Rafter Cut Calculator

How to Use the Rafter Cut Calculator

Our free Rafter Cut Calculator helps you estimate important measurements for laying out a rafter cut, including birdsmouth notch depth, plumb cut angle, seat cut length, heel cut depth, remaining rafter depth, and overhang rafter length.

1. Enter the roof pitch as rise per 12 inches of run.
2. Enter the actual rafter depth in inches.
3. Enter the wall plate width in inches.
4. Enter the desired overhang or eave in inches.
5. Click Calculate Rafter Cut to see the results.

The calculator provides the main measurements you can use as a starting point when laying out the cuts on a rafter.

What Is a Rafter Cut?

A rafter cut refers to the cuts made on a roof rafter so the rafter can properly meet the roof line, wall plate, and overhang. Accurate cuts are important because they affect how the rafter sits on the wall and how the roof structure follows the intended pitch.

Depending on the roof design, a rafter may require a plumb cut, seat cut, and overhang cut. The exact layout depends on the roof pitch, rafter dimensions, wall plate, and other construction requirements.

What Is a Birdsmouth Cut?

A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the wall plate. The notch normally consists of a vertical or plumb portion and a horizontal seat portion.

The seat allows the rafter to rest on the wall plate, while the plumb portion follows the roof angle. The size and depth of the notch depend on the rafter dimensions, roof pitch, and wall plate width.

This calculator provides an estimated birdsmouth notch depth based on the measurements entered above.

Rafter Cut Formula

Rafter cut measurements are based on the relationship between roof pitch, rafter depth, and the wall plate. For a roof pitch expressed as X:12, the rise is X inches for every 12 inches of horizontal run.

These formulas provide geometric estimates for the entered dimensions. Always verify the final rafter layout against your construction plans and applicable requirements.

What This Calculator Calculates

The Rafter Cut Calculator provides several useful measurements from your roof and rafter dimensions:

• Birdsmouth Notch Depth – estimated depth of the rafter notch.
• Roof Angle – roof angle calculated from the pitch.
• Plumb Cut Angle – the calculated angle used for the plumb cut.
• Seat Cut Length – based on the wall plate width entered.
• Heel / Plumb Cut – estimated vertical cut depth.
• Remaining Rafter Depth / HAP – estimated material remaining above the notch.
• Overhang Rafter Length – estimated rafter length through the overhang.

Understanding the Plumb Cut

The plumb cut is the angled cut at the top of a rafter where it meets the ridge or another roof framing member. Its angle is directly related to the roof pitch.

A steeper roof pitch produces a different plumb cut angle than a lower roof pitch. The calculator uses the entered pitch to determine the roof angle and corresponding plumb cut angle.

When laying out a rafter, make sure the direction and terminology used for the cut match your construction plans and framing method.

Understanding the Seat Cut

The seat cut is the horizontal portion of the birdsmouth notch that rests on the wall plate. It provides a bearing surface between the rafter and the wall plate.

In this calculator, the seat cut length is based on the wall plate width you enter. For example, if the wall plate is 3.5 inches wide, the calculator uses 3.5 inches as the seat cut length.

Actual framing requirements may vary depending on the construction method, lumber dimensions, and applicable building requirements.

Understanding Birdsmouth Notch Depth

Birdsmouth notch depth describes how deeply the notch extends into the rafter. The depth is important because removing too much material can reduce the remaining strength of the rafter.

The calculator estimates the notch depth from the roof pitch, seat cut geometry, and roof angle. It also shows the remaining rafter depth after the calculated notch.

What Is HAP (Height Above Plate)?

HAP, or Height Above Plate, refers to the remaining vertical depth of the rafter above the birdsmouth area after the notch is made.

In this calculator, the remaining rafter depth is estimated by subtracting the calculated birdsmouth notch depth from the original rafter depth.

The actual structural requirements for minimum remaining rafter depth should be checked before construction.

Rafter Cut Calculation Example

Suppose you have the following measurements:

With these measurements, the calculator estimates a roof angle of 26.57°, a seat cut length of 3.50 inches, a heel cut depth of approximately 1.75 inches, and a remaining rafter depth of approximately 5.68 inches.

The estimated overhang rafter length is approximately 13.42 inches.

How Roof Pitch Affects Rafter Cuts

Roof pitch has a direct effect on rafter geometry. A higher pitch creates a steeper roof angle, while a lower pitch creates a flatter roof.

As the pitch changes, the roof angle, plumb cut angle, rise, and other related measurements also change. This is why the roof pitch should be measured or confirmed before laying out rafter cuts.

For example, a 6:12 roof pitch rises 6 inches for every 12 inches of horizontal run. A 4:12 roof pitch rises 4 inches over the same 12-inch run and therefore produces a flatter roof.

Rafter Cut Measurements and Units

The Rafter Cut Calculator uses inches for the rafter depth, wall plate width, and overhang measurements. Roof pitch is entered as rise per 12 inches of run.

For accurate results, measure the actual dimensions of the lumber and framing components rather than relying only on nominal lumber sizes.

Rafter Cut vs. Rafter Length

A rafter length calculator and a rafter cut calculator serve different purposes.

For a complete rafter layout, you may need both the rafter length and the cut measurements.

Important Information