
Calculating
The Shed Roof Line length And Overhang
The rise and run are used in the Pythagorean Theorem to calculate the line length and overhang of the shed roof rafter shown in figure 1. The rafter rise of 500 mm is the difference between the height of the walls supporting the rafters. 2500 mm  2000 mm = 500 mm. The span of 2500 mm is also the run of the rafter. The rafter line length of 2549.5 mm is calculated by taking the square root of the sum of the rise (500) squared and the run (2500) squared. The line length lies between WP1 (working point1) and WP2. Both working points line up with an exterior surface of a wall. The overhang of 510 mm at each end of the rafter is calculated by dividing the line length of 2549.5 mm by the run of 2500 mm and multiplying the answer by the projection of 500 mm. Mark WP2 on the rafter. Use the line length and the overhang length to mark in the WP1 and the overhang tail cut locations on the top surface of the rafter. Draw a line, 90 degrees to the rafter sides, through each central working point and tail cut location. Extend plumb lines down from each line. See "How To Frame Hip Roofs  Figure 3" 
Figure 1 The Dimensions of a Shed Roof 