IRSCOOBY2: Treat it as a liquid that places a pressure on the roof. Since the drift probably is irregular in depth, you will have to take averages for the depth. Since 11 inches of snow equals 1 inch of water the density is one eleventh that of water. Note that this 11:1 ratio is only typical for new snow. If the snow is compacted, "wet",or partially ice the ratio will be different.
Section 7 of ASCE 7-02 addresses snow loads on buildings. Section 7.7 shows you how to determine drift lengths, intensity, etc. Density is defined in this section as a function of the ground snow for your area.
I don't actually own the following publication, but ASCE also sells a book from their website titled "Snow Loads: A Guide to the Use and Understanding of the Snow Load Provisions of ASCE 7-02". I have the similar wind loading guide and it is pretty helpful, so I would assume the snow load guide is equally helpful.
