To simplify the matter, it can be said that with common, sufficiently insulated objects (sc. cold roofs) with an under-roof gulley, the melting of snow starts only at temperatures close to the freezing point. For usual gutters with an unfolded housing of 33cm an output of cca 30W/m is used for both gutter and drain. On insufficiently thermally insulated roof constructions (sc. hot roofs) the melting starts already at temperatures way below the freezing point because of the heat leaking from the lining. For this reason it is necessary to use a bigger installed output, usually 40 - 60W/m.
In some cases it is also necessary to heat the valleys, where the snow and ice accumulates. In tough climatic conditions or abnormal winters, the snow and ice can also accumulate on roof edges. Gutters are then blocked by the snow and the outflowing water overruns the edge of the gutter creating icicles. In these cases it is necessary to proceed to roof edge heating. Usually the strip under the snow arresters is heated. The required output for valleys and roof edges is cca 200 W/m2.
In industrial objects and other vast buildings, between-roof gutters often serve for water draining. These gutters are often wider, which is why it is necessary to calculate with an output of 200 W/m2 for the area of the gutter.
