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SnowFlurry 9 2020/21 | The snow load on roofs

Half a ton of snow on every square meter

by Lukas Ruetz 01/30/2021
Over the last few days, the media has repeatedly reported on collapsed roofs in the snowy regions south of the main Alpine ridge. How much snow load must a roof actually be able to withstand and how many kilograms of snow are currently on Austria's roofs?

Many parts of the Alps are now covered in snow, often right down to the valleys. At the same time, there are more and more reports of roofs collapsing. Mostly large halls or mountain barns are affected, but sometimes also residential buildings, as is currently the case in East Tyrol.

What does a roof have to withstand? - The standards using Austria as an example

Depending on the location of the building, i.e. the altitude above sea level and the region, a roof in Austria must be able to bear between 84 and 1080 kilograms per square meter of snow load.

The values are regulated by ÖNORM EN 1991-1-3, which was last revised in 2006. In addition to the climate zone, the altitude is also decisive, whereby a fixed formula is applied up to 1,500m - above this, other rules apply.

The lowest load applies to the Pannonian Plain - the climate zone in the far east of Austria with the federal states of Burgenland and Vienna.

In contrast, the roofs of St. Christoph am Arlberg or Obertilliach in East Tyrol must be able to withstand more than ten times this amount, i.e. a good ton of snow per square meter. With a well-settled old snow cover, a density of around 300 to sometimes 400 kg per cubic meter is expected. This means that on the Arlberg Pass, it is only necessary to shovel off the roof when the snow cover is about three meters thick.

How are the values determined?

The Central Institute for Meteorology and Geodynamics, with its decades of measurement series from every conceivable location in Austria, is the most important institution in terms of snow load expertise in Austria. The ZAMG carries out regular snow load measurements in several locations in Austria and makes recommendations as soon as roofs should be shoveled off.

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Common misconception: Rain on snow increases the load extremely

When it rains, the snow and therefore the roof load becomes heavier by a maximum of the weight of the fallen rain. One liter/m² of rain is one kilogram of water. And it usually doesn't rain more than 20 or 30 liters per square meter in winter, especially at higher altitudes.

The impression that the snow becomes many times heavier is only created by the change in density due to the strong settling of the snow cover with the rain. The snow mainly becomes denser, but only heavier by that little bit of rain. In everyday life, however, the density of a material is often equated or confused with its weight. If it rains so heavily that the entire snow cover is soaked and water can even run off it, the snow cover often becomes lighter instead of heavier due to the rain. The lost weight then simply runs off as water via the gutter, while at the same time people have the feeling that the load is increasing due to the higher density on the roof. Read more about this in SnowFlurry 16 2019/20.

Collapsing roofs - quite normal?

The standard values are based on a certain annuality of snow loads. The question is: statistically speaking, how often does this snow load occur in this region? This is based on the assumption of many decades.

Roofs built according to the standard would therefore only really need to be skimmed off in a fraction of the winters in which people actually remove snow from them. In most cases, people just play it safe, as you never know how the winter will develop, especially with a thick layer of snow in early winter. In addition, with more snow, it becomes increasingly difficult and time-consuming to get the snow off the roofs.

The current situation in Tyrol

In East Tyrol, it has snowed two to three times as much so far this winter than in an average winter. The hotspot at the moment is Obertilliach in southern East Tyrol. Values of 550 kilograms of snow per square meter have been measured there. At an altitude of 1,400 meters above sea level with a snow depth of around 170 cm. The snow density is therefore approx. 330 kg/m³. Or to put it simply: there is a large cow on every square meter.

In the North Tyrolean municipalities at a similar altitude, the snow load usually does not even come close to 300 kilograms per square meter.

The area around Lienz in East Tyrol is particularly unusual. The standards have already been exceeded there and hundreds of roofs have been skimmed off for weeks. Most of the reports of collapsed roofs also came from the lower valleys of East Tyrol and from Upper Carinthia. By mid-January, three times the amount of fresh snow had already been measured there than in an average winter! At an altitude of 660m, Lienz currently has 435 kilograms of snow per square meter. However, the normal load is 360kg/m².

Links:

Snow load calculator for Austria

Snow water value measurements Hydrographie Tirol

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This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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