Snow Melting and Freezing on Older Townhouses

The snowy winter of 2009/2010 in Scandinavia prompted many newspaper articles on icicles falling from buildings and the risk this presented for people walking below. The problem starts with snow melting on the roof due to heat loss from the building. Melt water runs down the roof and some of it will freeze on the overhang. The rest of the water will either run off or freeze in gutters and downpipes or turn into icicles. This paper describes use of a model for the melting and freezing of snow on roofs. Important parameters are roof length, overhang length, heat resistance of roof and overhang, outdoor and indoor temperature, snow thickness and thermal conductivity. If the snow thickness is above a specific limit value – the snow melting limit- some of the snow will melt. Another interesting limit value is the dripping limit. All the melt water will freeze on the overhang, if the snow thickness is between the two limit values. Only if the snow thickness is above the dripping limit, will we get icicles. The model is used on an old townhouse without much thermal insulation and compared with newer townhouses. A discussion on attic temperatures is included. The results show that better thermal insulation or ventilation with outdoor air is the best way to reduce the risk of icicles.