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Winter sun and row spacing on a flat roof
On a flat roof, the rows of panels shade each other when the sun is low. How far apart they go depends on two choices: the tilt, and how low a sun you design for. Here are Runcorn's numbers.
Checked against its sources on 3 November 2026. Published by SEO Dons; we do not supply or install solar.
Midday sun height, month by month
| Month | Midday sun height |
|---|---|
| January | 16.6° |
| February | 25.8° |
| March | 36.6° |
| April | 48.3° |
| May | 56.7° |
| June | 60.1° |
| July | 57.3° |
| August | 49.0° |
| September | 37.7° |
| October | 26.3° |
| November | 16.9° |
| December | 13.2° |
Our calculation: 90° minus latitude (53.33° N) plus the sun's declination (Spencer's series).
Gap behind each row
| Tilt | Clear from late February | Clear from late January | Clear at midwinter |
|---|---|---|---|
| 10° | 0.41 | 0.66 | 0.84 |
| 15° | 0.61 | 0.99 | 1.25 |
| 20° | 0.80 | 1.30 | 1.65 |
| 30° | 1.17 | 1.90 | 2.41 |
Gap = panel length × sin(tilt) ÷ tan(sun height). For panels mounted upright (1.72 m up the frame), multiply by about 1.5.
What the date costs in panels
The gap is only half the story; what matters is how many panels the roof then holds. For an 8 m by 5 m flat roof in Runcorn, with 0.4 m clear at the edges and the example 440 W panel laid whichever way fits more:
| Rows clear from | 10° | 15° | 20° | 30° |
|---|---|---|---|---|
| Midwinter: midday on 21 December | 8 | 8 | 8 | 6 |
| Late January or late November | 8 | 8 | 8 | 8 |
| Late February or late October | 12 | 8 | 8 | 8 |
| The spring equinox, 21 March | 12 | 12 | 12 | 12 |
Our layouts with the flat-roof planner's rules.
At 10 degrees, choosing late February rather than midwinter lifts this roof from 8 panels to 12, half as many again, while December brings only about 3% of a south roof's yearly output here. At 30 degrees the roof holds no more than 12 whatever the date, and the frames would stand above the 0.6 m permitted development limit in any case.
Mornings and evenings
The spacing here is worked out for the sun due south at midday. Early and late in the day the sun is lower still, so rows do shade each other for a while after sunrise and before sunset, even in summer. Little is lost, because a south-tilted panel makes little at those hours anyway, but it is why the planner's figures are a careful estimate rather than a promise.
East-west pairs sidestep the question
Back-to-back east-west pairs do not cast a shadow onto the next pair in the way south rows do, because each pair leans away from its neighbour and the low tilt keeps the shadow short. The planner spaces them with a 0.4 m walkway for access rather than by sun height, so the "clear until" date does not apply to them. That is one reason they fit so many panels on a small roof.
Choosing
Spacing for midwinter costs a lot of roof to protect very little energy: December brings only a few per cent of a year's output in Runcorn. Spacing for late February or late October fits far more panels and gives up a little winter output to row shading. Lower tilts shrink the gaps further. The flat-roof planner lets you try each choice on your own roof.
Shading from other things, such as a parapet, a taller part of the house or a neighbour's extension, follows the same geometry: an obstacle rising 1 m above the panels throws a shadow about 4.2 m long at midday on 21 December.
Keep reading
Where the figures come from
Questions about row spacing
How far apart should rows of solar panels be on a flat roof?
It depends on the tilt and on how low a sun you want the rows to stay clear for. For a 1.13 m panel at 15 degrees in Runcorn, about 0.61 m behind each row keeps it clear at midday from late February, and about 1.25 m keeps it clear even at midwinter.
Why not space rows for midwinter?
Because the gap needed is several times wider, so far fewer panels fit, to protect a few weeks of the weakest sunshine of the year.
Does row spacing matter for east-west pairs?
Much less. Low east-west pairs face away from the midday sun and cast short shadows, so a walkway between pairs is usually enough.
A flat roof, a quote, or a question about your Runcorn home?
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