One thing to think about is the potential weight of snow that may accumulate on the roof. Historically, there's been a good amount of snow on Aspö in the winter, but Janne says there's been very little in recent years. (We visited in February 2008 and it was almost balmy!)
In any case, we need to be prepared for whatever snow that may fall. It's fairly easy to determine the weight of snow on any surface. The metric system is fantastically elegant in this regard: one millimeter of water covering one square meter weighs one kilogram. Using a standard snow water content of 10:1, that means 1cm of snow equals 1mm of water.
Our roof is 100 square meters, so a snowfall of 8 cms (or 3 inches) would weigh about 800kg, or 1760 pounds. A water ratio of 6:1, which is more typical of the wet snow we're likely to see, makes the weight 1330kg/2930 lbs!
Roofs in snowy climates are typically highly pitched (like the Colorado example above), to ensure snow runs off and doesn't accumulate into very heavy loads. The roof on our house is pitched at 14 degrees, which is well within the building codes, but isn't near the 45 degrees of an alpine chalet.
We want to be sure snow slides off as easily as possible, which is one advantage metal or even tile has over a tarpaper or shingle roof. This is worth keeping in mind as we decide on what kind of roof we will install.