How to Calculate the Rise and Run of the Roof's Angle

If you want to add another room onto your house, you need to extend the roof and make it fit seamlessly with the rest of the roof. Trying to eyeball the roof measurements will likely result in skewed and inaccurate measurements and make your project look mediocre. Roof pitch, or the roof's slope, is the rise and run of the roof over a given length.

Framing square
Level
Yardstick

Instructions

- 1
  Place your ladder on the ground and lay it against a flat, dry surface near the roof.
- 2
  Climb up and place the framing square underneath the roof against a rafter, aligning the bottom inside edge horizontally with the bottom edge of the roof. Ensure that the bottom leg of the framing square is horizontal with the help of the level. The two edges of the framing square should form a right triangle with the roof. You can measure from the peak of the roof downward, but the roofing layers may skew your results.
- 3
  Measure twelve inches horizontally along the framing square. At this point you will measure up until you hit the roof.
- 4
  Measure upwards from the twelve-inch mark until you get to the roof. You should get a measurement of several inches. The pitch of the roof is written as this number divided by twelve. For example, if the roof is three inches above the framing square at twelve inches from the starting point, its pitch is 3/12, where the rise of the roof is three inches and the run is twelve.

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