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Coal tar is an aromatic hydrocarbon and a by-product from the coking process of the coal industry. It is historically in abundance where coal is used in steel manufacturing. 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 tar paper. It is typically limited to applications on dead level or flat roofs with slopes of in 12 (1:48) or less. It is the only roofing material permitted by the International Building Code to be applied to slopes below in 12; the code allows its use on roofs with slopes as low as in 12 (1:96). It has a tendency to soften in warm temperatures and "heal" itself. It is typically surfaced with gravel to protect the roof from UV rays, hail, and foot traffic, as well as for fire protection. Coal tar provides an extremely long life cycle that is sustainable and renewable. It takes energy to manufacture and to construct a roof with it but its proven longevity with periodic maintenance provides service for many years, with ages from 50 to 70 years not uncommon, with some now performing for over a century. Currently, there are cold process (no kettle is used) coal tar pitch products that almost eliminate all fumes associated with its typical hot process version.