At some point in every commercial rooftop PV project, someone asks: can this roof take the weight? The formal answer comes from a structural engineer’s assessment. But the sensible question for estates teams and advisers is earlier: is this roof worth assessing? That is what structural screening answers — and it is the second of four engineering flags in every Stage1Energy feasibility dossier.
Screening vs assessment — the distinction matters
Structural screening at feasibility stage is a desk-based comparison of proposed array mass against typical capacity for the declared roof type. It produces a risk flag and a named next step. It does not visit the site.
Structural assessment is a formal engineering exercise. A qualified structural engineer inspects the roof, reviews construction details, and produces calculations or a written opinion on whether the array — including mounting system, ballast, and wind uplift — can be safely accommodated.
The screening tells you whether the assessment is likely to pass, fail, or need conditions. The assessment is the sign-off that landlords, insurers, and lenders may require. Conflating the two creates liability exposure: a feasibility dossier is screening-level, not a certificate.
Our methodology separates these stages deliberately. The example report shows a low structural flag with “confirm with structural engineer as standard due diligence” as the next step — the expected outcome for a well-suited warehouse roof.
What a structural assessment covers
A typical commercial solar structural assessment includes:
- Site inspection — deck type, build-up, condition, existing plant, drainage, and access.
- Load review — dead load from panels, rails, and ballast; wind uplift transferred to the deck; point loads from penetrations if applicable.
- Capacity check — comparison against the deck’s design capacity or inferred capacity from construction type and age.
- Report — pass, pass with conditions (e.g. restrict ballast in certain zones), or fail with recommendations.
The engineer may reference Eurocode actions (dead load, wind, snow where relevant) and the UK’s National Annexes. MCS 012 requires mounting systems to be installed per manufacturer guidance — the structural assessment confirms the roof can handle what the mounting system imposes.
Where wind uplift drives high ballast demand, the assessment becomes more critical. A roof that screens as low structural risk under a light penetrated system may flag differently under a heavy ballast layout on an exposed site.
When to commission the assessment
Commission a structural assessment when:
- The feasibility structural flag is medium or elevated.
- The roof is old, lightweight, or of unknown construction — asbestos cement, timber deck, or undocumented refurbishments.
- The landlord, insurer, or lender requires engineer certification as a condition of approval.
- The mounting approach changes after feasibility — switching from penetration to ballast, or increasing array density.
Do not commission assessments across a portfolio before feasibility screening. On ten warehouses, screening might show six with low structural risk, two that fail on DNO grounds, and one where planning is blocked. That leaves two surveys — not ten.
Cost, timeline, and programme
A structural survey for a commercial roof typically costs £2,000–£5,000 and takes two to four weeks from instruction — longer where access is difficult or the engineer needs original building records.
That timeline runs alongside DNO application (three to six months), planning verification, and installer procurement. Starting the structural assessment before the feasibility verdict is confirmed risks sunk cost on roofs that fail on other flags.
Commercial solar feasibility sequences the spend: £1,250 for a full dossier with all four engineering flags and 25-year financials, reviewed before release. The verdict tells you whether £5,000 in structural engineering is the sensible next step — or whether the roof should be deprioritised.
How the assessment fits the four flags
Structural load is one of four engineering screens. A roof can pass structurally but fail financially, on grid connection, or on planning. Conversely, a marginal structural flag might be acceptable where the financial case is strong and the landlord accepts engineer conditions.
The Stage1Energy dossier reports all four flags together so the verdict reflects the whole picture:
| Flag | What it screens |
|---|---|
| Wind uplift | BS EN 1991-1-4 estimated uplift vs mounting capacity |
| Structural load | Array mass vs typical roof capacity |
| DNO | G98/G99 route and grid risk |
| Planning | Permitted development vs full application |
No single flag overrides the others. For board papers and investment committees, that integrated view is the point.
What to give the structural engineer
When you commission the assessment, provide:
- The feasibility layout — panel zones, capacity, mounting approach assumed.
- Roof construction details — as-built drawings, survey records, or landlord knowledge.
- Mounting system specification — once selected, the supplier’s load data and wind calculation.
- Wind screening output — uplift assumptions from the dossier.
This avoids the engineer starting from a blank sheet and reduces iteration. The site assessment dossier includes layout and flag detail designed to hand off to consultants.
Limits and liability
Stage1Energy feasibility outputs are early-stage feasibility screenings. They are not design, not certification, and not a substitute for professional structural sign-off. Generation figures are feasibility-grade; engineering flags name next steps rather than confirming compliance.
The structural assessment is where professional liability sits — with the engineer who inspects and signs. Screening is how you decide whether to put that engineer on the roof in the first place.
Start with free screening for a verdict in three working days. Upgrade to the full dossier when the roof deserves engineering and financial depth.