Many houses in Vermont are quite old, and were built before insulation existed. This meant that during the winter, the heat from the fire would heat up the roof and consistently encourage the snow to shed. The rafters were at most designed to carry the weight of the slate, but snow was not really considered an issue due to this heating effect and the steep pitched roofs. After insulation and air sealing became widespread in the 1950s, people began to became more concerned with the snow load, since the roof remained cold and the snow did not shed as readily. That is why now many people with old houses have to be careful in the winter to remove excessive amounts of snow from their roofs. Hope this helps!