Roof Pitch and Rafter Length Explained

Common rafter length depends on three things: the horizontal run, the roof pitch, and the overhang past the wall. For a 24-foot-wide building with a 6/12 pitch and a 12-inch overhang, each common rafter measures about 14.47 feet from ridge to tail. The geometry behind that number is simple once you see the steps, and verifying it yourself takes less time than fixing a bad cut.

This guide covers common rafters only, the straight members that run from the ridge board down to the wall plate. Hip rafters, valley rafters, jack rafters, and trusses follow different geometry and are not covered here.

How Pitch Determines Rafter Length

Roof pitch is expressed as rise over run in inches. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. Steeper pitches produce longer rafters because the diagonal between the wall plate and the ridge gets longer.

A right triangle forms between the run (horizontal), the rise (vertical), and the rafter (hypotenuse). The Pythagorean theorem gives you the rafter length:

Rafter length = square root of (Run² + Rise²)

At a 4/12 pitch on a 24-foot span, main rafter length is about 12.58 feet. Bump that to a 10/12 pitch on the same span and the rafter stretches to 15.54 feet. Pitch is the single biggest variable in rafter length calculations.

From Building Width to Run

Most plans show building width (also called span), measured from outside wall to outside wall. Run is half of that width because common rafters meet at the ridge in the center of the building.

For a 24-foot-wide building:

Run = 24 / 2 = 12 feet

If your plans already show run instead of full span, skip the division and use the run value directly. The rafter calculator accepts span and handles this conversion automatically.

Ridge Thickness Adjustment

Where opposing rafters meet at the ridge board, each rafter stops short by half the ridge board thickness. A standard dimensional ridge board is 1.5 inches thick, so each rafter’s run shortens by 0.75 inches (0.0625 feet).

Adjusted run = (Span / 2) - (Ridge thickness / 24)

For the 24-foot span with a 1.5-inch ridge board:

Adjusted run = 12 - (1.5 / 24) = 11.9375 feet

This adjustment matters more than it looks. Skip it and opposing rafters will be about 3/4 inch too long on each side. That’s 1.5 inches of overlap at the ridge, which forces the ridge board out of plane or leaves a visible gap at the wall plate. Experienced framers who’ve dealt with a bowed ridge on a 40-foot building don’t skip this step twice.

Calculating Rise from Pitch

Once you have the adjusted run, multiply by the pitch ratio to get rise:

Rise = Run × (Pitch rise / 12)

For an 11.94-foot run at 6/12 pitch:

Rise = 11.94 × (6 / 12) = 5.97 feet

The rise tells you how high the ridge sits above the wall plate (before adding wall height). It also feeds directly into the rafter length formula.

Adding Overhang

The overhang (also called the tail or eave projection) extends past the wall plate to protect siding and windows from rain. Most residential overhangs run 12 to 24 inches.

Overhang adds length to the rafter as a second right triangle using the same pitch. The overhang run is the horizontal projection, and the overhang rise follows the roof slope:

Overhang run = Overhang (inches) / 12

Overhang rise = Overhang run × (Pitch rise / 12)

Overhang rafter length = square root of (Overhang run² + Overhang rise²)

For a 12-inch overhang at 6/12 pitch:

Overhang run = 1.0 foot

Overhang rise = 1.0 × 0.5 = 0.5 feet

Overhang rafter length = square root of (1.0² + 0.5²) = 1.12 feet

Total rafter length = main rafter + overhang rafter length.

Worked Example: 24-Foot Span at 6/12 Pitch

Here’s the full calculation for a common residential scenario.

Given: 24-foot building span, 6/12 pitch, 12-inch overhang, 1.5-inch ridge board.

Step	Calculation	Result
Adjusted run	(24 / 2) - (1.5 / 24)	11.94 ft
Rise	11.94 × (6 / 12)	5.97 ft
Main rafter	square root of (11.94² + 5.97²)	13.35 ft
Overhang rafter	square root of (1.0² + 0.5²)	1.12 ft
Total rafter	13.35 + 1.12	14.47 ft

Each common rafter needs to be at least 14.47 feet long. Lumber comes in 2-foot increments, so you’d buy 16-foot stock and trim to length. That extra length gives you room for clean end cuts and covers any slight measurement variance on site.

At 12 rafter pairs (24 individual rafters), that’s 347 lineal feet of lumber. At $3 to $9 per lineal foot for dimensional lumber in 2026, budget roughly $1,040 to $3,125 for the rafter stock alone.

How Pitch Changes the Numbers

The same 24-foot span produces very different rafter lengths depending on pitch:

Pitch	Roof Angle	Main Rafter	Total with 12” Overhang
4/12	18.4 degrees	12.58 ft	13.64 ft
6/12	26.6 degrees	13.35 ft	14.47 ft
8/12	33.7 degrees	14.35 ft	15.55 ft
10/12	39.8 degrees	15.54 ft	16.84 ft

Going from a 4/12 to a 10/12 pitch adds over 3 feet per rafter. On a 12-pair roof, that’s 77 extra lineal feet of lumber. Pitch selection affects material cost, board length requirements, and whether you can use 14-foot stock or need to step up to 16s or 18s.

When to Use the Rafter Calculator

Running these steps by hand once is straightforward, but comparing multiple pitch and overhang combinations gets tedious fast. The rafter calculator handles all of this instantly. Enter your span, pitch, overhang, and ridge thickness, and it returns main rafter length, total rafter length, roof angle, and a lumber cost estimate.

It’s especially useful early in design when you’re weighing pitch options. Change the pitch input and see exactly how each option affects rafter length and lumber totals, without re-running the Pythagorean theorem each time.

Common Mistakes to Watch For

Forgetting ridge thickness. Even 1.5 inches changes the run by 0.75 inches per side. On a steep pitch, that translates to a noticeable length error where the rafters meet.

Using span instead of run. The full building width is not the rafter run. Divide by two first, then subtract the ridge adjustment.

Ignoring overhang in the cut list. Overhang adds 1 to 2 feet per rafter depending on pitch and projection. If you size lumber based on main rafter length alone, your tails will come up short at the fascia line.

Ordering exact-length boards. A 14.47-foot rafter needs 16-foot stock, not 14-foot. Always round up to the next available lumber length and account for end trim. Paying an extra $15 for longer boards beats a second delivery fee because two rafters came up short.