Rafter Cutting: How Do You Measure Roof Rafters?

Framing a new roof is the ultimate test of a carpenter’s mathematical ability. If you are building a backyard shed, a detached garage, or an addition to your house, you cannot simply hold a heavy 2x6 piece of lumber up in the air, trace a line with a pencil, and hope it fits. To ensure the roof is structurally sound and perfectly level, every single rafter must be calculated, measured, and cut with absolute trigonometric precision on the ground before it is ever lifted into the air.

Before you start ordering massive stacks of expensive structural lumber, you must calculate the exact surface area of the roof you are about to build. Use our Free Roofing Calculator right now to compute your exact square footage and generate a mathematically flawless estimate of your sheathing and shingle requirements.

Table of Contents

1. What is a Roof Rafter?
2. The Three Measurements: Span, Rise, and Run
3. How to Measure Rafters (The Pythagorean Theorem)
4. Rafter Cutting: The Birdsmouth Joint
5. Conclusion: Measure Twice, Cut Once
6. Frequently Asked Questions

---

What is a Roof Rafter?

A roof rafter is the primary structural spine of a roof.

When you look at the skeleton of a pitched roof, the rafters are the heavy diagonal wooden beams (usually 2x6s, 2x8s, or 2x10s) that run from the absolute peak of the roof (the ridge board) all the way down to the exterior walls of the house. The rafters carry the massive dead weight of the plywood decking, the heavy asphalt shingles, and the immense winter snow loads, transferring that weight safely down into the foundation of the house.

The Three Measurements: Span, Rise, and Run

To calculate exactly how long to cut your diagonal rafter, you do not use a tape measure in the air; you use a calculator on the ground. You must know three numbers:

1. The Span: The total horizontal width of the building from the outside of one wall to the outside of the opposite wall. (Example: The shed is 10 feet wide).
2. The Run: Exactly half of the span. This is the horizontal distance the rafter must cover. (Example: 5 feet).
3. The Rise: The total vertical height the roof will go up. If your roof has a 4/12 pitch (meaning it rises 4 inches vertically for every 12 inches horizontally), and the run is 5 feet, the total vertical rise is 20 inches.

How to Measure Rafters (The Pythagorean Theorem)

Because a pitched roof forms a perfect right triangle, you calculate the exact diagonal length of the rafter using simple high school geometry: A² + B² = C².

• A is the horizontal Run.
• B is the vertical Rise.
• C is the exact diagonal length of the rafter (the hypotenuse).

By squaring the Run, squaring the Rise, adding them together, and finding the square root, you now have the exact mathematical length of the rafter from the peak of the roof to the edge of the wall. (Note: You must also add an extra 12 to 24 inches to this measurement if you want the roof to overhang the walls to create an eave).

Rafter Cutting: The Birdsmouth Joint

You cannot simply rest the flat diagonal edge of a rafter on top of a flat horizontal wall; it will slide right off the building. You must make a complex geometric cut known as a Birdsmouth.

A birdsmouth is a small, triangular notch cut into the bottom edge of the rafter exactly where the rafter intersects the exterior wall.

• The cut features a horizontal “seat cut” that rests perfectly flat on top of the wall plate.
• It features a vertical “plumb cut” that sits perfectly flush against the outside edge of the wall.
• When the massive weight of the roof pushes down, the birdsmouth locks the rafter permanently onto the wall, preventing the roof from spreading outward and collapsing.

Professional carpenters use a heavy steel “Framing Square” to trace the exact angles of the pitch onto the wood before cutting the birdsmouth with a circular saw.

Conclusion: Measure Twice, Cut Once

Rafter cutting is an intimidating, high-stakes math problem. If you miscalculate the pitch or cut the birdsmouth notch one inch too deep, the structural integrity of the heavy lumber is destroyed, and the wood must be thrown in the dumpster. By taking your time, understanding the geometry of the run and rise, and perfectly tracing your angles with a framing square, you guarantee that every rafter will drop perfectly into place on the first try.

Once the complex wooden skeleton is nailed together, you must cover it with shingles. Head over to our Free Roofing Calculator right now. By inputting your new roof’s dimensions, the algorithm will instantly output your exact total square footage, allowing you to quickly order the perfect amount of underlayment and shingles to waterproof your new framing.

Related Guides

• Pneumatic Frame Nailer: What is the Best Framing Gun?
• Patio Roof Pitch: Minimum Slopes Explained

Frequently Asked Questions

What is the difference between a rafter and a truss?

A rafter is a single, heavy, diagonal piece of lumber cut by hand on the job site. A roof built with rafters leaves a massive, open cavern in the attic. A “truss” is a complex, pre-engineered web of smaller 2x4s built in a massive factory using heavy steel gusset plates. Trusses are incredibly strong, cheap, and fast to install, but the complex webbing completely destroys any usable attic storage space.

How deep should a birdsmouth cut be?

Building codes strictly dictate this to prevent the rafter from snapping. The horizontal “seat cut” (the part resting on the wall) should never exceed the width of the wall plate (usually 3.5 inches for a 2x4 wall). Most importantly, the notch must never cut through more than one-third (1/3) of the total depth of the rafter. If you notch it deeper, you severely weaken the structural capacity of the beam.

Do I need to cut a birdsmouth on both ends of the rafter?

No. The birdsmouth is only cut at the bottom where the rafter rests on the exterior wall. At the absolute top peak of the roof, the rafter is given a simple vertical “plumb cut” so it can sit perfectly flush against the central ridge board.