It seems to me that it's going to depend on the water content of the snow. I did a search and found:
"The weight of snow varies. Light fluffy snow may weigh about 7 pounds per cubic foot. Medium snow might weigh about 15 pounds per cubic foot. Snow in a drift could weigh about 20 pounds per cubic foot."
So, if you know the approximate number of square inches on the roof and the water content of the snow, you can figure out the weight per square inch for every inch in height of the snow.
I seem to remember reading a few posts about over a foot of snow on the vert. And I imagine that they get lots of snow in Sweden... Ron