What a Satellite Image Cannot Tell You: Auditing a UK Roof for Solar

Solar panels are a long-term addition to a roof, not a temporary appliance. Yet many quotations are still prepared from a satellite image, a postcode and an assumed annual yield. That approach can be useful as an initial screen, but it is not a substitute for a measured roof audit. The difference matters because the performance of a domestic solar array depends on conditions that an aerial image cannot reliably show: the true pitch of each roof plane, the azimuth relative to south, the pattern of shading through the year, the usable area between chimneys and rooflights, the condition of the covering, and the ability of the roof structure to carry additional load.

In 2026, as more homeowners compare multiple solar quotations, the quality of the underlying roof data becomes as important as the panel brand or inverter specification. A measured audit allows quotations to be compared on a consistent basis and reduces the risk of an installation that looks neat on paper but underperforms in service.

Why an aerial image is a starting point, not a survey

Satellite and drone imagery can show the outline of a roof, the position of chimneys and the presence of large trees. It cannot confirm the roof pitch to the degree needed for panel layout, nor can it show the age and condition of the covering, the size and spacing of rafters, or the route a cable will take from the roof to the inverter. Image resolution, tree cover and the angle of capture can all obscure details that matter at the design stage.

A remote design may also assume a single continuous roof plane where the property actually has hips, valleys, dormers or a change in roof level. Those features reduce the area available for panels and affect how strings are configured. For a homeowner, the practical question is simple: has the installer measured the roof, or has the quotation been generated from a screen?

Pitch and orientation: the two numbers that frame yield

Roof pitch should be measured in degrees on each relevant plane, not estimated from a loft or inferred from the age of the house. In the UK, a pitch in the region of 30 to 40 degrees is often discussed as a useful baseline for annual yield, but the optimum depends on the array, the tariff or export arrangement and the load profile of the property. A lower pitch may suit a different design; a steeper pitch may still perform well but require different mounting and access arrangements.

Orientation should be recorded as an azimuth for each plane. A roof described simply as south-facing may contain one plane facing south-east and another facing south-west, with different yields and different shading patterns. East and west arrays can work well, particularly where generation is spread across the day, but the quotation should reflect the measured orientation rather than a general assumption. Where a roof has multiple planes, the panel layout and string design should be assessed plane by plane.

Shading is a measured condition, not a seasonal impression

Shading from chimneys, dormers, trees and neighbouring buildings is one of the most common reasons for a solar array to fall short of expectations. A roof that appears clear on a summer afternoon may be shaded by a chimney in winter, when the sun is lower and the effect on output is more pronounced. A measured shade assessment should record the position and height of obstructions, the time of year and the likely growth of any trees.

Shading does not automatically rule out solar. Optimisers or microinverters can mitigate some effects, but they add cost and complexity. The point of measurement is to establish whether mitigation is necessary and whether the quoted yield is realistic. A quotation that ignores a prominent chimney or a mature tree should be treated with caution until the shading has been properly considered.

Usable area versus gross roof area

The gross area of a roof is not the area available for panels. Set-backs are needed at ridges, eaves, verges and party walls, and current installer guidance and building control requirements should be confirmed for the specific property. Chimneys, flues, rooflights, vents, satellite dishes and dormers all interrupt the plane. Panel dimensions and clamping zones also affect how many modules can be fitted.

A measured audit should produce a roof plan with dimensions, a panel layout and a realistic module count. This allows the homeowner to compare quotations that may otherwise quote different panel sizes or different assumptions about usable area. It is also the moment to assess the condition of the roof covering. If a tiled or slate roof is nearing the end of its service life, replacing it before panels are installed is usually more economical than removing the array later.

Structural load: the question the quotation often skips

Solar panels and mounting systems add dead load to the roof, often in the region of 15 to 25 kilograms per square metre depending on the system, although the installer should confirm the figure for the proposed equipment. That additional load must be considered alongside the existing roof structure, the condition of the timbers and the local snow and wind loading.

Many UK roofs will carry a domestic solar array without difficulty, but not all will. Older properties may have rafters that are undersized by modern standards, have been altered, or show signs of sagging, cracking or previous movement. Heavy concrete tiles, dormer additions and long rafter spans are further reasons to seek a structural assessment. A solar installer is not a structural engineer, and a measured roof audit should identify where a engineer's calculation is required rather than assume that the roof is adequate.

Cable routes, inverter location and the less visible works

The performance and safety of a solar installation depend on more than the panels. The route from the roof to the inverter, the position of isolators, the earthing arrangement and the distance to the consumer unit all need to be planned. Long DC cable runs should be avoided where possible, and the inverter needs a suitable location with ventilation and access. In some properties, the meter position and the intention to export to the grid may require an application to the distribution network operator.

These are measured considerations, not administrative afterthoughts. A cable route that passes through a cold loft, a narrow eaves void or a finished wall may affect cost and programme. The audit should record the practical route and any constraints before the quotation is finalised.

Consent, heritage and the condition of the covering

Roof-mounted solar is often treated as permitted development in England, subject to conditions, but the position differs across the UK and depends on the property. Listed buildings, conservation areas, flats and some other settings require additional consent. A measured roof audit should flag these issues early so that the homeowner can take advice before committing to a design.

The condition of the covering is equally important. Slate, clay tile, concrete tile and metal roofs each require different fixing methods. A roof that is fragile, poorly detailed or due for replacement should not be treated as a simple mounting surface. The audit should note the covering type, its apparent condition and any repairs likely to be needed before or during installation.

What a measured roof audit should contain

A useful audit should include a roof plan with dimensions, pitch and azimuth for each plane, a record of shading obstructions, a usable area calculation, an indicative panel layout, a note on structural considerations, a cable route assessment and a summary of consent and covering issues. It should state its assumptions clearly. Not every property will need a full structural calculation, but the audit should identify where one is required.

For homeowners, the practical benefit is leverage. A quotation built on measured data can be compared with another on the same basis. Claims about annual yield, panel count and payback become easier to test. The roof is the most expensive part of the system to alter later, so it deserves to be measured before the first panel is ordered.