Re: Tar to repair a leaking 1 yr old roof?
Howdy tar exposed to sunlight oxidieses and weathers away. But if applied under shingles it will last the life of the roofing - on average. Ice dambing, darn if they would of run ice an water guard or similar product up the valley then the flashing is a superior method. So i would attempt to seal only the sides of the shingles that aline the valley i prefer a good silicone caulk. do not seal the bottoms of the shingles aligning the valley as trapping water is a outcome you wish to avoid. or another possible solution is to remove the rain gutter if there is one at the end of the valley or install a 5" gutter to get the melting off the roof. Perhaps a little more insulation in the attic or more soffit venting to avoid the ice dambing in the future make sure the exhaust venting ducting of plumbing an appliances are not into the attic- you just never know until you look.
Any an all of my comments are just my opinion and not to be confused with facts.