Determining the pitch of a roof for new construction is simple. The contractor consults the blueprints and finds the pitch, usually given as vertical rise in inches over horizontal run in inches, and then converts this ratio into a degree measurement for the guys running the miter saw. When you get into remodeling, finding the correct rafter angles can be a little trickier, but with the right tools, calculating the rafter angles for your ceiling is a snap. Here is how to measure the angles on preexisting rafters, so you can cut new rafters to the proper angle.
Things You'll Need
 Rafter Angle Square
 Level
 Pencil
 Scratch Paper

Scribe a vertical line on a preexisting rafter. Use the level to make sure the line is completely vertical and not canted off to either side. Any error you make with the line will be compounded when you take the angle for the rafter.

Place the pivot point of a rafter angle square on the bottom edge of the rafter. Place the pivot point at the exact place where the vertical line you made crosses the bottom edge of the rafter.

Align the ruler edge of the rafter angle square with the vertical line. A rafter angle square has two gradated edges. The first edge is set at a right angle to the flanged base of the rafter angle square, and is marked with a short ruler. The second edge is the hypotenuse of a right triangle and is marked with a degree scale.

Observe where the bottom edge of the rafter crosses the degree scale edge of the rafter angle square. Most rafter angle squares also have a slot cut parallel to the degree scale which displays the vertical rise in inches. This is usually marked on either side to accommodate common top cuts and hipvalley cuts. For example, a degree measurement of 30 degrees corresponds approximately to a vertical rise of seven for common cuts or a vertical rise of 10 for hipvalley cuts.

Note the degree measurement. Set your miter saw to that angle and let it rip.
Tips & Warnings
 Always double check measurements before cutting a test piece.
 Always dry fit a test piece to ensure proper angle before cutting a bunch of rafters.