Rafter Length Calculator
Find the length of a common rafter, its overhang tail, the roof rise and the plumb cut angle for a gable roof.
How to use the Rafter Length Calculator
- Enter the building span (outside of walls) and pick its unit.
- Enter the roof pitch (rise per 12).
- Enter the overhang (horizontal) and pick its unit.
- Enter the ridge board thickness and pick its unit – 1½ in for a 2× ridge board, 0 if there is no ridge board.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Rafter Length Calculator
A common rafter runs from the top of the wall plate to the ridge at a right angle to both. Its length depends only on the horizontal run and the pitch. Subtracting half the ridge board thickness from the run keeps the two opposing rafters from overlapping where they meet the ridge.
Mark the plumb cut at the top, the birdsmouth seat where the rafter sits on the wall (cut no deeper than one-third of the rafter depth), and the tail cut at the overhang. Cut one pattern rafter, test it on both sides, then use it to mark the rest. Rafter size and spacing must come from your local span tables.
Formula
Run = Span ÷ 2 − Ridge thickness ÷ 2
Pitch multiplier = √(1 + (pitch ÷ 12)²)
Rafter length = Run × multiplier + Overhang × multiplier
Plumb cut angle = arctan(pitch ÷ 12)
Lengths are measured along the top edge (the measuring line) of the rafter, from the ridge plumb cut to the tail. Stock lumber is sold in 2-ft increments, so the lumber length is rounded up to the next even foot.
Example calculation
Using the default values:
- Building span (outside of walls) = 24 ft
- Roof pitch (rise per 12) = 6 in 12
- Overhang (horizontal) = 12 in
- Ridge board thickness = 1.5 in
the calculator returns:
- Total rafter length: 14′ 5 9/16″
- Total rafter length (decimal): 14.465 ft
- Ridge to wall plate: 13′ 4 3/16″
- Overhang tail length: 1′ 1 7/16″
- Roof rise: 5′ 11 5/8″
- Plumb cut angle: 26.57 °
- Lumber length to buy: 16 ft
Frequently asked questions
How do I calculate rafter length?
Divide the span by two to get the run, multiply by the pitch multiplier √(1 + (pitch/12)²), then add the overhang multiplied by the same factor.
What is the pitch multiplier for a 6/12 roof?
About 1.118. Every foot of horizontal run needs 1.118 ft of rafter.
How long is a rafter for a 24 ft wide building at 6/12?
With a 1½ in ridge board and 12 in overhang, each rafter is about 14 ft 5 9/16 in long, so buy 16 ft lumber.
Last updated: · Category: Construction calculators