Source: My dad has his hobby car in a glorified tent during winter, works just fine, the roof is in a 45 degree-ish slant so any snow just falls off.
And I, every winter, see snow clinging to 45° roofs quite often. Slightly wet snow (also the heaviest snow) can even stick to vertical surfaces.
45° would also make an overhang more susceptible to wind forces, acting like a sail.
Source: My dad has his hobby car in a glorified tent during winter, works just fine, the roof is in a 45 degree-ish slant so any snow just falls off.