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Case Study · Industrial & Manufacturing
A tight margin, and the case where a coarse check fails the roof.
Rooftop solar was proposed across a corrugated-metal roof where the numbers were always going to be close. Most structural appraisals have comfortable headroom and the precision of the analysis barely matters. This one did not. At the margin, the quality of the assessment decides whether a roof stays in a scheme or is written out of it.
Sector
Industrial and manufacturing
Covering
Corrugated metal
Governing load
0.838 kN/m² against 0.90 kN/m² allowable
Outcome
Confirmed adequate. Retained in the scheme
The brief
There is a particular failure mode in structural appraisal that nobody talks about, because it produces no incident and no claim. It is the roof that gets rejected when it should have passed. A conservative assumption stacked on a generic wind figure stacked on a rounded-up dead load produces a number that fails, the roof comes out of the scheme, and nobody ever finds out it would have been fine.
This roof was close enough to its limit that the outcome depended entirely on whether the analysis was done properly or approximately.
Precision where the margin actually matters
When a roof sits close to its limit, the quality of the analysis is the difference between a scheme and a rejection. The assessment was built to remove approximation rather than absorb it.
- Site-specific wind actions derived to BS EN 1991-1-4 and its UK National Annex for the actual location, exposure and building geometry, rather than a banded regional figure.
- Site-specific snow actions to BS EN 1991-1-3, using the real altitude and roof form rather than a rounded zone value.
- Measured dead loads for the existing construction, established by survey rather than taken from a generic table.
- PV uplift to BRE Digest 489 against the actual proposed array and mounting arrangement.
Where the accuracy came from
None of this is exotic. It is the difference between deriving each input for the building in front of you and reaching for a value that is safely wrong. On a roof with a comfortable margin, the two approaches give the same answer and nobody notices. At 0.838 against 0.90, they do not.
What the analysis found
| Assessment | Finding | Consequence for the scheme |
|---|---|---|
| Governing load | Established at 0.838 kN/m² against an allowable of 0.90 kN/m². | Adequate, with the margin quantified rather than assumed away. |
| Margin | Approximately 7 per cent headroom on the governing combination. | Tight, but real, and demonstrable to a third party. |
| Sensitivity | The result depended on site-derived wind and snow actions and measured dead loads. | A generic or banded assessment would plausibly have returned a fail on the same roof. |
| Outcome for the scheme | Roof retained in the installation programme. | Array area preserved that a coarser check would have lost. |
The value here is the roof that stayed in the scheme because the numbers were done properly. A fail would have been easier to write and impossible to challenge, and it would have been wrong.
Outcome
The roof confirmed adequate at 0.838 kN/m² against a 0.90 kN/m² allowable.
Array area retained that a coarser or more conservative assessment would have removed from the scheme.
A quantified margin, not a vague reassurance. The client knows exactly how much headroom exists, which matters if the array specification changes later.
A defensible basis at the margin. Where the answer is close, the working has to withstand scrutiny. It does.
What this means for roofs near their limit
Most appraisals never test the quality of the engineering, because most roofs have enough headroom to survive a mediocre assessment. The ones at the margin are where the difference shows, and they are more common than the industry admits, particularly on lighter-gauge metal roofs and optimised modern frames.
Three points worth carrying:
- A fail is a finding, and it should be as well-evidenced as a pass. Rejecting a roof on generic inputs is not caution, it is a decision made without the information.
- Ask what the margin is, not just whether it passed. A roof at 93 per cent utilisation and one at 40 per cent are very different assets if the array is ever changed.
- Site-derived actions are not a luxury. On a marginal roof, banded wind and snow figures are the difference between a scheme and a write-off.
Got a roof that looks marginal? Get the number, not an assumption.