Hot tar is a durable waterproof option generally used for flat roofs. This kind of material is highly resistant to the ultraviolet rays from the sun and can substantially lower energy costs. Typically, tar is a dark/oily mixture made from coal tar and petroleum by-products making it an unsustainable product that can give off noxious fumes during the application process. When installing hot tar roofing, contractors create a seal that can completely withstand moisture and eliminate the risk of mold. Of all the flat roofing options, a tar and gravel roof is the most economical.