Snow Load = Snow Density × Depth × Slope Factor. Fresh snow: 5 pcf, packed: 20 pcf, ice: 57 pcf.
Most residential roofs are designed for 20-40 PSF. Remove snow above 25 PSF for safety.
Most roofs handle 20-40 PSF. 12 inches of fresh snow ≈ 5 PSF, but 12 inches of ice ≈ 57 PSF.