(1) Common rafters. These are framing members that extend at right angles from the plate
line to the roof ridge. They are called common rafters because they are common to all types of roofs
and are used as the basis for laying out other types of rafters.
(2) Hip rafters. These are roof members that extend diagonally from the corner of the plate
to the ridge.
(3) Valley rafters. These rafters extend from the plate to the ridge along the lines where two
roofs intersect.
(4) Jack rafters. These are a common rafter. The three kinds of jack rafter are the--
Hip jack, which extends from the plate to the hip rafter.
Valley jack, which extends from the ridge of the valley rafter.
Cripple jack, which is placed between a hip rafter and a valley rafter. The cripple jack
rafter is also part of a common rafter, but it touches neither the ridge of the roof nor the
rafter plate.
b. Rafter Layout. Rafters must be laid out and cut with the slope, length, and overhang exactly
right so that they will fit when placed in the roof.
(1) Scale or Measurement Method. The carpenter should first determine the length of the
rafter and the length of the lumber from which the rafter may be cut. If he is working from a roof
plan, he learns the rafter lengths and the width of the building from the plan. If no plans are available,
the width of the building must be measured.
To determine the rafter length, first find one-half of the distance between the outside plates. (The
amount of rise per foot has yet to be considered.) If the building is 20 feet wide, half the span will be
10 feet.
As an example, use a rise per foot of 8 inches. To determine the overall length of a rafter, measure on
the steel carpenter's square the distance between 8 on the tongue and 12 on the blade (8 is the rise, and
12 is the unit run). This distance is 14 5/12 inches. This represents the line length of a rafter with a
total run of 1 foot and a rise of 8 inches (see Figure 3-9, page 3-6).
Since the run of the rafter is 10 feet, multiply 10 by the line length for 1 foot (10 x 14 5/12 = 144 2/12).
The answer is 144 2/12 inches or 12 feet 1/6 inch. The amount of overhang, normally 1 foot, must be
added if an overhang is to be used. This makes the total length of the rafter 13 feet 1/6 inch. Use a
14-foot timber.
3-5
EN5156
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