Free Rafter Calculator — Common, Hip, Valley & Jack Rafters

How to Use the Rafter Calculator

How to Calculate Rafter Length

To calculate rafter length, multiply the horizontal run by the slope multiplier for your roof pitch. The slope multiplier equals √(1 + (pitch/12)²). For example, a 6/12 pitch has a slope multiplier of √(1 + (6/12)²) = √1.25 = 1.118. If your common rafter run is 12', the rafter length is 12' × 1.118 = 13' 5-3/16". This calculator handles common, hip, valley, and jack rafter lengths instantly — including birdsmouth cuts, overhang, and ridge deductions.

Understanding Roof Pitch

Roof pitch describes the slope of a roof as a ratio of vertical rise to horizontal run, expressed over a 12" base. A 6/12 pitch means the roof rises 6" for every 12" of horizontal run. Most residential roofs fall between 4/12 and 8/12 pitch. Lower pitches (3/12–4/12) are common on ranch-style homes and require modified roofing materials rated for low slope. Steeper pitches (8/12 and above) shed snow and rain quickly, allow usable attic space, and give the roof a more dramatic profile — but they also require more lumber and sheathing.

To measure an existing roof pitch, place a level horizontally against the underside of a rafter (or along the roof surface) and mark the 12" point. Hold a tape measure vertically from that 12" mark down to the level — the vertical distance in inches is your pitch. Alternatively, set a speed square on the rafter edge and read the pitch directly from the "Common" scale on the square body. Always take this measurement from inside the attic or at a gable end where you have a clear view of the rafter stock.

Common vs Hip vs Valley vs Jack Rafters

A common rafter runs perpendicular from the ridge board to the exterior wall top plate. It is the simplest rafter to calculate — straight run, single pitch, one birdsmouth. Every gable roof is framed entirely with common rafters, and they form the reference for every other rafter type in a hip or intersecting roof.

A hip rafter runs diagonally from the ridge board to an outside corner where two walls meet at 90°. Because it travels at a 45° angle in plan view, its horizontal run is longer than a common rafter's run by a factor of √2 (1.414). The hip rafter also has a shallower effective pitch angle and requires a compound (two-plane) cut at the ridge known as a cheek cut.

A valley rafter runs diagonally from the ridge to an inside corner where two roof planes intersect — typically where an addition or wing meets the main roof. Geometrically, valley rafters mirror hip rafters: same length formula, same √2 adjustment to the run. The difference is in the orientation of the cheek cut and the direction the rafter slopes.

Jack rafters are shortened common rafters that intersect a hip or valley rafter instead of running the full distance to the ridge. Hip jacks run from the top plate up to the hip rafter; valley jacks run from the ridge down to the valley rafter. All jacks on one side of a hip or valley are cut at the same check cut angle, and their lengths follow a constant increment based on rafter spacing and the common rafter slope multiplier.

The Birdsmouth Cut

A birdsmouth is the notch cut into the bottom of a rafter where it bears on the wall's top plate. It consists of two cuts: a plumb cut (vertical line following the rafter slope) and a seat cut (horizontal, flat against the plate). The resulting L-shaped notch locks the rafter in place and transfers the roof load directly into the wall framing.

The depth of the birdsmouth is measured as HAP (Height Above Plate) — the vertical distance from the top of the plate to the top edge of the rafter at the birdsmouth. IRC R802.6 requires that enough rafter material remain above the seat cut to resist the forces at the bearing point. Standard practice is to maintain at least 3-1/2" of HAP for 2×6 rafters, and many framers use a full 2/3 of the rafter depth. A consistent HAP across all rafters is essential — any variation will produce a visible wave in the roof plane.

Plumb Cut vs Level Cut

A plumb cut is a vertical cut made at the top of the rafter where it meets the ridge board (or at the tail for a fascia cut). On a framing square, the plumb cut line runs along the rise side. A level cut (also called a seat cut) is horizontal — it forms the bottom of the birdsmouth and sits flat on the top plate.

On a miter saw, angles are measured from the fence, which represents the horizontal plane. This means the miter saw setting for a plumb cut is (90° − pitch angle). For a 6/12 pitch with a geometric angle of 26.57°, set the miter saw to 90° − 26.57° = 63.43°. For the seat cut (level cut), set the saw to the pitch angle itself: 26.57°. A protractor or angle finder, on the other hand, reads the actual geometric angle directly. Keep this distinction clear to avoid mis-cuts — a plumb cut and level cut on the same rafter are complementary angles that always sum to 90°.

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

Consider a gable roof with a 24-foot span (outside wall to outside wall), a 6/12 pitch, 16" on-center rafter spacing, a 2×8 ridge board, and an 18" overhang at the eaves.

Step 1: Common Rafter

• Run: 24' ÷ 2 = 12' (144")
• Slope multiplier for 6/12: √(1 + 0.5²) = √1.25 = 1.1180
• Theoretical rafter length: 144" × 1.1180 = 160.99"
• Ridge deduction: 1/2 of ridge board thickness = 3/4"
• Adjusted length: 160.99 − 0.75 = 160.24" (13'4-1/4")
• Overhang: 18" × 1.1180 = 20.12"
• Total rafter stock: 160.24 + 20.12 = 180.36" (15'0-3/8") — order 16' lumber

Step 2: Hip Rafter

• Hip run: 12' × √2 = 12' × 1.4142 = 16.97' (203.6")
• Hip slope multiplier: √(1 + (6/16.97)²) = approximately 1.0607
• Hip rafter length: 203.6" × 1.0607 = 215.95" (18'0") plus overhang
• Order 20' lumber for hip rafters

Step 3: Jack Rafters at 16" OC

• Jack decrement per 16" spacing: 16" × 1.1180 = 17.89"
• Longest hip jack: common rafter length minus one decrement = 160.24 − 17.89 = 142.35"
• Each subsequent jack is 17.89" shorter: 142.35, 124.46, 106.57, 88.68, 70.79, 52.90, 35.01, 17.12
• All jack rafters get the same cheek cut angle at the hip

This is a straightforward calculation, but it involves enough steps that errors are easy to make. Enter your dimensions into our calculator above and it computes all of these values instantly, including HAP, check cut angles, and the complete jack cut list.

Common Rafter Calculation Mistakes

• Forgetting ridge deduction: Every common rafter must be shortened by half the ridge board thickness. On a 2× ridge (1-1/2" actual), that's 3/4" off each rafter. Skipping this pushes every rafter tail past the wall.
• Measuring from inside of walls: The run is measured from the outside face of the wall (sheathing line) to the centerline of the ridge. Measuring from the inside of the wall frame adds the wall thickness to your run, making every rafter too long.
• Inconsistent HAP: If the birdsmouth depth varies from rafter to rafter, the roof surface will have visible waves. Use a consistent HAP dimension — mark a story pole and check every rafter against it.
• Wrong cheek cut angle on jacks: Hip jack and valley jack cheek cuts are different from common rafter plumb cuts. The cheek cut angle accounts for the 45° approach to the hip or valley rafter. Using the common plumb cut angle produces a gap.
• Not crowning lumber: Always sight down each rafter and install it with the crown (natural bow) facing up. Gravity and roof load will flatten the crown over time. If you install a crowned board crown-down, it creates a permanent low spot.

Pro Tips

• Always verify the roof pitch with a level and speed square before cutting any lumber. Field conditions often differ from plan dimensions — existing structures may have settled, and new framing may not be perfectly plumb or level.
• Account for the ridge board thickness when calculating rafter run. A standard 2× ridge is 1-1/2" thick, so each common rafter's run is reduced by half the ridge thickness (3/4"). Forgetting this deduction is one of the most common framing mistakes and will cause the rafter tails to overshoot the wall.
• Hip and valley rafters are significantly longer than common rafters — roughly 1.414 times the common rafter length. Order lumber that is long enough for the full hip/valley run, including overhang. Running short by even a few inches means scrapping an expensive board.
• Mark and cut all jack rafters on one side at the same time. They share the same cheek cut angle and differ only in length by a constant increment. Gang-cutting saves time and ensures a consistent fit against the hip or valley.
• Double-check overhang measurements from the outside face of the wall sheathing, not from the inner edge of the top plate. The overhang dimension on plans almost always references the point where the fascia or soffit attaches, which is the sheathing line.