When do you shovel snow from the roof

Too much snow on the roof: don't shovel yourself

How much snow can a roof really take? A civil engineer gives answers.

The motto "Do it yourself" is anything but popular with heavy snow loads on the roof. "Don't climb up yourself", warned the civil engineer Anton Pech on Friday, "but call the community or the fire brigade." But first you should know how much snow a roof can really take. And that depends on many factors, explained Pech.

One of the deciding factors is where a building is located. The altitude plays a major role, but so does the region, as the expert explained, who is also chairman of committee 176 "Load assumptions in the building industry" at the Austrian Standards institute.

The basis for the static calculations of snow loads are the values ​​of ÖNORM EN 1991-1-3 and ÖNORM B 1991-1-3. The Eurocode EN 1991-1-3 is valid throughout Europe and provides a framework for the guide values. Each country then addresses its specific circumstances with a national standards appendix. In Austria this is ÖNORM B 1991-1-3.

This divides Austria into four load zones. For example, two of these zones are striking for Vienna because there is a completely different amount of precipitation in the Vienna Woods than in the Simmeringer Haide, for example. The limit for the two load zones in Vienna is roughly the southeast tangent. The classification is made according to Austrian standards on the basis of meteorological records from many years. However, this can only be used below 1,500 meters above sea level. Above this, these calculation values ​​can be requested directly from ZAMG or from the local building authorities.

An example of the differences: According to Austrian standards, a house roof in Vienna-Simmering has to withstand 84 kilograms of snow load. In St. Christoph am Arlberg, the roofs have to be designed for 1,080 kilos of snow per square meter.

In addition to the location of the house, the nature of the snow is a decisive criterion: ten centimeters of fresh powder snow weighs ten kilograms per square meter, ten centimeters of wet snow four times as much. A ten centimeter thick layer of ice weighs 90 kilograms. "The worst case scenario for the roof is fresh snow on which it rains. Then the whole thing picks up and freezes. And then fresh snow again," emphasized Pech. Because the fresh snow can then no longer bind, in addition to the exorbitant load on the roof, there is also the danger of an avalanche.

Roofs without snow noses or rakes that have an incline of more than 60 degrees do not have these problems. The snow can slide off them, explained the civil engineer. The opposite of this are flat roofs, which are also not infrequently built in high alpine regions. Pech's comment: "Architecture builds everything everywhere."

The expert also had a tip for more experienced homeowners who are also familiar with the load-bearing capacity of their roof: "Cut out snow, put it in a ten-liter bucket and weigh it." So you can extrapolate the snow load on the roof area.