After you determine the rise, span, run, line length and pitch, the next step is to lay out the rafters, or mark the cuts on a pattern rafter to create the roof. This article will discuss a gable roof, without valleys or dormers, because it’s the most common and the easiest for a first-time builder to tackle. A framing square is traditionally used for laying out the roof and determining pitch. This is denoted two ways, a 1/3 or 1/4 pitch in fractions, and a 6/12 pitch in inches, which means the roof rises 6 inches for each 12 inches of run. Pitch is the amount of angle or slope the roof has.
For lower pitches, a built-up or continuous roll roofing must be applied to keep the roof waterproof. 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). Those in the southern climates may utilize lower pitches. 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. Pitch is determined by snow loads, other weather factors and the covering to be applied to the roof. In many instances, a certain pitch may be necessary or even required by local codes. For instance a 4/12 pitch denotes a roof rising 4 inches for each 12 inches. Pitch also has its own denotation, determined by the rise in inches in 12 inches. 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 can vary a great deal, from a shallow slope up to a very steep pitch.
Pitch is the slope or angle from the wall plate to the roof ridge line. You will also need to know the desired pitch of the roof. 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. The framing of a basic gable roof is based on a right-angle triangle, and the various roof framing components fit the triangle. Basically you’re working with a triangle with two legs and the line length for a hypotenuse. 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. Rise is the measurement from the centerline of the span to the top of the roof line. Span is the measurement across the building from outside supporting wall to outside supporting wall. For a shed roof you only need to know the rise, span and line length. 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. The gable is the most common, and it can be complicated with multiple roof lines, including valleys and dormers. Roofs are framed in five basic designs: shed, gable, hip, gambrel and mansard. 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. Piece-by-piece rafter/roof construction, however, is still the more common construction method for many buildings.
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You can also build your own trusses if you have the equipment, or can rent the equipment to install them. They do, however, require extra manpower and lifting equipment to install in place. They are constructed at a factory to match your building and delivered on site. Pre-constructed trusses have also become increasingly popular. In many cases with purchased house plans, the details of the roof construction, including rafter design, are included. These days “cut-ups,” or roofs with a lot of valleys, dormers and other features, are increasingly popular. Another common design in the Northeast is the “saltbox,” which is a gable roof with one longer side. Roofs are basically five types: shed, gable, hip, gambrel and mansard. Roof framing is one of those carpenter skills that appears quite complicated, and indeed, some roof designs are difficult.
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