Two roof build-ups on one site, and nearly double the dead load between them

The lightweight build-up totals 0.55 kN/m² dead: 0.25 for the insulated cladding panel, 0.15 for the metal purlins and 0.15 for services. The heavy build-up totals 1.02 kN/m²: 0.32 for cement sheets, 0.55 for concrete purlins and the same 0.15 for services. The concrete purlins alone weigh more than the entire lightweight roof above them.

That matters because headroom is what an added array consumes. A roof already carrying 1.02 kN/m² of permanent load has substantially less spare capacity than one carrying 0.55 before anything is added, so the same panel layout is a comfortable proposition on one building and a marginal one on the next.

Dead loads were built from the actual specified make-up for the existing roofs rather than from a default allowance, which is the only way that distinction surfaces at all. Imposed roof loading was taken at 0.60 kN/m² to BS 6399 Part 3 and Eurocode 1.

Roof geometry was recorded per building: Roof 1 spans 15 metres over 77 metres of length with purlins at 1.254 metre centres, Roofs 2 and 3 span 20 metres over 36 with purlins at 1.43, and Roof 4 spans 25 metres over 34 with purlins at 1.35. Purlin spacing is what determines where an array can actually be fixed, so it is measured rather than assumed.