Figure 6-27.-Bird’s-mouth on a rafter with projection.ridgeboard or, if the ridgeboard is omitted,against the opposite rafters (fig. 6-25).SEAT, BOTTOM, OR HEEL CUT—The cutmade at the end of the rafter that is to rest onthe plate.SIDE, OR CHEEK, CUT—A bevel cut on theside of a rafter to fit against another framemember.EAVE OR TAIL—The portion of the rafterextending beyond the outer edge of the plate.Figure 6-26 shows additional terms used inconnection with rafter layout.RAFTER LENGTH is the shortest distancebetween the outer edge of the plate and the centerof the ridgeline.MEASURE LINE is an imaginary referenceline laid out down the middle face of the rafter.PLUMB LINE is any line that is vertical whenthe rafter is in its proper position.LEVEL LINE is any line that is horizontalwhen the rafter is in its proper position.A rafter with a projection often has a notchin it called a BIRD’ S-MOUTH (fig. 6-27). Theplumb cut of the bird’s-mouth that bears againstthe side of the rafter plate is called the HEELCUT, whereas the SEAT CUT bears on top ofthe bird’s-mouth. COLLAR TIES (fig. 6-28)are horizontal members used as reinforcementin gable or double-pitch roof rafters. In afinished attic, these ties may function as ceilingjoists.When the rafters are placed farther apart,horizontal members called PURLINS are placedacross them to serve as the nailing or connectingmembers for the roofing. Purlins are generallyused with standard metal roofing sheets, such asgalvanized iron or aluminum sheets.Several methods of roof framing and types ofrafter arrangement6-29 through 6-36.are further shown in figuresFigure 6-28.-Layout of a collar tie.6-23
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