The building
Re-roofing and rooftop solar share the same access, scaffolding, fixings, and structural allowance. Once the new covering is on, that decision is effectively fixed for the life of the roof.
Estates with more than a handful of buildings replace coverings by age and condition. Solar and the roof programme are usually owned by different people, on different timetables. They rarely meet before the scaffolding is booked.
Adding solar while a planned re-roof is open is not the same cost as adding it to a roof finished last year. A new specification can account for the array: fixing method, membrane compatibility, extra dead load, and the warranty. After the tender is out, you are working around a roof that was designed without it, even if the scaffold is still standing.
The useful moment is before the specification is written. That is judgement about how estate programmes run. It is not a regulation. A desk screen is most useful when there are more roofs coming up than anyone can assess one by one. One building, with a roof survey already in hand, can often be settled with the roofing contractor and a structural engineer.
The programme is the point. A single roof is a different job.
The decision window is the specification, not the day the contractor arrives.
Facilities owns the roof. Energy owns the solar. The screen is what puts them on the same page.
Desk screening names the risk and the next step. Formal sign-off on load and fixings sits with a qualified structural engineer. Structural screening and wind loading are the neighbouring questions.