Soft spots over eaves

First off, I'm very broke and can't afford even the cheap options but it needs to happen but I don't want to do it completely wrong and waste what little money I have. I have an asphalt roof with two layers on a traditional ranch, not too steep. I want to put metal over the asphalt, but... I have a couple small soft spots in the decking one one side in say the bottom foot of the roof (2 ft overhang on eave). I assume this is from ice dams. So, lets say I fix the ventilation in the attic and add insulation to stop the ice dam problem can I just lay the metal over the soft spots or do I need to tear it out? If so, can I just replace that bottom 2 feet or so or would I need to tear off the whole thing? I've already done this in one spot and it was a terrific pain in the neck. I would almost rather tear off the whole side, but then would have to pay to get rid of shingles.