or garage from outside, less the thickness of the walls if you want the inside of the parapet. A floor plan or an aerial map gives the same figures.
Do I need to know which way the roof faces?
Yes, to tell the planner which side is south. On a flat roof the panels can face south or east and west whatever way the building runs, but the roof's width east to west and depth north to south decide how many rows fit.
What else should I note?
Rooflights, vents, aerials and anything that rises above the roof, the height of any parapet wall round the edge, and anything taller nearby to the south that could shade it.
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.