Financial modelling alone does not tell you whether a commercial roof can host solar. Irradiance and payback matter — but so do wind, structure, grid connection, and planning. Those four engineering risks are screened as flags in every Stage1Energy site assessment dossier, with a risk level and a named next step for each.
This article explains what the four flags are, how they are screened, and why they belong in commercial solar feasibility before you commission surveys or installer quotes.
Why four flags — and why early
Commercial rooftop PV projects fail late for predictable reasons:
- Wind — mounting system cannot resist uplift at exposed sites; ballast quantities become uneconomic.
- Structure — roof deck lacks capacity for the proposed array mass.
- Grid — G99 application reveals export limits or reinforcement costs not in the business case.
- Planning — permitted development does not apply; a full application adds months.
Each failure mode is discoverable before structural surveys, DNO applications, and planning submissions — if you screen early. That is what the four flags do. They are not a substitute for professional sign-off; they are a sequencing tool that tells you which professional steps are worth ordering.
The example report shows all four flags on a real warehouse assessment: low wind, low structural, G99 required for DNO, medium planning. Our methodology describes our screening approach for each flag.
Flag 1: Wind uplift
Wind is the force that tries to lift the array off the roof. Screening uses BS EN 1991-1-4 with site wind speed (P90), building height, and roof zone to estimate uplift per panel.
The flag compares that uplift against typical mounting system capacity for the proposed layout. Outcomes range from low — ballast or rail approach looks plausible — to elevated — supplier confirmation needed before layout is finalised.
Named next steps typically include “confirm ballast specification with mounting supplier.” Wind screening does not produce a ballast schedule; it tells you whether that conversation is urgent.
Full detail: wind loading for commercial roof solar.
Flag 2: Structural load
Structure is the question of whether the roof can carry the array — panels, rails, ballast, and cabling — without exceeding deck capacity.
Screening compares array mass per square metre against typical capacity bands for the declared roof type: steel deck warehouse, asbestos cement, flat built-up, and so on. The flag does not visit the site or produce Eurocode calculations.
Named next steps range from “confirm with structural engineer as standard due diligence” to “obtain roof construction details before proceeding.” Where the flag is elevated, a formal structural assessment is the sensible next step — not an optional extra.
Full detail: structural screening for commercial roof solar.
Flag 3: DNO grid connection
The grid flag screens the connection route under ENA G98/G99 standards. For commercial rooftops, G99 applies in virtually every case — the threshold is far below typical array sizes.
The flag answers whether G99 is required (yes), what the screening risk level is, and what programme allowance to build in. Named next steps typically include “submit G99 application; allow 3–6 months.”
This is not a DNO approval. It is screening that prevents board approval on a business case assuming twelve-week energisation when the grid route needs six months and a reinforcement quote.
Full detail: G98 and G99 explained and DNO solar application.
Flag 4: Planning route
The planning flag screens whether the proposed array appears to qualify under permitted development (Class J in England) or whether a full planning application is likely.
Conditions include panel projection above the roof plane, heritage constraints, conservation areas, and Article 4 directions. The flag does not submit applications — it names whether PD appears applicable or whether verification with the local planning authority is the next step.
Named next steps range from “verify planning route with local planning authority” to “planning application likely required.”
Full detail: permitted development for commercial solar.
How flags appear in the dossier
Each flag returns:
- Risk level — low, medium, elevated, or informational (e.g. “G99 required”).
- Named next step — a specific action, not generic advice.
- Sourced basis — referenced in the calculation workings section of the PDF.
Flags are reported together in a single engineering section of the dossier. The verdict — pursue, pursue with conditions, or deprioritise — reflects all four alongside the financial case. A roof with strong IRR but elevated grid risk is not automatically a yes; it is a roof where programme and cost assumptions need adjustment.
See the commercial solar engineering flags hub for an overview, or the example report for a worked example.
Flags vs surveys — sequencing the spend
A structural survey costs £2,000–£5,000 whether the roof works or not. Screening first means that spend only lands where it can pay back.
| Stage | What you get | Typical cost |
|---|---|---|
| Free screening | Verdict, layout, monthly generation | £0 |
| Full dossier | Four flags, financials, 25-year model, workings | £1,250 |
| Structural survey | Engineer sign-off | £2,000–£5,000 |
| DNO G99 application | Connection offer | Variable |
| Planning application | Permission (if PD does not apply) | £1,000+ and time |
The dossier sits between “should we look at this roof?” and “commission professional surveys.” That is the gap commercial solar feasibility is designed to fill.
What flags do not do
Be explicit about limits:
- Flags are screening flags, not certification.
- They do not replace structural engineers, DNOs, or planning authorities.
- They do not visit the site or open the roof.
- They do not guarantee outcomes — they name risks and next steps.
They do prevent the common failure mode: approving capital on financials alone, then discovering engineering blockers after surveys are commissioned.
Getting the four flags for your site
The free screening gives a verdict and layout in three working days — enough to know whether a roof deserves deeper analysis. The full site assessment adds all four engineering flags, scenario-optimised financials, and sourced calculation workings reviewed before release.
For portfolios, the same four flags run per site so you can rank roofs by engineering risk as well as return. For advisers packaging board papers, the flags are the engineering section that technical reviewers expect before financial sign-off.
Screen all four. Pursue with eyes open.