Birdsmouth Cut Calculator

Calculate roof angle, seat cut, heel cut, horizontal run, and rafter tail dimensions for accurate roof framing layouts.

Rafter birdsmouth layout

Enter the rafter, pitch, wall plate, cut depth, and overhang measurements.

Rafter Length (ft)

Pitch Rise

Pitch Run

Wall Thickness (in)

Birdsmouth Depth (in)

Rafter Width (in)

Horizontal Overhang (in)

About birdsmouth cuts

A birdsmouth is the notch where a sloped rafter bears on the top plate of a wall. It combines a horizontal seat cut with a vertical heel or plumb cut. A carefully laid out notch provides a flat bearing surface, keeps rafters aligned at the intended roof pitch, and transfers roof loads into the supporting wall. Poorly sized cuts can reduce bearing, weaken the rafter, or make the eaves and ridge difficult to align. Roof pitch is entered as rise over run, such as 6 over 12. The calculator converts that ratio to an angle with the arctangent function. It then divides wall thickness by the cosine of the roof angle to estimate the sloped seat dimension and divides notch depth by the sine of the angle to estimate the corresponding heel-cut length along the rafter. Horizontal run is the entered sloped rafter length multiplied by the cosine of the angle. Conversely, a horizontal eave overhang is divided by cosine to obtain the longer sloped rafter-tail dimension. Cut depth deserves special attention because removing too much wood sharply reduces the rafter's effective section. A widely used preliminary rule limits the notch to no more than one-third of the actual rafter depth, although local codes, engineering details, and connector requirements may impose stricter limits. The calculator enforces that one-third screening rule. Rafter width here means the actual face depth being notched, not the nominal lumber name. For example, a nominal two-by-six commonly has an actual depth near 5.5 inches. Use calculated dimensions as layout guidance, then verify them against field measurements before cutting production rafters. Actual wall plates may vary in width, roof framing can include ridge offsets, and sheathing or fascia details may change the required tail. Make a test pattern from straight stock, place it on the building, and confirm full bearing and alignment. Roof framing carries significant loads and falls under building codes, so structural sizes, fastening, uplift resistance, and unusual geometries should be approved by a qualified designer or building official.

Birdsmouth cut examples

Roof configuration	Calculated cuts	Use
6 over 12 pitch; 5.5 in wall; 1.5 in notch	26.57° angle; 6.149 in seat; 3.354 in heel	A common residential roof layout using actual two-by-six dimensions.
12 over 12 pitch; 5.5 in wall; 1.75 in notch	45° angle; 7.778 in seat; 2.475 in heel	A steep roof creates equal horizontal and vertical pitch components.
3 over 12 pitch; 5.5 in wall; 1.25 in notch	14.04° angle; 5.67 in seat; 5.154 in heel	A low pitch produces a comparatively long heel dimension.

How to calculate a birdsmouth cut

Measure the sloped rafter length and enter the roof pitch as separate rise and run values.
Enter the actual wall plate thickness, proposed notch depth, and actual rafter depth.
Enter the desired horizontal eave overhang, then select Calculate Cuts.
Transfer the angle and dimensions to a test rafter and verify its bearing and alignment before production cutting.

Birdsmouth cut calculator FAQ

How deep can a birdsmouth cut be?

A common preliminary limit is one-third of the actual rafter depth. Always follow the applicable building code and engineered framing details because local requirements can be more restrictive.

What does a 6 over 12 roof pitch mean?

It means the roof rises 6 units for every 12 units of horizontal run. That ratio corresponds to an angle of approximately 26.57 degrees.

Is wall thickness the same as nominal lumber size?

Not necessarily. Enter the actual bearing width measured at the top plate rather than relying only on a nominal lumber designation.

Why is the seat cut longer than the wall thickness?

The seat lies along geometry related to the sloped rafter, so its calculated dimension includes the roof angle. Dividing wall thickness by cosine converts the horizontal bearing width to that corresponding sloped dimension.

Should I cut every rafter from these results?

No. Cut and fit a test pattern first because framing dimensions, ridge details, and plate alignment can differ from plans. Confirm the pattern on site before copying it to the remaining rafters.