6 inches of snowfall would be equivalent to 6/11 inches of water or 6/132 feet. So the volume of the equivalent amount of water on the roof is 150 ft. x 45 ft. x 6/132ft. = 306.8 cubic feet.
Since there are 907185 grams in a ton and 28316.8 mL in a cubic foot, 1.00 g/mL is equivalent to 0.0312 tons/cubic foot
Multiplying the two results gives 9.58 tons of snow.