Roof Construction Components

Frame

• Common roof framing uses rafters or trusses. Trusses are the quickest and simplest way to put up a roof. Each truss arrives numbered and labeled to show where it goes. A truss engineering document explains how to connect the trusses to the building, how to reinforce them and how to connect them to each other if that is required for particular roof styles. For example, hip roofs framed with trusses have half-gable trusses, end jacks, that must hang from main full gable trusses, girder truss, used at the building's middle. The truss's bottom piece of lumber, the bottom chord, serves as either a ceiling joist, or it can be sized to serve as a floor joist to accommodate rooms in the attic.

  Frame with rafters when trusses will not work or rafters are more cost effective. Size rafters by their span and the expected live and dead loads. Live loads include wind and snow while dead loads are the other roof components the rafters support. Most building codes require wood rafters to be high quality with minimal knots and more heartwood so they remain stable and strong. Rafters run from the building's sidewall to the ridge board, the peak of the roof, on pitched roofs. On flat roofs and shed-style roofs rafters run from sidewall to sidewall. Rafters may have collar ties that run from a rafter on one side of the building to a rafter opposite it on the other side of the building.

Sheathing

• Common sheathing is ½-inch or 5/8-inch plywood or oriented strand board (OSB). The sheathing fastens on top of the trusses or rafters and forms the skin of the roof. There are specifications describing how the sheathing is attached. Each succeeding row must be staggered to the preceding one so there are no seams running from end to end on rafters or trusses. Most building codes also require metal clips used between sheets at their horizontal middle points. The sheathing is covered with a layer of roofing felt, tar paper, or other type of weather resistive barrier.

Covering

• The covering is the top component of a roof. The most popular for pitched roofs is shingles. Shingles are made of fiberglass with asphalt and mineral coatings and come in many grades, some even lasting up to 40 years. Metal roofing is another kind of roof finish that is extremely durable. It now comes in many colors. Flat roofs usually rely on a built-up system of asphalt, tar paper and gravel with shingle-type material used to cover duct work and the area between the roof edge and the building sidewall.