How to Judge the Roof Pitch

Place the steel carpenter's square flat on a work surface and put the level on top of the shorter arm of the square. Align the bottom edge of the level with the bottom edge of the arm of the carpenter's square and wrap duct tape around the level and carpenter's square in two different places to secure the level in place.

Set the ladder against the side of the house where you can see the slanted edge of the roof and climb to the edge of the roof.

Hold the steel carpenter's square so that the short arm is level and parallel with the ground and the longer arm is pointed up and closest to the roof apex.

Position the carpenter's square so that the short arm intersects with the bottom edge of the roof keeping the carpenter's square level. Use the marker to indicate where the roof intersects the long arm of the carpenter's square on the inner edge.

Read the distance from the inner corner of the carpenter's square to the mark on the long arm giving you the rise of the roof once you're back on the ground. Read the distance from the inner corner of the square to the end of the short arm, which was against the underside of the roof.

Divide the distance on the long arm by the length of the short arm. Multiply the result by 12. The number you obtain is now the rise over 12. For instance, if the short arm was 16 inches while your mark was at 10 inches your intermediary number is 0.625 and the final pitch of the roof is 7.5 / 12.