Geoff,
I have a friend who is a contractor - and cautions folks against getting up on the roof in the spring and walloping the roof with a fire axe, shovel, or similar tool.
His contention is that, if you don't fall and kill yourself, you will do damage to the shingles - then you will have a BIG problem - and one that lasts all year long. In fact, when he sees people pounding the crap out of their roofs with such tools, he often drops off a business card - and replaces the shingles the next summer for them.
He suggests leaving it alone until spring, then installing the heaters on the outside ( when they are on sale ), and doing the insulation thing.
Stewart
Fighting The Dam Ice in Beachville
( Sorry - "Ice Dam" )