Rafter Length Calculator

Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement. The calculation includes results for hip/valley factor, slope factor and the roof slope in degrees.

If you add an eaves overhang dimension, then the calculator will add the amount the rafter sticks passed the wall to the rafter length. If you enter a value for the ridge board, then the calculator will subtract half of this amount from the rafter length.

Example of how to enter the values:

The building width is 44 feet 6 and ¾ inches
the overhang for the rafter is 22 and ½ inches, and you are using a 1 ½ inch ridge board.
The roof pitch is 8:12 (it rises 8 inches in a one-foot run).

If you know what 44' 6 ¾" is in decimal, you can add this in the building width ft field
44' 6 ¾" in decimal format = 44.5625
otherwise, enter the 44 feet in the ft field
6 inches in the in field
¾ inch: the 3 here is the n value, the numerator, and the 4 is the d value, the denominator

For the overhang 22 ½" can be added to the in field as 22.5
or 22 in the in field, and the fraction ½, the 1 in the n field and 2 in the d field

Enter the ridge board the same way as the overhang using the appropriate fields.

And the 8 for the roof pitch has only one field.

Calculation

1. building width in feet and inches
2. overhang in inches
3. ridge board thickness in inches
4. slope ratio amount of rise per foot of run

Results

1. roof slope in degrees
2. slope factor
3. hip/valley factor
4. length of rafter in decimal feet
5. length of rafter in feet and inches