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.
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.
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
Published under Creative Commons Attribution 4.0. Reproduce, quote and build on this with attribution. Writing about it and want something clarified, or a point expanded? Ask. We would rather it was reported correctly than quickly.
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