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Chapter 2 Roof Framing | Up
Builder 3&2 Volume 02 - Construction manual for building structures | Next
Rafter |
other types and requires more time and labor to
construct.
Shed
The shed roof, or lean-to, is a roof having only one
slope, or pitch. It is used where large buildings are
framed under one roof, where hasty or temporary
construction is needed, and where sheds or additions are
erected. The roof is held up by walls or posts where one
wall or the posts on one side are at a higher level than
those on the opposite side.
FRAMING TERMS
Knowing the basic vocabulary is a necessary part of
your work as a Builder. In the following section, we’ll
cover some of the more common roof and rafter terms
you’ll need. Roof framing terms are related to the parts
of a triangle.
Roof
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,
Figure 2-2.—Roof framing terms.
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.
2-2
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