Framing A Flat Or Low-Pitch Roof
The two basic types of roofs--flat and pitched--have numerous variations. The so-called flat roof may actually have some slope for drainage. The slope is generally expressed as the inches of vertical rise in 12 inches of horizontal run. For purposes of definition, flat roofs might be classed as those having less than a 3-in 12-slope.
Post-and-beam construction is frequently used with flat or low-slope roofs. In conventional stud wall framing for buildings with flat or low-slope roofs, the rafters or roof joists usually serve as ceiling joists for the space below.
The flat or low-slope roof sometimes combines ceiling and roof elements in one system. This system serves as an interior finish, or as a fastening surface for the finish, and as an outer surface for application of the roofing. In mild climates flat or low-pitch roofs may be guilt with 2" matched planks for roof sheathing supported on large beams spaced about 6' apart. The planking and beams are exposed on the underside. The beams or thick roof decking which spans from the exterior walls to the ridge beam or center bearing partition. The roof is generally covered with a fiberboard insulation, and this in turn with a composition roof.
Roof joists for flat roofs are commonly laid level, with roof sheathing and roofing on top and with the underside utilized to support the ceiling. Sometimes a slight roof slope may be provided for roof drainage by tapering the joist or adding a cant strip ( a triangular piece of lumber) to the top. Insulation may be added just above the ceiling, and the space above the insulation should be ventilated to remove hot air in the summer and to provide protection against condensation in the winter.
Flat and low-pitch roofs generally require larger-sized rafters than pitched roofs, but the total amount of framing lumber required is usually less. In flat roof construction where rafters also serve as ceiling joists, the size of the rafters is based on both roof and ceiling loads. The size is given on the plans or determined from rafter span tables.
When there is an overhang on all sides of the house, lookout rafters are ordinarily used. The lookout rafters are nailed to a double header and toenailed to the wall plate. The distance from the double header to the wall line is usually twice the overhang. Rafter ends may be finished with an outside header, which will serve as a nailing surface for trim.