Roofing Facts

A leaky roof can damage ceilings, walls, and furnishings. To protect buildings and their contents from water damage, roofers repair and install roofs made of tar or asphalt and gravel; rubber or thermoplastic; metal; or shingles made of asphalt, slate, fiberglass, wood, tile, or other material. Repair and reroofing—replacing old roofs on existing buildings—makes up the majority of work for these workers.

There are two types of roofs—low- and steep-sloped. Roofs considered low-slope rise 4 inches per horizontal foot or less and steep-slope roofs increase more than 4 inches per horizontal foot. Most commercial, industrial, and apartment buildings have low-sloping roofs. Most houses have steep-sloped roofs. Some roofers work on both types; others specialize.

Most low-slope roofs are covered with several layers of materials. Roofers first put a layer of insulation on the roof deck. Over the insulation, they then spread a coat of molten bitumen, a tarlike substance. Next, they install partially overlapping layers of roofing felt—a fabric saturated in bitumen—over the surface. Roofers use a mop to spread hot bitumen over the surface and under the next layer. This seals the seams and makes the surface watertight. Roofers repeat these steps to build up the desired number of layers, called “plies.” The top layer either is glazed to make a smooth finish or has gravel embedded in the hot bitumen to create a rough surface.

An increasing number of low-slope roofs are covered with a single-ply membrane of waterproof rubber or thermoplastic compounds. Roofers roll these sheets over the roof’s insulation and seal the seams. Adhesive, mechanical fasteners, or stone ballast hold the sheets in place. The building must be of sufficient strength to hold the ballast. A small, but growing number of flat-roofed buildings are now having “green” roofs installed. A “green” roof begins with a single or multi-ply waterproof system. After it is proven to be leak free, a root barrier is placed onto it, and then layers of soil, in which trees and grass are planted. Roofers are generally responsible for making sure the roof is watertight and can withstand the weight and water needs of the plantings.

Most residential steep-slope roofs are covered with shingles. To apply shingles, roofers first lay, cut, and tack 3-foot strips of roofing felt lengthwise over the entire roof. Then, starting from the bottom edge, they staple or nail overlapping rows of shingles to the roof. Workers measure and cut the felt and shingles to fit intersecting roof surfaces and to fit around vent pipes and chimneys. Wherever two roof surfaces intersect, or shingles reach a vent pipe or chimney, roofers cement or nail flashing-strips of metal or shingle over the joints to make them watertight. Finally, roofers cover exposed nailheads with roofing cement or caulking to prevent water leakage. Roofers who use tile, metal shingles, or shakes follow a similar process.

Because of their expertise in waterproofing roofs, some roofers also waterproof and dampproof masonry and concrete walls and floors, including foundations. To prepare surfaces for waterproofing, they hammer and chisel away rough spots, or remove them with a rubbing brick, before applying a coat of liquid waterproofing compound. They also may paint or spray surfaces with a waterproofing material, or attach waterproofing membrane to surfaces. When dampproofing, they usually spray a bitumen-based coating on interior or exterior surfaces. Roofers also install equipment that requires cutting through roofs, such as ventilation ducts and attic fans.