So, you need some heat in your new garage to work on your vintage automobile, but you are not planning to finish the interior, what problems might you encounter?  Heating an uninsulated building is a very common situation and can cause serious issues during the snow months.  The heat is allowed to warm the roof sheathing which, in turn, heats the shingles and melts the snow.  The water then runs to the cold, unheated eaves where it can freeze and form an ice dam.  This dam blocks additional melt-water from exiting the roof and starts to freeze under the shingle layers and possibly onto the roof deck where rot can occur when it thaws.