Flat Roof Components

Framing

• While pitched roof framing consists of angled or triangular units, such as rafters or trusses, flat roof framing consists of flat, rectangular support beams called joists. Essentially the same as the joists that compose raised floors, flat roof joists are often solid lumber planks. Alternatively, builders use specially engineered rectangular trusses to frame flat roofs. They angle trusses or joists slightly toward one side of a structure or toward a flat roof's central drain. Alternatively, builders laminate small, angled strips of wood on top of horizontally level framing members.

Flashing

• The word "flashing" refers to protective covering along roofs' edges, hips and valleys. Roof flashing reinforces and seals roofs' most vulnerable areas. Flat roofs typically include flashing around their perimeters. The correct type of flashing for a particular flat roof depends on the roof's design. Common flat roof flashing includes edge flashing, stop flashing and roof cap. Flashing is usually made of rust-resistant metal, such as aluminum and galvanized steel. Occasionally, roofers use roof covering materials and sealants, such as mineral paper, in conjunction with metal flashing.

Built-Up Roofing Systems

• Built-up roofing systems consist of overlapping layers of asphalt-impregnated sheets or tar paper, vapor-retardant materials, insulation and sealants. Roofers lay several layers of the material, binding each layer with an adhesive and sealant compound, typically asphalt. Built-up systems are among the most common flat roof coverings and frequently are used in commercial and industrial structures. Many builders refer to built-up roofing systems as "hot mop" roofs because roofers traditionally applied hot asphalt to them with a mop. The final, exposed layer of a built-up roof often consists of gravel bound in asphalt.

Membrane Roofing

• Membrane roofing consists of large sheets of rubber, plastic or asphalt materials that cover large portions of a flat or low-slope roof. In some cases, a single sheet covers an entire roof. The size of the sheets minimizes lap joints and increases a flat roof's resistance to water penetration. Like built-up roof systems, membranous roof coverings consist of several layers bound by adhesives and sealants. Although the membrane might form a single, integral layer, insulation and vapor retardants beneath might consist of small overlapping sheets. Roofers often top roof membranes with bitumen compounds and gravel.