Step 2.Step 3.Step 4.Lay out the bird’s-mouth. Measure 1 1/2inches along the heel plumb line up fromthe bottom of the rafter. Set the blade ofthe square along the plumb line with theheel at the mark just made and strike a linealong the tongue. This line represents theseat of the bird’s-mouth.Lay out the tail line length. Measure thetail line length from the bird’ s-mouth heelplumb line. Strike a plumb line at thispoint in the same manner as the heelplumb line of the common rafter.Lay out the plumb cut at the ridgeboard.Measure and mark the point along the linelength half the thickness of the ridge-board. (This is the ridgeboard shorteningallowance.) Strike a plumb line at thispoint. This line represents the plumb cutof the ridgeboard.Step-Off Calculations and LayoutThe step-off method for rafter layout is old but stillpracticed. It combines procedures for laying out therafters with a procedure of stepping off the length of therafter (see fig. 2-21). In this example, the roof has an8-inch unit of rise, a total run of 5 feet 9 inches, and a10-inch projection.First, set gauges at 8 inches on the tongue and12 inches on the blade. With the tongue in the righthand, the blade in the left hand, and the heel awayfrom the body, place the square on the right end ofthe rafter stock. Mark the ridge plumb line along thetongue. Put a pencil line at the 12-inch point of the blade.Second, with the gauges pressed lightly against therafter, slide the square to the left. Line the tongue up withthe last 12-inch mark and make a second 12-inch markalong the bottom of the blade.Third, to add the 9-inch remainder of the total run,place the tongue on the last 12-inch mark. Draw anothermark at 9 inches on the blade. This will be the totallength of the rafter.Last, lay out and cut the plumb cut line and the seatcut line.Roof AssemblyThe major part of gable-roof construction is settingthe common rafters in place. The most efficient methodis to precut all common rafters, then fasten them to theridgeboard and the wall plates in one continuousoperation.The rafter locations should be marked on the topwall plates when the positions of the ceiling joists arelaid out. Proper roof layout ensures the rafters and joiststie into each other wherever possible.The ridgeboard like the common rafters, should beprecut. The rafter locations are then copied on theridgeboard from the markings on the wall plates(fig. 2-22). The ridgeboard should be the length of thebuilding plus the overhang at the gable ends.Figure 2-22.—Ridgeboard layout.2-15
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