Awkward Roofs, Useful Kilowatts: Solar Design for East-West, Flat and Shaded Homes

Walk down almost any residential street in Britain and you will see the same set of obstacles: chimneys, dormers, television aerials, a mature oak in a neighbouring garden, and a roof ridge that runs the wrong way. The classic solar sales image — a broad, empty, south-facing pitch — describes a minority of the housing stock. The good news is that the technology has moved on, and so has the design software.

A roof that is not ideal is not the same as a roof that is unsuitable. What changes is the expected yield, the shape of the generation curve and the way a battery should be sized. Those are engineering questions with answers, not reasons to walk away.

Start with the roof you have, not the roof you wish you had

Orientation and tilt determine how much of the sun's energy reaches the modules across the day and the year. In UK latitudes, a due-south pitch at roughly 35 to 40 degrees is the benchmark, and it is usually given an index of 100. South-east and south-west typically sit in the mid-90s. East and west fall further, and the precise figure depends heavily on pitch.

These are indicative relationships, not guarantees. A well-designed east-west array on a 30-degree pitch will comfortably outperform a badly shaded south array, and a flat roof with the right tilt frames can rival a mediocre pitched installation. Design software accounts for local weather data, horizon shading and pitch; a rule of thumb cannot.

East-west arrays: a broader day, not simply a weaker one

Splitting panels across two opposite pitches produces a flatter generation curve with a lower midday peak and longer shoulders in the morning and evening. Annual yield is typically a little lower than a single south array of the same capacity, but the timing often suits real household demand better.

This has three practical consequences. First, the system rarely reaches the inverter's maximum input, so a smaller inverter can be specified without much clipping loss. Second, self-consumption improves in homes where people are awake and using power in the early morning and evening. Third, battery behaviour changes: instead of one large midday surplus, you get a gentler spread of generation that may charge a battery more evenly and export less.

For households on a smart export tariff, that lower peak surplus is a modest trade-off. For households wanting to use as much of their own generation as possible, it is often an advantage.

Flat roofs: tilt frames, ballast and the space between rows

A flat roof is not a poor solar surface; it is a blank canvas with constraints. Panels are mounted on frames at around 10 to 15 degrees, either facing south or arranged as east-west pairs. Lower tilt angles reduce wind loading and allow rows to sit closer together, which matters because rows must be spaced to avoid one shading the next during winter, when the sun is low.

There are two common fixing approaches. Ballasted systems use weighted trays and avoid penetrating the waterproof membrane, which is attractive where the roof warranty is important. Mechanically fixed systems use brackets and are lighter but require careful sealing. In both cases, the structural capacity of the roof, the condition of the membrane and the building's wind exposure need proper assessment.

Parapets, plant rooms and neighbouring buildings can all cast shadows on a flat roof, and these are easy to miss in a quick survey. Ask for a layout drawing that shows row spacing and the shadow path at low winter sun angles.

Split arrays and multiple orientations

Many homes suit a split array: four panels on the south-east pitch, six on the south-west, perhaps two on a garage. Modern inverters often have two independent maximum power point trackers, allowing strings on different orientations to operate separately. Where that is not possible, panel-level electronics such as optimisers or microinverters can reduce the penalty from mismatch and shading.

More electronics means more components that could fail, so the design should justify them. A simple two-string layout on a dual-tracker inverter is often cleaner and cheaper than fitting optimisers to every panel. The right answer depends on the roof, not on a sales preference.

Shade: chimneys, trees and the nine o'clock problem

Shading losses are disproportionate. A shadow across part of one panel can reduce the output of an entire string, and the effect is worst in winter when the sun is low and household demand is highest. A chimney that casts a narrow shadow in June may cast a long one across the array in December.

A credible quote should include a shading analysis, not a guess. Reputable designers use modelling tools that account for obstructions and horizon profiles, and they will discuss whether a tree is deciduous or evergreen. Deciduous trees are forgiving in winter; a leylandii hedge is not. If a tree is due to be pruned or removed, say so before the design is fixed.

What orientation and shading do to battery strategy

With a south array, generation peaks around midday and a battery sized to absorb that surplus makes obvious sense. With east-west, the surplus is spread thinner across more hours, so a smaller battery may capture most of the usable energy. Over-sizing storage on a low-yield, split array ties up capital that could take many years to recover.

Export matters too. Under the Smart Export Guarantee, rates vary considerably between suppliers, and some time-of-use tariffs reward exporting at peak times. A flatter generation profile reduces the volume available for those evening exports. Model the economics on your actual predicted yield, not on an optimistic headline figure.

Questions to ask before accepting a quote

• What annual yield do you predict, and what assumptions about orientation, pitch and shading sit behind it?
• Can I see a panel-level layout drawing on my actual roof, including obstructions?
• How many strings and trackers will the inverter have, and why?
• What is the inverter-to-array ratio, and how much clipping is expected?
• Is the installer MCS certified, and who handles the DNO application and building regulations notification?
• What is included for scaffolding, bird proofing, cable routing and making good?
• How will the system behave during a power cut, and is battery retrofit straightforward?
• What warranties apply to panels, inverter, battery and workmanship, and who honours them?

Listed buildings, conservation areas and shared roofs

Permitted development rights for solar are broad but not unlimited, and they do not apply to listed buildings or to some flats and maisonettes. Conservation areas and World Heritage Sites can impose additional constraints on visible panels. For leaseholders and shared freeholds, the freeholder's consent is usually needed before any work begins.

These are administrative hurdles, not technical ones, but they are far easier to clear before a deposit is paid. A local authority planning enquiry costs nothing and can save months.

The bottom line

An east-west roof, a flat roof or a partly shaded roof can all host a worthwhile solar installation. What separates a good outcome from a disappointing one is design discipline: honest yield modelling, a layout that respects obstructions, an inverter and battery matched to the generation profile, and a quote that explains its reasoning. Ask for the workings, compare them, and treat any installer who promises a south-facing yield from a north-facing roof with caution.