I am hoping you can help me with a question..I drove to Castlegar recently and noticed something very unusual...You could clearly see the imprint of every rafter on the roofs of houses which still had the full snow load on them...I have never witnessed this before so am wondering why?

I have also seen these rafter lines through snow covered roofs occasionally. It is most likely that the interior space under the roof is a vaulted ceiling with insulation between the rafters. A typical example would be a second floor attic area with sloped ceilings that  has been converted to a living space by insulating between the rafters and then paneling or drywalling the underside of the rafters to make a finished living area. The heat from the room passes through the rafters much faster than it passes through the insulation because the rafters are much better thermal conductors than the insulation. The interior heat is quickly drawn from the room to the exterior through the rafters because of this thermal conductivity. The insulation acts in exactly the opposite manner,  reducing thermal conductivity to such an extent that the interior building heat melts the snow at a much slower rate.  So, the snow melts along each rafter line much more quickly than the insulated spaces between the rafters, creating visible snow-melt lines on the outside of the roof. 

This thermal transfer or "thermal bridging" as it is called in the construction industry, is the reason it has become very popular  to construct double staggered stud exterior wall cavities in new home construction. In other words, modern home builders, looking for energy efficiency often build "staggered stud" exterior walls ( 2"x4" studs offset on a 2"x8" or 2"x10" plates)  so no wall studs directly contact both the interior and exterior wall surfaces. This staggered stud construction has the added benefits of creating significantly more insulation space and subsequent superior sound control in a wall cavity than a conventional 2"x6" framed wall. The additional costs in materials are quickly recouped in heat savings and personal comfort from reduced noise transfer through the exterior walls.