The hairiest solvent commonly available is methylene chloride. Non- flammable, volatile, causes cancers in California. Buy it at your paint store in bodied formulations such as StripEase paint remover. Follow directions. It will cut old roofing tar/cement easily based on my experience. The resulting goop will need lots of paper towels preferably placed in the outdoors trash can ASAP. Buy the smallest amount you can as the stuff does tend to corrode the metal cans if some casual moisture is present. If this sounds too scary, try some of the solvents made by 3M and found at autobody supply stores. The pros that refinish cars have some really heavy duty stuff for cleaning prior to paint.
Joe