Figure 2-62.—Truss construction.
Roof trusses save material and on-site labor costs.
It is estimated that a material savings of about 30 percent
is made on roof members and ceiling joists. When you
are building with trusses, the double top plates on
interior partition walls and the double floor joists under
interior bearing partitions are not necessary. Roof
trusses also eliminate interior bearing partitions because
trusses are self-supporting.
The basic components of a roof truss are the top and
bottom chords and the web members (fig. 2-62). The top
chords serve as roof rafters. The bottom chords act as
ceiling joists. The web members run between the top and
bottom chords. The truss parts are usually made of 2- by
4-inch or 2- by 6-inch material and are tied together with
metal or plywood gusset plates. Gussets shown in this
figure are made of plywood.
TYPES
Roof trusses come in a variety of shapes. The ones
most commonly used in light framing are the king post,
the W-type (or fink), and the scissors. An example of
each is shown in figure 2-63.
King Post
The simplest type of truss used in frame con-
struction is the king-post truss. It consists of top and
bottom chords and a vertical post at the center.
W-Type (Fink)
The most widely used truss in light-frame con-
struction is the W-type (fink) truss. It consists of top and
bottom chords tied together with web members. The
W-type truss provides a uniform load-carrying capacity.
Scissors
The scissor truss is used for building with sloping
ceilings. Many residential, church, and commercial
buildings require this type of truss. Generally, the slope
of the bottom chord of a scissor truss equals one-half the
slope of the top chord.
DESIGN PRINCIPLES
A roof truss is an engineered structural frame resting
on two outside walls of a building. The load carried by
the truss is transferred to these outside walls.
Weight and Stress
The design of a truss includes consideration of snow
and wind loads and the weight of the roof itself. Design
also takes into account the slope of the roof. Generally,
the flatter the slope, the greater the stresses. Flatter
slopes, therefore, require larger members and stronger
connections in roof trusses.
Figure 2-63.—Truss types.
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