U values in the snow at Stonethwaite

I spotted this in Stonethwaite at the end of Borrowdale. Why has the snow fallen from the window but not from the roof? I remembered this table of U-values:

Rate of heat loss from a building = area x U-value x temperature difference.
The bigger the U-value, the faster the heat is lost. Even double glazing has a bigger U-value than an insulated roof so the window will have been leaking thermal energy more effectively hence the snow has melted.