CROSS Safety Report
Dangerous snow slides from 100 houses
This report is over 2 years old
A reporter raises concerns about the build up of snow on cold roof systems.
Key Learning Outcomes
For civil and structural design engineers:
Careful consideration should be given to the risk of snow loads especially for areas prone to heavier snow falls
Consider what reasonably foreseeable loads could be applied beyond the code minimum values
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A reporter is a structural engineer and was been involved in the investigation of gutters collapsing under snow load in Scotland in the winter of 2011. A noticeable feature was the influence of the roof construction on snow build up. All the houses that were investigated had cold roofs i.e. the insulation at ceiling level prevented any heat reaching the roof surface. The effect of this, says the reporter, was that the snow built up rapidly on the roofs (35 degree slope) and stuck to the roof tiles.
The build up continued until the weight of the snow caused a shear plane failure within the snow mass with all the snow, estimated between 2 and 4 tonnes, falling from the roof. This happened on over 100 houses at around the same time. There was clearly a significant danger of loss of life – fortunately in these cases the falls took place in the early morning.
If however the falls had taken place during the day, there could have been severe consequences with injuries or deaths as many children played adjacent to the houses. Such falls could be mitigated by the use of snow guards or trace heating in the roof. The number of cold roofs is increasing rapidly – action, says the reporter, needs to be taken.
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Expert Panel Comments
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Snow can be deadly as demonstrated by the number of fatalities from avalanches in mountainous areas. In countries with severe winters the risks from sudden roof snow slides are well known but it is not a frequent occurrence in the UK. If a roof is steep enough then high build up of snow is unlikely and UK practice has been to assume that above a 60 degree pitch snow does not lie.
The practice of installing snow guards might be thought a prudent precaution if the falling snow presents a hazard, whereas in other cases it may be better to actively promote snow sliding off roofs. The reporter is concerned that the level of thermal insulation could have been a contributory factor in preventing roof temperatures from being affected by rising warmth from within the houses.
Within the UK this may be an aspect that needs further review as it could be an unintended consequence of the practice of enhancing the levels of domestic thermal insulation.