How to Compute Roof Rafter Angles

When building a new roof or repairing an old roof, calculating the framing for the roof is an important part of the process. Without proper calculations, the angle of the roof may be wrong, and the material quantities and sizes may be off. Determining the angle, or pitch, of the roof requires some careful measurements. Always measure twice and double check your figures before cutting the rafters for your roof to prevent a bad cut.

Things You'll Need

• 18 to 24-inch spirit level
• Measuring tape
• Calculator

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Instructions

1. • 1

     Measure 12 inches from the end of the spirit level. Mark the level with a pencil.

   • 2

     Enter the attic. Hold one end of a level against the bottom of a roof rafter, adjusting the level until the spirit bubble is between the lines. Have a volunteer hold the level in this position.

   • 3

     Measure from the 12-inch mark on the spirit level to the roof. The measurement in inches is the amount of inches the roof rises for every 12 inches. This is your roof angle, or pitch.