Step-Off Calculations and Layout

Step 2. Step 3. Step 4. Lay out the bird’s-mouth. Measure 1 1/2 inches along the heel plumb line up from the bottom of the rafter. Set the blade of the square along the plumb line with the heel at the mark just made and strike a line along the tongue. This line represents the seat of the bird’s-mouth. Lay out the tail line length. Measure the tail line length from the bird’ s-mouth heel plumb line. Strike a plumb line at this point in the same manner as the heel plumb line of the common rafter. Lay out the plumb cut at the ridgeboard. Measure and mark the point along the line length half the thickness of the ridge- board. (This is the ridgeboard shortening allowance.) Strike a plumb line at this point. This line represents the plumb cut of the ridgeboard. Step-Off Calculations and Layout The step-off method for rafter layout is old but still practiced. It combines procedures for laying out the rafters with a procedure of stepping off the length of the rafter (see fig. 2-21). In this example, the roof has an 8-inch unit of rise, a total run of 5 feet 9 inches, and a 10-inch projection. First, set gauges at 8 inches on the tongue and 12 inches on the blade. With the tongue in the right hand, the blade in the left hand, and the heel away from the body, place the square on the right end of the rafter stock. Mark the ridge plumb line along the tongue. Put a pencil line at the 12-inch point of the blade. Second, with the gauges pressed lightly against the rafter, slide the square to the left. Line the tongue up with the last 12-inch mark and make a second 12-inch mark along 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 another mark at 9 inches on the blade. This will be the total length of the rafter. Last, lay out and cut the plumb cut line and the seat cut line. Roof Assembly The major part of gable-roof construction is setting the common rafters in place. The most efficient method is to precut all common rafters, then fasten them to the ridgeboard and the wall plates in one continuous operation. The rafter locations should be marked on the top wall plates when the positions of the ceiling joists are laid out. Proper roof layout ensures the rafters and joists tie into each other wherever possible. The ridgeboard like the common rafters, should be precut. The rafter locations are then copied on the ridgeboard from the markings on the wall plates (fig. 2-22). The ridgeboard should be the length of the building plus the overhang at the gable ends. Figure 2-22.—Ridgeboard layout. 2-15