How to Calculate Roof Pitch and Rafter Load

The pitch, or slope, of the roof is the number of inches a roof rises over a span of 12 inches. It often resembles a fraction on building plans such as 6/12 but is read as individual numbers as in a “6-12 pitch.” Roof pitch directly affects the cost of installing a new roof or replacing an old one. Therefore accurate roof pitch calculations are critical for accurate job estimates. Rafter load is the amount of weight the roof can bear based on the size and spacing of the supporting rafters. The roof has to be able to support the roofing materials as well as people working on the roof and seasonal loads created by rain and snow.

2-foot level
Tape measure
Pencil
Local building code
Span tables for construction

Instructions

Roof Pitch
- 1
  Measure and mark 1 foot on the level with a measuring tape.
- 2
  Position one end of the level on the roof while holding the other end in the air.
- 3
  Adjust the level until the bubble in the tube indicates it is level.
- 4
  Measure the distance from the 1-foot mark on the level straight down to the surface of the roof. This measurement in inches is the amount the roof rises over a 12-inch span.
Rafter Load
- 5
  Read the building plans to determine rafter span and on-center spacing. Determine the snow load for your area by referencing local building codes.
- 6
  Find the appropriate span table that lists the proper pounds per square foot or psf chart for the load requirements for your area, the lumber size and on-center spacing as determined by your building plans and the correct roof pitch.
- 7
  Follow the chart to the right until you reach the appropriate span in feet and inches needed for your rafters, according to your plans. The measurements listed in the chart are the maximum spans allowed for each type of lumber based on the building plan parameters and the woods strength and flexibility. Follow this column to the top of the chart to learn which type and grade of lumber is the best choice for carrying the necessary rafter load. You might have several options available to you. Choose the most appropriate lumber for your project based on availability, cost and preference.

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