Research · Paper 05 · August 2026
Characteristic ground snow load is derived from the location of the site and rises with altitude. Where an assessment takes a postcode or a town centroid as a proxy for the site, the resulting load is wrong in one direction only. This paper sets out the mechanism, where in Britain it bites, and what to require instead.
Ground snow load is a function of site location, including altitude, not of the nearest town.
A centroid used as a proxy understates the load on higher ground. It does not overstate it.
An array does not only add weight. It creates new places for snow to gather.
01 · The mechanism
Under BS EN 1991-1-3 and its UK National Annex, the characteristic ground snow load at a site is derived from that site’s own location, and it increases with altitude. This is not a marginal correction applied at the end of a calculation. It is one of the inputs the calculation begins from.
Where this bites hardest in BritainConurbations built against high ground, where altitude changes sharply inside a single travel-to-work area: Greater Manchester toward the Saddleworth moors, Sheffield toward the Peak District, West Yorkshire on the Pennine flank, the South Wales valleys, and much of Scotland. In flat country the effect is a rounding error. In these places it is not.
A site on higher ground assessed using a value drawn from lower ground receives a load that is too small. The reverse case — a low site assessed on a high value — produces a conservative answer that fails safe. The asymmetry matters: this class of error errs towards optimism, never towards caution, and an optimistic structural answer is the one that does not get discovered until something happens.
A report produced from a centroid looks identical to one produced from the site. The figure is present, the standard is cited, the conclusion is stated. Nothing on the face of the document reveals which value was used or where it came from.
Figure 1
Two sites, one postcode district
02 · The array
Snow accumulates against obstructions. On a roof carrying an array the obstructions include parapets, upstands, plant, adjacent higher roofs and the modules themselves. The governing case is frequently not uniform snow across the roof but accumulated snow in a bay, and the array is what creates the bay.
The dead load of the modules and their mounting is modest and easily calculated. The load that governs is the combination — snow, wind and imposed actions together with that dead load, combined under BS EN 1990. A calculation performed on panel weight alone answers a question nobody needed answered.
03 · What to require
Stated explicitly, so that the basis of the figure is on the record and can be checked. Not the town, not the postcode district, not the nearest weather station.
Against the proposed layout rather than the roof as a bare plane, including accumulation against parapets, upstands, adjacent higher roofs and the modules.
Which combination governs, and by what margin. A report that states adequacy without identifying the governing case has not shown its working.
How Solar Surveys approaches this
Generic figures are quick and they are occasionally very wrong. Solar Surveys derives the actions for the site in front of us, because the difference between a postcode-level assumption and the actual site can be the difference between a scheme that works and one that does not.
Characteristic ground snow load under BS EN 1991-1-3 and its UK National Annex is a function of both the snow zone and the site altitude. Two buildings sharing a postcode district can sit at materially different heights, and the load follows the height.
An array changes the surface. Obstructions, level changes and the modules themselves create local accumulation that a bare-roof figure does not describe, and that is where the load concentrates.
Snow is one action among several. What matters is the governing combination under BS EN 1990, and our reports state which case governs rather than listing actions and leaving them unresolved.
A figure taken from a postcode is an assumption wearing the clothes of a calculation. We derive the actions for the site, because that is the only version you can build on.
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04 · Basis and limitations
05 · Citation
Suggested citation
Raihan, S. (2026) Snow, Altitude and the Postcode Problem. Solar Surveys Research, Paper 05. Solar Surveys Ltd. Available at: https://solarsurveys.co.uk/research/snow-altitude-and-the-postcode-problem
© 2026 Solar Surveys Ltd. All rights reserved. Journalists, researchers, industry bodies and educators may quote from this paper with attribution and a link, and no permission is needed to do so. Republication in full, adaptation, redistribution or any commercial use requires written permission — ask, and it is normally given. Writing about this and want something clarified, or a breakdown we have not published? Ask. We would rather it was reported correctly than quickly.
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