Biggest problem will probably be the fact that when the snow melts during the day and the water is held in place under the snowpack only to refreeze at night. The expansion of the water as it freezes will occasionally cause damage. That is the best reason to keep the roof cleared. The weight will generally not be an issue. Also, be aware that if you have some ice built up on the roof and you move the rig, that ice can cause damage to anything it hits, be it stuff mounted on your roof, or someone or something behind you.