More Basic Concepts
Roof framing is indeed sometimes quite complicated. However it helps to be able to organize the different types of roofs mentally before approaching this skill for the first time. Keep in mind that there are five basic roof types: hip, gable, shed, gambrel and mansard. In New England and other parts of the northeastern U.S., the “saltbox” design is also popular, and consists of a gable roof that has one side longer than the other. Recently more complicated roof designs referred to as “cut ups” have become popular. The cut-up designs contain lots of dormers, valleys, and other features. They are found mostly in new housing developments in which the homes appear to be “all roof”. Roof construction plans are included in house building plans which are purchased rather than custom-designed.
Trusses are a big part of the project. Prefabricated trusses are available and are made to order and delivered at the job site. These are exceedingly heavy, and require cranes and extra manpower to place them. Properly equipped persons can build their own trusses on site. This equipment is available for rental. The most common method of roof construction is still the rafter/roof piece by piece route. If the roof is of one of the simpler designs like shed or gable, they are fairly easy to construct and can be put together without plans if necessary. Of course this requires that the builder have a grasp of the basics of roof design and possess a bit of geometry.
To begin with any roof type excepting a shed roof, a few pieces of information are required. These are the span, run, rise, and line length. The run can be omitted when beginning a shed roof.
Definitions of these terms are:
- Span – the width from outside wall to outside wall.
- Run – half the span.
- Rise – the height from the center of the span to the peak of the roof.
- Line length – a hypotenuse running from the outside of the wall line to the center line of the roof at the top of the rise.
Pitch is also important. This is how steep the roof is, measured as an angle from the vertical wall plate to the ridge line of the roof. These vary a lot from roof to roof. The commonest pitch is a gable roof which is usually 1/4 or 1/3 and this is the same as a fourth or a third the entire span of the building. This does not include overhang.
Pitch is expressed as inches per 12 inches. For example a pitch of 4/12 would mean that the roof rises 4 inches for every 12 inches of linear travel. Local codes sometimes determine what the pitch of a given roof needs to be. The weight of snow, wind force, and the type of covering all are factors in determining the correct pitch of a roof.
Northern climates commonly see 8 in 12 pitch or more. This assists with snow weight, as the snow tends to slide off of the steep roof. Lower pitches are seen in the south.
Note, however, that a 3 in 12 pitch is the minimum required when using materials like corrugated metal or asphalt shingles. Lower pitches also require the use of roll roofing to maintain waterproofing.