Is Your Roof Suitable for Solar? Direction, Tilt, Shade and Age

Not every roof is a good solar roof, and the reasons are usually visible before anyone climbs a ladder: which way it faces, how steep it is, what shades it, and how many years of life it has left. This guide explains how to judge each factor, so you know what an installer is looking at and can spot a quote that ignores a problem.

Direction: which way the roof faces

In the northern hemisphere, a south-facing roof generally receives the most sunlight over the year, because the sun tracks across the southern sky. East-facing roofs catch the morning sun and west-facing roofs catch the afternoon sun; both produce less over a year than a south-facing roof of the same size, but their production is spread differently across the day, which can suit a household that uses most electricity in the morning or the evening.

North-facing roofs in the northern hemisphere receive the least direct sun and are usually the weakest option, though they are not always useless — a shallow north-facing roof in a very sunny climate can still produce a useful amount. In the southern hemisphere the pattern is reversed, with north-facing roofs receiving the most sun. The key point is that direction is a property of your roof, not a choice, and a good quote states the direction it assumed and how much production it expects from it.

Tilt: how steep the roof is

Tilt matters less than direction, but it still counts. Panels produce most when sunlight hits them close to square-on, and the ideal tilt for yearly production is roughly the same as the local latitude, give or take. Most residential roofs sit within a workable range, and installers can adjust panel tilt slightly with mounting hardware, though steep adjustments add cost and wind load.

Flat roofs are a special case: panels are mounted on tilted frames to reach a useful angle and to let rain wash dust off. That works well, but it needs space between rows so panels do not shade each other, and the mounting must be properly secured against wind. A very steep roof is also workable, though installers may charge more for the extra safety measures involved.

Shade: the factor that matters most

Shade is the single biggest threat to a roof's solar potential, and it is the factor most often underestimated. A tree, a chimney, a neighbouring building or even a dormer window can cast shade that moves across the roof through the day and changes with the seasons — a roof that is clear in June may be partly shaded in December, when the sun is lower.

With a plain string inverter, shade on one panel can reduce the output of the whole string, so shaded roofs need microinverters or power optimizers to limit the damage. Even with those, a heavily shaded roof may simply not be worth covering: the panels in permanent shade produce little, and the money is better spent on the unshaded part of the roof, or on reducing the home's electricity use first. Ask the installer for a shade analysis — a survey that models shade across the year — rather than a glance at the roof on a sunny afternoon.

Age, material and structure

Panels are expected to stay on a roof for twenty-five years or more, so the roof underneath needs at least that much life left. A roof that will need replacing in five or ten years should usually be replaced before panels go on, because removing and reinstalling an array is expensive and disruptive. This is the most common reason a roof is judged unsuitable despite facing the right way.

Roof material affects the mounting method and the cost. Concrete and clay tiles, metal roofing and asphalt shingles each have their own fixings, and some materials — slate, for example — need specialist care. The roof structure must also carry the extra weight of panels and mounting hardware, plus wind and snow loads, which a structural check confirms. A reputable installer inspects the roof and, where needed, the structure, before quoting a firm price.