Finding Your Local Snow Load Before You Choose a Veranda

Every veranda specification eventually runs into snow. The manufacturers publish load tables telling you what a given configuration can carry, and those figures vary enormously — one louvred system takes 428 kilograms per square metre in its shallowest configuration and 101 in its deepest. Useful numbers, but only if you know what your roof is actually going to be asked to hold.

That figure is not a weather forecast and it does not come from the Met Office. It comes from the UK National Annex to BS EN 1991-1-3, the Eurocode covering snow loads, and it is a calculation you can do yourself in about two minutes. This is how.

Ground Snow Load Is Where It Starts

The Eurocode works from a characteristic ground snow load, written as sk and measured in kilonewtons per square metre. It represents the weight of snow on the ground at your location with a defined probability of being exceeded in any year, derived from decades of records rather than from a forecast.

The UK National Annex gives it as a map divided into numbered zones, plus a correction for how high above sea level you are. The relationship is straightforward: the base value is 0.15 plus (0.1 times the zone number plus 0.05), which produces a tidy sequence — zone 1 gives 0.30, zone 2 gives 0.40, zone 3 gives 0.50, zone 4 gives 0.60, zone 5 gives 0.70 kilonewtons per square metre.

Broadly, the low numbers cover the south and west of England, rising as you move north and east, with the higher zones over Scotland. The zone map is what settles it for your postcode, and it is published in the National Annex itself.

Then Add Your Altitude

The map value applies at 100 metres above sea level. Above that, snow load increases with height, and the National Annex handles it with a simple addition: take your site altitude in metres, subtract 100, and divide by 525. Add the result to the base figure.

At or below 100 metres, there is no adjustment — you use the map value as it stands.

The effect is larger than people expect. A site in zone 2 at sea level has a characteristic ground snow load of 0.40. The same zone at 400 metres works out at 0.40 plus (400 minus 100) divided by 525, which is 0.40 plus 0.57, giving 0.97 — nearly two and a half times as much, for the same part of the country. Altitude matters more than latitude across most of Britain.

Your altitude is on any Ordnance Survey map and in most online mapping tools. Use the actual site rather than the nearest town, because a few hundred metres of hillside makes a real difference.

From Ground to Roof

The snow on your roof is not the same as the snow on the ground, and the Eurocode converts between them with a shape coefficient and two multipliers: s = μ1 × Ce × Ct × sk.

For the UK, the National Annex sets both the exposure coefficient Ce and the thermal coefficient Ct to 1.0 — for all topographies and all roofing materials respectively — so in practice they drop out of the sum.

The shape coefficient depends on pitch. For a monopitch roof between 0 and 15 degrees, μ1 is 0.8, and every Deponti roof is set to 15.7 centimetres per metre, which is nine degrees, so 0.8 is your figure.

Which reduces the whole thing to: multiply your ground snow load by 0.8. A zone 2 site at 80 metres gives 0.40 × 0.8 = 0.32 kilonewtons per square metre, or about 33 kilograms per square metre. A zone 4 site at 300 metres gives 0.98 × 0.8 = 0.78, or about 80 kilograms.

Comparing It With the Tables

Now the manufacturer's figures mean something.

Take that first example — 0.32 kilonewtons per square metre. A louvred roof pergola in its shallowest configuration is rated at 4.20, which is more than thirteen times the requirement. Even its deepest configuration, at 0.99, has three times the margin. On a low-lying southern site, snow is simply not the governing consideration and you can choose on other grounds.

Now take the second — 0.78 on high ground in Scotland. Suddenly that 0.99 rating has very little left in it, and the shallower configurations become the sensible choice, or the structure wants an additional centre post. The manufacturers' own tables show this: on one bioclimatic system, adding a centre pillar takes the deepest configuration from 98 kilograms per square metre to 243.

That is the whole point of doing the sum. It tells you whether you are choosing freely or choosing carefully.

The Cases That Need More Care

Two site conditions produce loads above the general calculation, and both are common on domestic verandas.

Snow sliding off a taller roof above. A veranda tucked under the eaves of a two-storey house can receive a whole roof slope's worth of snow arriving in one place, and the Eurocode treats this as a separate load case rather than folding it into the general figure. If your structure sits below a large pitched roof, say so.

Drifting against a wall or a taller structure. Snow accumulates where wind slows down, which is exactly what happens in the angle between a veranda roof and the house, so the load there can be well above the average across the roof.

Neither rules anything out. Both are reasons to be some way inside a rating rather than at it, and reasons to mention the geometry when you ask for figures.

What to Ask For

Work out your zone and your altitude, do the two-line sum, and multiply by 0.8. Then ask your supplier for the published load figures for your exact width, projection and height — not for the product in general, since those figures change by a factor of four across configurations.

Check the basis they quote, too. The tables for these systems state a fifty-year reference period, consequence class CC1 and a deflection limit of one two-hundredth of the span, calculated to EN 1990, at a stated maximum height. A figure without its basis is not comparable with anything.

Wind Is the Other Half of the Question

Snow is the load people worry about. On a lightweight structure, wind is frequently the one that governs, and it works in the opposite direction.

Air moving over a roof produces suction above it and tries to lift the whole plane, which is why the load tables list a maximum wind suction alongside the downward load and why posts are bolted down rather than simply stood on their bases. Wind loading in the UK is covered by BS EN 1991-1-4 and its National Annex, and it depends on your basic wind velocity, your terrain, your altitude and how exposed the specific site is — a garden ringed by mature trees and neighbouring houses is a very different case from an open plot on a hillside. It is a more involved calculation than the snow one, which is a reason to describe your site honestly rather than attempt it yourself.

And if you are anywhere above about 200 metres, treat the sum as the start of the conversation rather than the end of it. On a wide-span structure at altitude, the difference between comfortable and marginal is a single extra post, and it is far cheaper to add now.