How do I calculate common rafter length?

Use half the building width as run, multiply the run by pitch rise over 12 to get rise, then use the Pythagorean theorem and add the overhang.

Can this replace rafter span tables?

No. It is a planning calculator. Actual rafter size must be checked against span tables, local code, lumber species, grade, loads and deflection limits.

What rafter spacing should I use?

Common residential layouts use 16 inches or 24 inches on center, but roof sheathing, load, span and local requirements can require a different spacing.

Rafter Calculator

Estimate common rafter length, roof pitch factor, rafter count, nominal lumber size, board feet and rough material cost from U.S. framing inputs.

Roof framing inputs

Building width, ft

Roof length, ft

Overhang, ft

Roof load, psf

Roof pitch

Rafter spacing

Lumber price, $/board ft

Common rafter section

Calculate common rafter length from run and pitch

English rafter-calculator intent is usually practical roof framing: convert span and pitch into common rafter length, rise, pitch factor, rafter count, nominal lumber size and a rough lumber quantity.

R u n = \frac{B u i l d in g W i d t h}{2}

A simple gable roof uses half the building width as the horizontal rafter run.

R i se = R u n \times \frac{P i t c h R i se}{12}

Pitch rise is the inches of rise for each 12 inches of run.

R a f t er L e n g t h = R u n^{2} + R i s e^{2} + O v er han g

The common rafter length adds the tail or overhang to the sloped length.

R a f t er C o u n t = 2 (⌊ \frac{R oo f L e n g t h \times 12}{S p a c in g _{in}} ⌋ + 1)

Rafter count is rounded by layout spacing along the ridge and doubled for the two roof planes.

Rafter sizing limits

The nominal size suggestion is a planning guide, not a stamped structural design. Final framing depends on species, grade, snow and wind loads, dead load, ceiling attachment, bearing, connections, deflection and local code.

Use code span tables or an engineer for the actual allowable span.
Long runs may need larger lumber, purlins, intermediate supports, LVL or trusses.
Roofing material, sheathing, ceiling load, solar panels and local snow load can change the size.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: May 29, 2026

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