a. General. Rafters will seldom be put up singly; they are usually assembled into trusses, as shown in figure 2-19. Two rafters are connected at the top by using a rafter or collar tie well nailed into both rafters. Before any ties or chords are nailed, accuracy of the rafter cuts should be checked by spreading the lower ends of the rafters to correspond to the width of the building. This may be accomplished by a template, or by measuring the distance between the seat cuts with a tape.
b. TO type trusses. Trussed rafters for a 20-foot-wide, TO type building are made from the following members (fig. 2-19).
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Figure 2-19. Typical rafter truss for a TO type building.
(1) Rafters. The rafters or upper chords are the same as the rafters in the end panel.
Their primary function is to carry the roof load to the studs. NOTE: rafters in end panels are an integral part of the panel and are not considered as rafter trusses.
(2) Lower chord. The lower chord or ceiling joist is a 1 x 6 member spanning from the lower ends of the two rafters in the truss. When a ceiling is used, these members are increased to 2 x 6's and the ceiling is attached to them.
(3) Hanger. The hanger is the 1 x 6 vertical member in the center of the truss that ties in the peak of the rafters to the center of the lower chord.
(4) Web members. The web members are the diagonal braces which tie in the center of the lower chord with the centers of the rafters. They are cut from 2 x 4's.
(5) Rafter tie. The rafter tie, normally a piece of 1 x 8, connects the two rafters at the peak of the truss as in the end panel.
(6) Knee braces. Knee braces are the members which tie in the roof system with the wall
system. You can see that they are attached to the rafter, to the lower chord, and to the stud in the sidewall panel.
c. Advantages. The principal advantages of a trussed rafter system over a plain rafter system are that smaller size lumber can be used and that the truss can be fabricated as a unit for later erection.
This results in economy of labor and material, and saves time except on very small projects.
d. Templates or jigs. Rafter framing constructed without the use of ridgeboards may be rapidly completed by use of a truss assembly jig or template. The template is laid out (1, fig. 2-20) to form a pattern conforming to the exact exterior dimensions of the truss. Templates or jigs must be constructed accurately to assure proper dimensions of the rafter truss. Lay out a jig in the following manner (2, fig 2-20):
(1) Measure and mark a straight line on a selected surface, the exact length of the joists which will form the chord of the truss. This is the base line A.
(2) From the center of the base line and at right angles to it, lay out a line the length required to form the leg of a right
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Figure 2-20. Rafter-truss jigs.
triangle, the base of which is half the length of the base line, A, and the hypotenuse, B, which is the length of the rafter measured as indicated. This is the centerline, C.
(3) Nail 2 x 4 by 8-inch blocks flush with the ends of base line A and centerline C as shown. Mark centerline on center jig blocks.
(4) Start assembly by setting a rafter in the jig with plate cut fitted over jig block at one end of base line. Peak is flush with centerline on peak jig block. Nail a holding block outside rafter at point D.
Repeat for other rafter at E.
(5) Assemble trusses in the following order: Lay one 2 x 4 joist or chord in place across base blocks. Lay two 2 x 4 rafters in place over joist.
Center one end of a 1 x 6 hanger under rafter peak.
Center rafters against peak block. Nail through rafters into hanger with six 8-penny nails. Line up one end of chord. Nail through rafter with 16-penny
spikes. Line up other end of chord. Nail as above.
Center bottom of hanger on top of chord and nail with 8-penny nails.
e. Placement. After the rafters have been assembled into trusses, they must be placed on the building. The first set of rafters may be assembled in the end section of the building or at the center as indicated in figure 2-21. The rafter trusses are raised by hand into position and nailed to the plate with 16- or 20-penny nails. These trusses are temporarily braced to the end section of the building, until the sheathing is applied. Building temporary platforms or work-benches for the workers to stand on while erecting these trusses will save time. The knee braces may be applied before or after the sheathing is applied. Knee braces are not used on every rafter truss unless severe wind conditions are anticipated.
In fact, as figure 2-19 shows, they may be spaced as widely as 16 feet OC. There are several methods for
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Figure 2-21. Erection of rafter trusses.
the actual installation. The following procedures may be used in the actual installation of trusses:
(1) Mark proper positions of all truss assemblies on top plate. The marks will show the exact position of a given face of all rafters (south or north, etc.).
(2) Rest one end of a truss assembly, peak down, on an appropriate mark on top plate on one side of structure (1, fig. 2-21).
(3) Rest other end of truss on opposing mark on top plate on other side of structure (2, fig. 2-21).
(4) Rotate assembly into position by means of a pole or rope (3, fig. 2-21).
(5) Line up rafter faces flush against marks and secure.
(6) Raise, aline, and nail several truss assemblies into position. Nail temporary 1 x 6 braces across these assemblies (4, fig. 2-21) and other assemblies as they are brought into position. Check rafter spacing at peaks as braces are nailed on.
(7) Braces may be used as a platform when raising those trusses when there is too little room to permit rotation.