Remove snow from the roof
Even a light covering of snow on the greenhouse roof reduces the light getting to your plants inside. If the snow persists the structure also has to stand the weight and when it begins to thaw, there will be slippage and the danger of a big dump of snow onto whatever is near the outside of the house. Brush the snow off daily with a long-handled broom and give it a final clean with a window cleaning blade when the snow is over.