Roof Framing 101
Roof framing is one of those carpenter skills that appears quite complicated, and indeed, some roof designs are difficult. Roofs are basically five types: shed, gable, hip, gambrel and mansard. Another common design in the Northeast is the “saltbox,” which is a gable roof with one longer side. These days “cut-ups,” or roofs with a lot of valleys, dormers and other features, are increasingly popular. In many cases with purchased house plans, the details of the roof construction, including rafter design, are included. Pre-constructed trusses have also become increasingly popular. They are constructed at a factory to match your building and delivered on site. They do, however, require extra manpower and lifting equipment to install in place. You can also build your own trusses if you have the equipment, or can rent the equipment to install them. Piece-by-piece rafter/roof construction, however, is still the more common construction method for many buildings. Simple roofs, such as a shed or the common gable, are fairly easy to construct, even without plans, if you understand the basics and a little geometry.
Roofs are framed in five basic designs: shed, gable, hip, gambrel and mansard. The gable is the most common, and it can be complicated with multiple roof lines, including valleys and dormers.
In order to build any but a shed-type roof, including trusses, you’ll need to first determine a few factors; the span, rise, run and line length. For a shed roof you only need to know the rise, span and line length. Span is the measurement across the building from outside supporting wall to outside supporting wall. Run is half the distance of the span. Rise is the measurement from the centerline of the span to the top of the roof line. Line length is the measurement from the outside of the supporting wall line to the centerline of the roof at the top of the rise. Basically you’re working with a triangle with two legs and the line length for a hypotenuse.
The framing of a basic gable roof is based on a right-angle triangle, and the various roof framing components fit the triangle. The rise, or height of the roof at its peak, is the altitude of the triangle; the run, or half the building span, is the base of the triangle; and the line length, measurement from the roof peak to the building wall, is the hypotenuse.
You will also need to know the desired pitch of the roof. Pitch is the slope or angle from the wall plate to the roof ridge line. Pitch can vary a great deal, from a shallow slope up to a very steep pitch. Pitch for a gable roof, the most common, is generally 1/4 or 1/3; which is equal to 1/4 or 1/3 the total span of the building, not counting any overhang. Pitch also has its own denotation, determined by the rise in inches in 12 inches. For instance a 4/12 pitch denotes a roof rising 4 inches for each 12 inches. Having the correct pitch is important. In many instances, a certain pitch may be necessary or even required by local codes. Pitch is determined by snow loads, other weather factors and the covering to be applied to the roof. For those in the northern parts of the country, an 8 in 12 pitch, or more, is commonly used to keep excessive snow loads off the roof. Those in the southern climates may utilize lower pitches. The minimum pitch, however, that can be used with many roofing materials such as asphalt shingles or corrugated metal is 3 in 12 (3/12 pitch). For lower pitches, a built-up or continuous roll roofing must be applied to keep the roof waterproof.