Laying Out And Cutting Rafter Birdsmouths At Each Rafter End
The rafter birdsmouth at WP1 is laid out first. See figure 2. Slide the Frederickson Metric Framing Square down the rafter until the bottom edge of the rafter intersects the exterior 50 mm mark on the tongue of the square, and intersects the exterior 250 mm mark on the blade at WP1. Mark the seat cut line from WP1 along the bottom edge of the blade for a distance of 89 mm. 89 mm is the width of the top plate. At the 89th mm mark draw a plumb line and lay out the birdsmouth.
Slide the square down the rafter to layout the seat cut of the birdsmouth at WP2. Use the same rise and run units, and rafter and metric square edges as WP1. Position the blade's 250 mm mark along the bottom edge of the rafter so the length along the bottom edge of the blade, from the 250 mm mark to the plumb line at WP2, is 89 mm. Layout the birdsmouth.
See "How to Frame Gable Roofs" for instructions on how to layout a birdsmouth.
of a Shed Roof Rafter