The primary object of a roof in any climate is protection from the elements. Roof slope and rigidness are for shedding water and bearing any extra additional weight. Roofs must also be strong enough to withstand high winds. In this section, well cover the most common types of roofs and basic framing terms.
Knowing the basic vocabulary is a necessary part of your work as a Builder. In the following section, well cover some of the more common roof and rafter terms youll need. Roof framing terms are related to the parts of a triangle.
Features associated with basic roof framing terms are shown in figure 2-2. Refer to the figure as you study the terms discussed in the next paragraphs.
Span is the horizontal distance between the outside top plates, or the base of two abutting right triangles.
Unit of run is a fixed unit of measure, always 12 inches for the common rafter. Any measurement in a horizontal direction is expressed as run and is always measured on a level plane. Unit of span is also fixed, twice the unit of run, or 24 inches. Unit of rise is the distance the rafter rises per foot of run (unit of run).
Total run is equal to half the span, or the base of one of the right triangles. Total rise is the vertical distance from the top plate to the top of the ridge, or the altitude of the triangle.
Pitch is the ratio of unit of rise to the unit of span. It describes the slope of a roof. Pitch is expressed as a fraction, such as 1/4 or 1/2 pitch. The term "pitch" is gradually being replaced by the term "cut." Cut is the angle that the roof surface makes with a horizontal plane. This angle is usually expressed as a fraction in which the numerator equals the unit of rise and the denominator equals the unit of run (12 inches), such as 6/1 2 or 8/12. This can also be expressed in inches per foot; for example, a 6- or 8-inch cut per foot. Here, the unit of run (12 inches) is understood. Pitch can be converted to cut by using the following formula: unit of span (24 in.) x pitch = unit of rise. For example, 1/8 pitch is given, so 24 x 1/8 equals 3, or unit of rise in inches. If the unit of rise in inches is 3, then the cut is the unit of rise and the unit of run (12 inches), or 3/12.
Line length is the hyptenuse of the triangle whose base equals the total run and whose altitude equals the total rise. The distance is measured along the rafter from the outside edge of the top plate to the centerline of the ridge. Bridge measure is the hypotenuse of the triangle with the unit of run for the base and unit of rise for the altitude.
Figure 2-2.Roof framing terms.
The members making up the main body of the framework of all roofs are called rafters. They do for the roof what the joists do for the floor and what the studs do for the wall. Rafters are inclined members spaced from 16 to 48 inches apart. They vary in size, depending on their length and spacing. The tops of the inclined rafters are fastened in one of several ways determined by the type of roof. The bottoms of the rafters rest on the plate member, providing a connecting link between the wall and the roof. The rafters are really functional parts of both the walls and the roof.
The structural relationship between the rafters and the wall is the same in all types of roofs. The rafters are not framed into the plate, but are simply nailed to it. Some are cut to fit the plate, whereas others, in hasty construction, are merely laid on top of the plate and nailed in place. Rafters usually extend a short distance beyond the wall to form the eaves (overhang) and protect the sides of the building. Features associated with various rafter types and terminology are shown in figure 2-3.
Figure 2-3.-Rafter terms.
Common rafters extend from the plate to the ridgeboard at right angles to both. Hip rafters extend diagonally from the outside corner formed by perpendicular plates to the ridgeboard. Valley rafters extend from the plates to the ridgeboard along the lines where two roofs intersect. Jack rafters never extend the full distance from plate to ridgeboard. Jack rafters are subdivided into the hip, valley, and cripple jacks.
In a hip jack, the lower ends rest on the plate and the upper ends against the hip rafter. In a valley jack the lower ends rest against the valley rafters and the upper ends against the ridgeboard. A cripple jack is nailed between hip and valley rafters.
Rafters are cut in three basic ways (shown in fig. 2-4, view A). The top cut, also called the plumb cut, is made at the end of the rafter to be placed against the ridgeboard or, if the ridgeboard is omitted, against the opposite rafters. A seat, bottom, or heel cut is made at the end of the rafter that is to rest on the plate. A side cut (not shown in fig. 2-4), also called a cheek cut, is a bevel cut on the side of a rafter to make it fit against another frame member.
Figure 2-4.Rafter layout.
Rafter length is the shortest distance between the outer edge of the top plate and the center of the ridge line. The cave, tail, or overhang is the portion of the rafter extending beyond the outer edge of the plate. A measure line (fig. 2-4, view B) is an imaginary reference line laid out down the middle of the face of a rafter. If a portion of a roof is represented by a right triangle, the measure line corresponds to the hypotenuse; the rise to the altitude; and, the run to the base.
A plumb line (fig. 2-4, view C) is any line that is vertical (plumb) when the rafter is in its proper position. A level line (fig. 2-4, view C) is any line that is horizontal (level) when the rafter is in its proper position.
|David L. Heiserman, Editor||
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Revised: June 06, 2015