# Common Rafter Calculator

Common rafter calculator for framing: line length, total rafter length with overhang, plumb and seat cut angles, and height above plate for any pitch and span.

## What this calculates

A common rafter runs at right angles from the exterior wall plate up to the ridge, forming the backbone of any gable roof. This common rafter calculator computes the run, rise, line length (ridge to birdsmouth), total length with overhang, plumb cut angle, seat cut angle, and height above plate (HAP) for any building span and roof pitch. It matches the framing-square and speed-square methods used by carpenters, so the numbers transfer directly to the lumber and cut lines.

## Inputs

- **Building Span** (ft) — min 0 — Outside wall to outside wall (narrow dimension)
- **Roof Pitch** (:12) — min 0.5, max 24 — Rise per 12 inches of run (e.g. 8:12 = 8 inches rise per foot of run)
- **Ridge Board Thickness** (in) — min 0, max 4 — 2x ridge = 1.5 in, LVL ridge beam often 1.75-3.5 in
- **Eave Overhang** (in) — min 0, max 48 — Horizontal overhang past the wall (common: 12-24 in)
- **Rafter Depth** (in) — min 3.5, max 14 — Actual depth: 2x6 = 5.5, 2x8 = 7.25, 2x10 = 9.25, 2x12 = 11.25

## Outputs

- **Rafter Run** (ft) — Horizontal distance from wall to ridge
- **Rafter Rise** (ft) — Vertical height gained
- **Line Length** (ft) — Rafter length from ridge to birdsmouth (no overhang)
- **Total Rafter Length** (ft) — Ridge to tail including overhang
- **Total Length** — formatted as text — Feet and inches format
- **Plumb Cut Angle** (°) — Ridge and tail cut angle from vertical = roof angle from horizontal
- **Seat Cut Angle** (°) — Horizontal cut at birdsmouth (90 - plumb)
- **Height Above Plate** (in) — HAP: distance from top plate to top edge of rafter above the seat cut

## Details

## What Is a Common Rafter

A common rafter is the "standard" sloped roof framing member that connects the wall's top plate to the ridge board at a 90 degree angle to both walls. Every gable roof has dozens of common rafters, evenly spaced along the building length (usually 16 or 24 inches on center). Hip, valley, and jack rafters are named differently because they run at other angles.

## How the Common Rafter Calculator Works

The rafter forms the hypotenuse of a right triangle:

- **Run** (horizontal leg) = span / 2 minus half the ridge thickness
- **Rise** (vertical leg) = run x (pitch / 12)
- **Line length** (hypotenuse) = sqrt(run² + rise²)
- **Total rafter length** = line length + overhang rafter

The overhang rafter extends the rafter past the wall plate for the eave. Its length uses the same pitch applied to the overhang distance.

## Worked Example

24 ft span, 8:12 pitch, 1.5 in (2x) ridge board, 12 in overhang:

- Run = 24/2 - 1.5/24 = **11.9375 ft**
- Rise = 11.9375 x 8/12 = **7.9583 ft**
- Line length = sqrt(11.9375² + 7.9583²) = **14.3419 ft**
- Overhang rafter = sqrt(1.0² + (1.0 x 8/12)²) = **1.2019 ft**
- Total rafter length = **15.5438 ft**, order 16 ft lumber

## Plumb Cut and Seat Cut Angles

The plumb cut (vertical cut at the ridge end and tail end) equals the roof angle from horizontal:

**Plumb cut = atan(pitch/12)**

For common pitches:

| Pitch | Plumb Cut | Seat Cut |
|---|---|---|
| 3:12 | 14.04° | 75.96° |
| 4:12 | 18.43° | 71.57° |
| 6:12 | 26.57° | 63.43° |
| 8:12 | 33.69° | 56.31° |
| 10:12 | 39.81° | 50.19° |
| 12:12 | 45.00° | 45.00° |

To mark the plumb cut with a framing square, set the tongue at 12 inches and the blade at the pitch rise (e.g., 8 inches for 8:12). The edge of the tongue gives the plumb line.

## Height Above Plate (HAP)

HAP is the vertical distance from the wall plate to the top edge of the rafter above the birdsmouth seat cut. It determines how much of the rafter stands "above" the wall.

**HAP = rafter_depth / cos(plumb_angle) - seat_cut_depth**

For a 2x8 rafter (7.25 in depth) on an 8:12 pitch with a standard 1.5 in seat cut:

HAP = 7.25 / cos(33.69°) - 1.5 = 7.25 / 0.8321 - 1.5 = 8.71 - 1.5 = **7.21 inches**

HAP affects soffit and fascia sizing. A larger HAP gives room for a continuous soffit vent; a smaller HAP makes the eave tighter.

## Marking the Birdsmouth

1. Mark the plumb line at the ridge end of the rafter using a framing square set to the pitch.
2. Measure along the rafter edge by the line length distance from that plumb line.
3. Mark a second plumb line at that point; this is the outside of the wall plate.
4. From that plumb line, measure 1.5 inches horizontally into the rafter; this is the seat cut.
5. Mark the seat cut perpendicular to the first plumb line at that 1.5 in depth.

Never cut more than one-third of the rafter depth for the seat cut, or you compromise structural strength.

## Common Rafter vs Hip and Jack Rafters

- **Common rafters** run from wall plate to ridge at 90 degrees to both.
- **Hip rafters** run from a corner at 45 degrees to the ridge, used in hip roofs.
- **Jack rafters** are shortened commons that frame into hip or valley rafters.
- **Valley rafters** run in the opposite direction of hips, at the intersection of two roof sections.

This calculator computes common rafters only. For hip or valley rafters, multiply by 17/12 for hip length per foot of run, and apply different cut angles (33.56° for a 45-degree hip on an 8:12 pitch).

## Frequently Asked Questions

**Q: What is a common rafter?**

A: A common rafter runs at 90 degrees from the exterior wall plate to the ridge board. It's the primary sloped roof framing member in a gable roof. Common rafters connect to the ridge at the top (with a plumb cut) and sit on the wall plate at the bottom (with a birdsmouth notch containing a plumb and seat cut).

**Q: How do I calculate common rafter length?**

A: Common rafter length = sqrt(run² + rise²) + overhang rafter. Run = span/2 minus half the ridge thickness. Rise = run x pitch/12. For a 24 ft span, 8:12 pitch, 1.5 in ridge, 12 in overhang: run = 11.94 ft, rise = 7.96 ft, line length = 14.34 ft, plus 1.2 ft overhang = 15.54 ft total. Order 16 ft lumber.

**Q: What is the plumb cut angle for a common rafter?**

A: Plumb cut angle equals the roof angle from horizontal, computed as atan(pitch/12). For common pitches: 3:12 = 14.04 degrees, 4:12 = 18.43 degrees, 6:12 = 26.57 degrees, 8:12 = 33.69 degrees, 10:12 = 39.81 degrees, 12:12 = 45 degrees. The seat cut is the complement (90 minus plumb cut).

**Q: How is common rafter line length different from total length?**

A: Line length is the distance from the ridge plumb line to the birdsmouth plumb line (outside of wall plate). Total length adds the overhang rafter portion, which extends from the birdsmouth to the tail end at the eave. Line length determines where you cut the birdsmouth; total length determines how much lumber to order.

**Q: What is HAP (height above plate) for a common rafter?**

A: HAP = rafter_depth / cos(plumb_angle) - seat_cut_depth. For a 2x8 common rafter (7.25 in actual) on an 8:12 pitch with 1.5 in seat cut: HAP = 7.25 / cos(33.69) - 1.5 = 7.21 inches. HAP determines how much of the rafter stands above the wall plate and affects soffit, fascia, and ceiling heights.

**Q: How do I mark a common rafter with a framing square?**

A: Set the framing square with the tongue at 12 inches on the top edge of the rafter and the blade at the pitch rise (e.g., 8 inches for 8:12 pitch). Mark along the tongue edge to get the plumb cut line. Slide the square along the rafter, stepping off the run in 12-inch increments, to mark subsequent positions. A calculator like this one is faster and more accurate.

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Source: https://vastcalc.com/calculators/construction/common-rafter
Category: Construction
Last updated: 2026-04-08