Figure 2-62.—Truss construction.Roof trusses save material and on-site labor costs.It is estimated that a material savings of about 30 percentis made on roof members and ceiling joists. When youare building with trusses, the double top plates oninterior partition walls and the double floor joists underinterior bearing partitions are not necessary. Rooftrusses also eliminate interior bearing partitions becausetrusses are self-supporting.The basic components of a roof truss are the top andbottom chords and the web members (fig. 2-62). The topchords serve as roof rafters. The bottom chords act asceiling joists. The web members run between the top andbottom chords. The truss parts are usually made of 2- by4-inch or 2- by 6-inch material and are tied together withmetal or plywood gusset plates. Gussets shown in thisfigure are made of plywood.TYPESRoof trusses come in a variety of shapes. The onesmost commonly used in light framing are the king post,the W-type (or fink), and the scissors. An example ofeach is shown in figure 2-63.King PostThe simplest type of truss used in frame con-struction is the king-post truss. It consists of top andbottom 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 andbottom chords tied together with web members. TheW-type truss provides a uniform load-carrying capacity.ScissorsThe scissor truss is used for building with slopingceilings. Many residential, church, and commercialbuildings require this type of truss. Generally, the slopeof the bottom chord of a scissor truss equals one-half theslope of the top chord.DESIGN PRINCIPLESA roof truss is an engineered structural frame restingon two outside walls of a building. The load carried bythe truss is transferred to these outside walls.Weight and StressThe design of a truss includes consideration of snowand wind loads and the weight of the roof itself. Designalso takes into account the slope of the roof. Generally,the flatter the slope, the greater the stresses. Flatterslopes, therefore, require larger members and strongerconnections in roof trusses.Figure 2-63.—Truss types.2-39
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