Figure 2-18.—The actual (versus theoretical) length of a commonrafter.Figure 2-19.-Steel square used to lay out plumb and seat cuts.SHORTENING.— Rafter length found by any ofthe methods discussed here is the measurement from theheel plumb line to the center of the ridge. This is knownas the theoretical length of the rafter. Since a ridgeboard,usually 1 1/2 inches thick, is placed between the rafters,one-half of the ridgeboard (3/4 inch) must be deductedfrom each rafter. This calculation is known as shorteningthe rafter. It is done at the time the rafters are laid out.The actual length (as opposed to the theoretical length)of a ratler is the distance from the heel plumb line to theshortened ridge plumb line (fig. 2-18).LAYING OUT.— Before the rafters can be cut, theangles of the cuts must be marked. Layout consists ofmarking the plumb cuts at the ridge, heel, and tail of therafter, and the seat cut where the rafter will rest on thewall. The angles are laid out with a framing square, asshown in figure 2-19. A pair of square gauges is usefulin the procedure. One square gauge is secured to thetongue of the square next to the number that is the sameas the unit of rise. The other gauge is secured to the bladeof the square next to the number that is the same as theunit of run (always 12 inches). When the square is placedon the rafter stock, the plumb cut can be marked alongthe tongue (unit of rise) side of the square. The seat cutcan be marked along the blade (unit of run) side of thesquare.Rafter layout also includes marking off the requiredoverhang, or tail line length, and making the shorteningcalculation explained earlier. Overhang, or tail linelength, is rarely given and must be calculated beforelaying out rafters. Projection, the horizontal distancefrom the building line to the rafter tail, must be locatedfrom drawings or specifications. To determine tail linelength, use the following formula: bridge measure (ininches) times projection (in feet) equals tail line length(in inches). Determine the bridge measure by using therafter table on the framing square or calculate it by usingthe Pythagorean theorem. Using figure 2-20 as a guide,you can see there are four basic steps remaining.Figure 2-20.—Laying out a common rafter for a gable roof.2-13