Roof snow load calculator
Characteristic snow load to EN 1991-1-3 with the Polish National Annex (5 snow zones) plus the real weight of the snow lying on your roof. Instant result, no sign-up.
1. Location
2. Roof
3. Snow lying on the roof now
Result
How much does the snow on your roof weigh?
At what depth does the snow reach the code load s?
Depth measured on the slope. Wet snow is 4× heavier than fresh snow – 10 cm of wet snow equals 40 cm of fresh.
How we calculate
s = μ1 · Ce · Ct · sk (EN 1991-1-3, eq. 5.1). sk from Table NB.1 of the Polish National Annex: zone 1: 0.007·A − 1.4 (min 0.70), zone 2: 0.9, zone 3: 0.006·A − 0.6 (min 1.2), zone 4: 1.6, zone 5: 0.93·e^(0.00134·A) (min 2.0) kN/m², A = altitude. μ1 from Table 5.2: 0.8 up to 30°, 0.8·(60−α)/30 between 30° and 60°, 0 from 60°. Ct = 1.0. Actual weight = depth × density (Annex E) × roof surface area.
The calculator gives the characteristic load for a typical roof without drifts, parapets or snow pockets. It does not replace a structural design – consult a structural engineer if in doubt.
How the calculator works – questions and answers
What are the snow zones in Poland and where do I find mine?
The Polish National Annex to EN 1991-1-3 divides the country into 5 zones (map NB.1). Zone 1 is the west and south-west (Lower Silesia, Opole, Lubusz), zone 2 covers most of the country (Warsaw, Kraków, Łódź, Poznań, Gdańsk), zone 3 is the east and north-east (Lublin, Białystok, Olsztyn), zone 4 the Suwałki region and higher mountains, zone 5 the Tatra and Giant Mountains.
How much does snow on a roof weigh?
Per Annex E of the standard: fresh snow about 100 kg/m³, settled 200 kg/m³, old 250–350 kg/m³, wet 400 kg/m³. 30 cm of wet snow is about 120 kg per m² of roof surface – on a 100 m² roof that is roughly 12 tonnes. The same weight needs 120 cm of fresh powder.
Does the roof pitch reduce the snow load?
Yes. The shape coefficient μ1 is 0.8 for pitches up to 30°, falls linearly to 0 at 60° (e.g. 0.4 at 45°), and from 60° the snow slides off and the standard takes 0. For a duo-pitch roof the unbalanced case with 0.5·μ1 on one slope is also checked.
When should a roof be cleared of snow?
When the actual weight approaches the load the roof was designed for (in the calculator: a ratio above about 0.8), and especially after a thaw or rain, which raise the weight sharply. In Poland the building owner must remove excess snow (Building Law, art. 61). Clear evenly, with fall protection, and do not leave drifts along one edge.
Does the calculator replace a structural engineer?
No. It gives the characteristic load for the persistent situation on a typical roof (no drifting at parapets, steps or dormers, no accidental loads). A structural design needs load combinations and partial factors per EN 1990 – a job for a licensed structural engineer.
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