Standards Library · 02
BS EN 1991-1-3
Snow actions
The Eurocode that governs snow actions. It matters more than people expect on rooftop solar, because the characteristic ground snow load rises with site altitude and an array creates new places for snow to gather.
What it is
BS EN 1991-1-3 covers snow loading on structures, used with the UK National Annex. The characteristic ground snow load is taken from the site's location, and it increases with altitude.
The standard also covers drifting: snow does not lie evenly. It accumulates against obstructions, and the shape coefficient applied depends on the roof geometry and what stands on it.
Why it matters for rooftop solar
A building at 150 metres carries a materially higher snow action than an otherwise identical building near sea level. Assessment carried out without site-specific altitude data therefore tends to understate the load, and the understatement is directional: it errs towards optimism, never towards caution.
The array itself becomes an obstruction. Snow drifts against parapets, upstands and adjacent higher roofs, and now also against the panel rows. A roof that looked viable on generic assumptions can fail once the site-specific snow action is applied.
We record this as snow load plus altitude bias, and it is set out as a named failure mode on the 575-roof dataset.