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How are roofs framed?

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Nearly all roofs are framed using one of two methods: standard stick framing or newer truss framing. Stick-framed roofs utilize individual rafters that span from the top of exterior walls to the ridge. Truss-framed roofs are built from triangular-shaped, pre-made truss units. Gable and hip roofs may be built primarily of trusses; other roof shapes, particularly those with dormers or on houses with cathedral ceilings, attic rooms or attic storage areas are stick built. Stick framing creates a triangle between rafters and ceiling joists. Collar beam adds strength at the middle.

Like wall studs and floor joists, rafters and trusses are spaced every 16 or 24 inches from center to center. Most roofs utilize 16-inch spacing for strength and rigidity. Rafters are usually positioned directly above wall studs. A truss is one contiguous double rafter/ ceiling joist unit. Truss construction is just as strong but is lighter weight and uses smaller sizes of lumber than stick framing. Because trusses are carefully engineered units that shouldn't be cut, they are not a good choice for roofs that may be modified at a later date. And because they have several intermediate support members, they don't allow use of the attic space.

The angle of a roof is measured and referred to by either slope or pitch. The slope of a roof is the number of inches a roof rises in 12 inches of horizontal run. A roof with a 5-in-12 slope rises 5 inches for every 12 inches of horizontal run. Pitch expresses the same two measurements as a fraction. The same roof has a 5/12 pitch.

You've probably noticed that different roofs have different shapes. Houses can have gable, hipped, mansard, gambrel, flat or shed roofs. Many homes combine varying roof types on one roof. It's quite common, for example to see a hipped roof with gable dormers. Roof shape is one of the key factors in setting the architectural style of a house.

Roof shape also dictates how difficult and costly a roof is to build and how it will serve. For example, flat, shed and-in some case-gable roofs tend to be relatively affordable to build. Gambrel and mansard roofs offer more head height for attic rooms. Shed roofs are usually the easiest type to connect to an existing roof when adding-on.

Roof framing can be simple or complex, depending on the roof. Overhangs, hips and dormers add greatly to the complexity of the framing. Here the major framing components are shown.

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