Birdsmouth Cut Calculator

How to use the Birdsmouth Cut Calculator

What this calculator does

This calculator turns basic roof geometry into practical rafter layout dimensions. It estimates the horizontal rafter run from the building width and ridge board thickness, converts roof pitch into an angle, calculates the rise and sloped rafter lengths, and derives the heel depth and related minimum-depth checks for a birdsmouth seat. It is useful for preliminary layout, material takeoff, shop planning, and checking hand calculations. It does not select a lumber species or grade, verify span capacity, design fasteners, account for unusual loading, or replace an engineer or building official.

When to use it

Use it when laying out common rafters for a simple symmetrical gable roof, comparing how a steeper pitch changes rafter length, checking the effect of a longer eave overhang, or estimating whether a planned seat cut removes too much depth. For structural work, compare the proposed notch with the applicable provisions on cutting, drilling, and notching roof members in the International Residential Code.

How to calculate

1. The calculator opens with a complete demonstration: a 24 ft building, 1.5 in ridge board, 6:12 pitch, 3.5 in seat, and 18 in overhang. The displayed results and Excel workbook are ready immediately.
2. Replace each sample with measurements for your project. Choose the Pitch format that matches your plans, then enter the corresponding Roof pitch.
3. Read the primary Total rafter length, then review run, rise, notch, and minimum-depth values. The layout table repeats the same canonical model in current units and inches.
4. Use Unit system to convert the live values between imperial and metric. Use Download Excel to create a fresh workbook from the current validated controls.
5. Reset clears the demonstration data rather than restoring it. Export is then disabled until a complete valid set is entered again.

Input guide

Building width is required and must be a positive decimal. In imperial mode it is entered in feet; in metric mode it is metres. A realistic example is 24 ft. Increasing width increases run, rise, and rafter length. Do not enter half the building width – the calculator divides the full width after subtracting the ridge thickness.

Ridge board thickness is required and must be greater than zero but less than the building width. Enter actual thickness, such as 1.5 in or 38.1 mm. A thicker ridge slightly shortens each rafter run. Do not confuse ridge depth with ridge thickness.

Pitch format is required. “Rise per 12” accepts the vertical rise in inches for 12 inches of horizontal run; “Degrees” accepts an angle greater than 0° and less than 89°; “Percent grade” accepts rise divided by run times 100. Switching formats converts the current slope rather than merely relabeling it.

Roof pitch is required and must match the selected format. Examples are 6 for 6:12, 26.565 for degrees, or 50 for percent. A higher pitch increases rise and sloped lengths and deepens the heel cut. Avoid typing “6:12” into the numeric field; enter only 6 when Rise per 12 is selected.

Seat cut length is required and represents horizontal bearing on the wall plate. Enter a positive value such as 3.5 in. A longer seat increases heel depth and every derived minimum-depth check. Confirm the seat fits the plate and does not violate notch limits.

Overhang distance is required, may be zero, and is measured horizontally from the outside wall line to the eave. An example is 18 in. Increasing it adds sloped tail length to the total rafter. Do not enter the sloped tail measurement.

Unit system changes all length controls and displayed lengths. Imperial uses feet for building width and inches for smaller measurements; metric uses metres and millimetres. The underlying model remains in inches, so a round trip preserves the geometry apart from display precision.

Output guide

Total rafter length is the full sloped length from ridge to eave, including the overhang. Rafter run is the horizontal half-span after allowing for ridge thickness. Rafter rise is the vertical rise over that run. Rafter segment length is the sloped ridge-to-wall length before the overhang. Higher width, pitch, or overhang usually increases these values.

Heel cut depth is the vertical depth created by the horizontal seat at the chosen pitch. Minimum rafter depth, Minimum ridge board depth, and Height above plate are geometric checks based on the one-quarter-depth notch assumption used by this model; they are not a complete structural design. Pitch angle is the common angular form of the selected pitch. The summary pills repeat pitch, run, and total rafter length, while the table lists each dimension in current units and canonical inches.

Worked example

With the startup values, the clear horizontal span is 24 ft minus 1.5 in. Half of that is 143.25 in, or 11 ft 11.25 in. A 6:12 pitch has a slope of 0.5 and an angle of about 26.57°. The rise is 143.25 × 0.5 = 71.625 in. Dividing the run by cos(26.57°) gives a ridge-to-wall segment of about 160.16 in. The 18 in horizontal overhang adds about 20.12 in along the slope, producing a total rafter length of about 180.28 in, displayed as 15 ft 0.28 in. The 3.5 in seat creates a 1.75 in heel depth; the model then reports 6.26 in minimum rafter depth, 7.00 in minimum ridge depth, and 5.25 in above the plate.

Learn more

For broader wood-member behavior and material properties, consult the USDA Forest Products Laboratory's Wood Handbook: Wood as an Engineering Material. Engineered wood products can have manufacturer-specific bearing and notching restrictions; the American Wood Council manual for engineered wood construction discusses sloped bearing conditions and why product instructions matter.