Roof Rafter Length

Rafter² = run² + rise². Pythagorean from pitch + run.

Inputs

6/12 = standard. 4/12 = low. 12/12 = steep (45°).

Result

Loading calculator…
—

How to use this calculator

  • Enter pitch (X/12) + horizontal run.

About this calculator

Roof pitch is rise per 12 inches of run. 6/12 pitch = 6" rise per 12" run = ~26.6° angle. Rafter length = hypotenuse of right triangle: √(run² + rise²). Add foot for overhang + tail cut. Common pitches: 4/12 (low slope), 6/12 (standard), 12/12 (steep). Higher pitch sheds water/snow better; eats more material. Source: International Residential Code (IRC) §R802.

Frequently asked

What's a "12-pitch" roof?+
Each unit of run goes up 12 units. 12/12 = 45° = "12 pitch."
Common pitches?+
Low: 2-4/12. Standard: 6/12. Steep: 9-12/12. Very steep: above 12/12.
Why /12?+
Carpentry standard. Inches per foot of run. Universal in US construction.
Overhang?+
Add eave overhang to rafter (typically 18-24"). Account for plumb cut at ridge, bird's mouth, tail.
Walking on roof?+
Up to 6/12: relatively safe with grip shoes. Above 8/12: rope or harness; above 12/12: pro only.

Related calculators

Angle of Elevation / Depression
tan(angle) = opposite / adjacent. Find angle from height + distance.
Distance to Object (Angle + Height)
distance = height / tan(angle). Range from observer to base of object.
Ladder Safe Angle (4:1 Rule)
For every 4 ft of height, base is 1 ft from wall. Angle ≈ 75.5°.
Roofing Square Count Calculator
1 roofing square = 100 ft². Bundles = squares × 3 (for 3-tab) or × 4 (architectural).
Roofing Replacement Cost Calculator
Roof replacement = squares × $/square (asphalt / metal / tile) + tear-off & disposal.
Right Triangle Solver
Given any 2 of (a, b, c, angle), solve all remaining sides + angles.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Right Triangle Solver
Calc

Given any 2 of (a, b, c, angle), solve all remaining sides + angles.

Angle of Elevation / Depression
Calc

tan(angle) = opposite / adjacent. Find angle from height + distance.

Distance to Object (Angle + Height)
Calc

distance = height / tan(angle). Range from observer to base of object.

Ladder Safe Angle (4:1 Rule)
Calc

For every 4 ft of height, base is 1 ft from wall. Angle ≈ 75.5°.

Roofing Square Count Calculator
Calc

1 roofing square = 100 ft². Bundles = squares × 3 (for 3-tab) or × 4 (architectural).

VO2 Max Calculator (Cooper + Rockport)
Health

Estimate your VO2 max from the 12-min Cooper run OR the Rockport 1-mile walk test, with age- and gender-adjusted fitness rating.