First, I'll apologize for this discussion, but I'm a structural engineer and investigating roof collapses was one on my jobs.
If you had 2' on the ground, that equates (on average) to 40 pounds per square foot (PSF). In New Hampshire roofs are probably designed for 50 to 60 and more, but older structures may be less. The actual load that would "break" a roof considering current safety factors is 3 to 4 times that much or maybe 150 psf. That equates to over 7 feet of snow to collapse a properly designed roof.
So how do roofs fail you ask? The ones that fail always have something wrong with them. Bad design, rot and poor construction. Most of the failed roofs I've investigated did so around 25 psf when it was measured. So, it can never hurt clearing a roof, but most will stand considerable more snow than we've had with no sweat ... just don't ask me which ones unless you pay me to come look at yours!