warning on roofing cement product
Coal tar is still used on flat roofs, but not as often as it once was. Many flat roof products are asphalt based. Asphalt is made from oil and is lighter than water. Coal tar is is made from a coal derivative and is heavier than water. The only thing they have in comon is that both are black. Otherwise they are not compatible. When somebody decided that coal tar was carcinogenic, many companies drifted away from it, although it is an excellent roof product. A roofing supply house can probably fix you up with some repair products, but I doubt you'll find it in a box store.