Structural load is the second of four engineering flags in every Stage1Energy feasibility dossier. For UK commercial rooftop PV, it is also one of the most misunderstood: landlords often assume a structural survey is the first step, when in practice a desk-based screen can tell you whether that survey is worth ordering at all.
What structural screening actually checks
Structural screening at feasibility stage is not a calculation to Building Regulations. It is a comparative assessment: given the proposed array mass, the mounting system type, and the likely roof construction, does the additional dead load appear within a reasonable range for that roof type?
The screen considers:
- Array mass per square metre — panels, mounting rails, ballast or penetrations, and cabling.
- Roof construction type — steel deck, concrete, built-up felt, asbestos cement, standing seam, and so on.
- Building age and span — older portal frames and lightweight decks carry more uncertainty.
- Existing load — plant, HVAC, and access walkways already on the roof.
The output is a risk flag — low, medium, or elevated — with a named next step. That might be “confirm with structural engineer as standard due diligence” or “obtain roof construction details before proceeding.” It is never a certificate of structural adequacy.
Our methodology describes the structural screening approach and stated limits. Technical basis and full workings appear in the example report.
Why screening comes before surveying
A structural survey for a commercial roof typically costs £2,000–£5,000 and takes several weeks. On a portfolio of ten warehouses, that is a significant upfront commitment — before you know whether the roof passes wind screening, whether G99 applies, or whether the financial case stacks up.
Commercial solar feasibility exists to sequence that spend. The dossier answers whether the roof is worth pursuing before you commission surveys. If the structural flag is low and the financial verdict is positive, a survey becomes informed due diligence. If the flag is elevated and the roof is a 1970s asbestos deck with unknown capacity, you may decide the site is not worth the engineering cost — regardless of irradiance.
This is the same logic estates teams apply to other capital projects: screen wide, survey narrow.
Roof types and typical screening outcomes
Modern steel-deck warehouses (post-2000 portal frame, trapezoidal deck) are the most common commercial solar host. Array loads of 15–25 kg/m² are typically within the range these roofs were designed to accommodate, especially where ballast mounting avoids penetration loads. Screening usually returns low risk — with a reminder that engineer sign-off remains standard practice.
Older metal roofs and asbestos cement carry more uncertainty. Asbestos cement sheets have limited residual capacity and often trigger landlord, insurer, and CDM considerations beyond pure load. Screening tends to flag medium or elevated risk, naming structural assessment as a prerequisite.
Flat roofs with unknown build-up — common on retail and office stock — depend heavily on whether the deck is concrete or timber. Without construction details, the screen states its assumptions and flags the gap.
Roofs with heavy existing plant may have limited spare capacity even if the deck type is favourable. Screening accounts for declared plant where data is available.
For a deeper treatment of when to commission formal engineering, see commercial solar structural assessment.
How structural load interacts with wind
Dead load and wind uplift are separate physics, but they interact on the roof. A ballasted system adds mass — helpful for structural dead load screening but demanding for wind uplift analysis. A penetrated rail system reduces ballast but introduces point loads and weatherproofing obligations.
The feasibility dossier screens both flags independently. A roof that passes structural screening may still flag elevated wind risk if the mounting approach needs confirmation with the supplier. Conversely, a roof with ample structural capacity may fail on DNO or planning grounds.
That is why Stage1Energy reports all four flags together rather than as isolated checks.
What a structural screen does not replace
Be explicit about limits. A feasibility dossier:
- Does not visit the site or open the roof.
- Does not measure deck thickness, purlin spacing, or deflection.
- Does not produce calculations to Eurocode or sign off for Building Control.
- Does not satisfy a lender’s or insurer’s requirement for a structural engineer’s certificate.
It does tell you whether those steps are likely needed and whether the roof looks plausible for PV at the proposed scale. A structural survey costs £2,000–£5,000 whether the roof works or not. Screening first means that survey spend only lands where it can pay back.
Portfolio screening — why order matters
Estates teams with ten or twenty commercial roofs rarely commission structural surveys on every building at once. The sensible sequence is feasibility first, surveys second. A portfolio batch might show three roofs with low structural flags and strong financials — prime candidates for engineer instruction. Two roofs might flag elevated structural risk on older decks — survey only if the financial verdict still supports pursuit. Five roofs might fail on grid connection or yield before structure becomes the binding constraint.
Without screening, landlords default to surveying the largest roofs first — which are not always the best suited. Structural screening reorders the queue by engineering plausibility as well as capacity. That is how commercial solar feasibility earns its place in portfolio workflows: not by replacing the engineer, but by telling you which engineer appointments are worth making.
Screening in practice
The free screening gives a verdict and panel layout in three working days — enough to know whether a roof is worth pursuing. The full site assessment adds all four engineering flags, 25-year financials, and sourced calculation workings reviewed before release.
On a recent warehouse assessment (see the example report), the structural flag returned low — array mass within typical capacity for a modern steel deck — with the next step named as standard structural engineer confirmation. That is the expected outcome for a well-suited commercial roof: not a pass, but a clear route.
If you are advising a landlord or reviewing a portfolio, structural screening is how you avoid ordering surveys on roofs that fail before the engineer arrives.