Birdsmouth Rafter Calculator

If you’ve ever framed a roof, you know the birdsmouth cut matters. It’s the small notch that lets a rafter sit snug on the wall plate. A wrong cut can weaken the rafter or throw off the roof angle. That’s why we built a free Birdsmouth Rafter Calculator. It helps you find the exact heel cut, notch size, and angle in seconds.

What is a Birdsmouth Cut?

A birdsmouth cut is a notch in the rafter that has two parts. The seat cut is horizontal and rests on the wall plate. The heel cut is vertical and matches the roof pitch. Together, they lock the rafter in place. Without it, the rafter would slide or not align with the ridge beam.

Think of it like a puzzle piece. The wall plate is one part, and the rafter’s birdsmouth is the other. When they match, you get a strong and stable fit.

Why Use a Birdsmouth Rafter Calculator?

Doing the math by hand takes time. You need to work out angles and lengths with trigonometry. A wrong number means wasted wood. Our calculator uses proven formulas to give instant results. Whether you’re building a shed roof or a house, it saves time and avoids mistakes.

Birdsmouth Calculation Formula

Here’s how it works in plain terms. First, find the roof angle. If you know the pitch (rise over run):

Angle (degrees) = arctan(rise ÷ run)

Then, find the heel cut:

Heel cut = Seat cut × tan(angle)

Next, get the notch size:

Notch size = Heel cut × cos(angle)

Finally, find the remaining rafter depth:

Remaining depth = Rafter depth − Notch size

This tells you if the cut is within safe limits. Many builders follow the rule that the notch should be less than one-third of the rafter depth.

How to Use the Calculator

1. Choose pitch method: rise/run or direct angle.
2. Enter the rise, run, or angle.
3. Add seat cut length and rafter depth.
4. Pick your units: inches, cm, or mm.
5. Click calculate.

You’ll get the heel cut, notch size, remaining depth, and safety checks instantly.

Final Verdict

The birdsmouth cut is small, but it has a big role in roof framing. A good fit means your roof sits solid and lasts longer. Our Birdsmouth Rafter Calculator makes it easy for pros and DIYers. No more guesswork, no wasted lumber, and no math headaches.

FAQs

How do you calculate a birdsmouth rafter?

Use the formula: Heel cut = Seat cut × tan(angle). Then find notch size with Heel cut × cos(angle).

How deep should a 2×8 rafter birdsmouth be?

It should be less than one-third of the rafter’s depth, so under about 2⅝ inches.

What’s the rule for birdsmouth cuts?

Keep the notch shallow for strength. Ensure the seat cut has at least 1½ inches of bearing on the wall plate.

Can I use this for a shed roof?

Yes. The calculator works for sheds, houses, and any sloped roof.