Slope is the most significant limitation of coal tar roofing systems when compared to asphalt BUR’s. When using organic coal tar saturated felts (Tarred Felt), the maximum slope allowed is 1/2″ per foot. When using coal tar coated glass felts (Tar Glas), the maximum slope allowed is 1/4″ per foot. Coal tar has a relatively low softening point, so it must always include a gravel surface. Gravel provides many benefits, including protecting the roof from the sun’s rays, protection from foot traffic, weight as additional protection for wind uplift, reduced surface temperature, and protection from burning brands. But most importantly, gravel allows us to include an additional minimum average of 70 lb/square of coal tar in the installation of the roof.