A roof that passes its structural check can still need repair before panels go on. Data from 1,568 roof planes shows why a solar business case should price both questions from the start. Sahir Raihan, Structural Engineer and Managing Director, Solar Surveys Ltd.
Most rooftop solar business cases are built on yield, tariff and payback. The roof tends to get a single line: structural survey required. That line hides two questions, and they have different answers and different costs. Can the structure carry the array? And is the roof fit to carry it for the life of the system?
We have put numbers on both. The UK Roof Plane Dataset 2026 records 1,568 roof planes on 651 buildings and homes assessed for solar between January and September this year, with every figure taken from an issued structural report. It is not a random sample of UK roofs. These are roofs somebody intended to put solar on, which is the population an energy manager deals with.
The array is rarely the deciding load
A typical array adds about 0.13 kN/m², around a fifth of a commercial roof’s allowable imposed load. What decides whether a roof can take it is how much of its capacity is already spoken for, and in the dataset it was snow, not the solar, that decided how close a roof ran to its limit. Snow load rises with altitude, so the same building can give a different answer in a different place. On the commercial roofs, the median share of capacity used by PV and snow together rose from 61% below 50 m above sea level to 92% at 150 m and above.
Some building types run closer to the limit than others. Of 112 commercial buildings with a stated figure, 20 used 90% or more of their capacity once PV and snow were combined. Schools and faith and community buildings reached that level most often, 36% of each. A business case that assumes the same headroom across an estate will be most optimistic at exactly the sites with least to spare.
Condition is a separate question
The second finding is that capacity tells you nothing about condition. Roofs with a visible condition issue in their survey photographs used a median 75% of their capacity; roofs without one used 73%. A pass on the loading check is no evidence that the covering will last, and a sound covering is no evidence of structural headroom.
Condition follows the covering. A visible issue was photographed on every natural slate roof we recorded, 93% of felt flat roofs and 83% of fibre cement, against 51% of profiled metal and 38% of standing seam. Across 109 commercial buildings, the photographs showed biological growth on 41% and ponding or drainage marks on 8%, and rooflights, which are fragile and limit where an array can go, on 68%. Very few of the reports recorded a roof’s age, so at portfolio scale the covering is the most reliable guide to it.
Flat roofs deserve particular care. Seven of the nine buildings with visible ponding had felt or bitumen membranes, and 37 of the 42 flat-roofed commercial buildings with a stated fixing method named a ballasted system, which is a permanent dead load.
What it means for the business case
Once panels are on, repairing the roof beneath them means taking them off first. The covering’s remaining life therefore belongs in the business case alongside the structural answer, not after it. In practice that means:
- asking for capacity and condition to be reported together, before procurement, so remedial work is priced in rather than discovered;
- treating any repair the covering needs within the system’s life as part of the project cost, done before installation;
- checking that feasibility work uses each site’s own snow load, corrected for altitude, rather than one figure for the whole estate;
- screening the whole estate first and concentrating survey effort where the roof area is. In our data, one commercial building in five carried more than half of the commercial roof planes.
A roof that passes its structural check has answered one question. A sound business case needs the answer to both.
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Sahir Raihan is a structural engineer and the Managing Director of Solar Surveys Ltd, which assesses whether roofs can carry solar PV across the UK and Ireland. The UK Roof Plane Dataset 2026 is free to read and cite at solarsurveys.co.uk/roof-plane-dataset-2026
Photographs: Solar Surveys
This article appeared in the Nov/Dec 2026 issue of Energy Manager magazine. Subscribe here.
