The rubber crumbs are 100% manufactured from used tires. Once the crumb rubber is mixed with Sikalastic 720 Base, it becomes part of the semi-self-leveling resurfacing for the refurbishment of aging gravel roofs. The crumb rubber adds to the durability of the whole roof system. They do not decompose or compact. 100% recycled used tires add to the environmentally friendly system and keep the tires out of a landfill.