Coal tar is an aromatic hydrocarbon and a by-product from the coking process of the coal industry. It ages very slowly through volatilization and is an excellent waterproofing and oil resistant product. Roofs are covered by heating the coal tar and applying it between layers of roofing belts.
It has limitations in regards to its application on dead level or flat roofs with slopes less than 1/4 in 12. It has a tendency to soften in warm temperatures and "heal" itself. It is typically ballasted with gravel to provide a walking surface. Coal tar provides an extremely long life cycle that is sustainable and renewable.
This product has proven longevity and with periodic maintenance provides service for many years. There are also cold process coal tar pitch products available.