Roofers work on a variety of types of buildings, protecting those facilities against water damage to the structure and it contents. Roofing in the commercial and industrial sector is generally of the built-up type or the single-ply category. In built-up roofing, layers of felt are set in hot bitumen over insulation boards to form a waterproof membrane. A separate category of Roofing is the modified bitumen system that may be applied with hot bitumen or torched-on with high intensity propane burners. Another type of roofing is the residential type, although these applications can also be done in the commercial and industrial sector as well. They include composition shingles, slate, tile and metal rods.