Roof Rafter Calculator

Building a roof can feel tricky. One wrong cut and the whole project suffers. That’s where our Roof Rafter Calculator comes in. It helps you find the exact rafter length in seconds. Whether you’re building a shed, a garage, or a house roof, you’ll get the right numbers every time.

Why a Roof Rafter Calculator Matters

If you’ve ever tried to figure out roof rafters by hand, you know it’s easy to mess up. Roof pitch, span, and overhang all affect the length. Guessing can cost time, wood, and money. Our calculator removes the guesswork. You just enter your measurements, and it gives you an accurate cut length — even with a birdsmouth allowance.

The Roof Rafter Formula

The most accurate way to calculate a rafter length uses the Pythagorean theorem.

Formula:

Rafter Length = √( (Half Span)² + (Rise)² )

Where:

• Half Span = Horizontal distance from the wall plate to the roof ridge.
• Rise = Vertical distance from the top of the wall plate to the roof peak.

Example:

If the total span is 20 ft and the rise is 4 ft:

Half Span = 10 ft

Rafter Length = √(10² + 4²) = √(100 + 16) = √116 = 10.77 ft (≈ 10' 9")

If you need an overhang, just add that length to the result.

How to Use Our Roof Rafter Calculator

1. Measure the total building span.
2. Find your roof pitch or rise height.
3. Enter your numbers into the calculator.
4. If you want an overhang, add it in.
5. The calculator shows the exact cut length.

Choosing the Right Rafter Size

The span length decides the rafter size.

• For spans up to 12 feet, a 2×6 works in most cases.
• For longer spans, you may need 2×8 or bigger.
  Always check local building codes before buying lumber.

How Many Rafters Do You Need?

The number of rafters depends on spacing. Most builders use 16-inch centers.

Formula:

Number of Rafters = (Roof Length ÷ Spacing) + 1

Example: If your roof is 20 ft long with rafters spaced 16 inches apart:

20 ft = 240 inches

240 ÷ 16 = 15 rafters per side

Final Verdict

A roof rafter calculator is more than a tool — it’s peace of mind. No more wasting wood or cutting twice. You’ll work faster, save money, and get a roof that fits perfectly.

FAQs

Can I use this for a shed roof?

Yes. Just enter your shed’s span and pitch.

Does it work with a birdsmouth cut?

Yes. Just include the birdsmouth deduction in your measurements.

Is this better than a rafter length table?

Yes. It’s faster, more accurate, and works for any pitch.