The birdsmouth cut calculator takes building width (w), heel cut depth (ch), overhang distance (o), roof pitch angle (θ), ridge board thickness (tridge board), roof pitch x 12, tan and cos and returns minimum ridge board depth (dmin ridge board), minimum rafter depth (dmin rafter) and total rafter length (lrafter). It is built for quick answers, for checking somebody else’s arithmetic, and for the moments when a formula book is not within reach.
What Is the Birdsmouth Cut Calculator?
The birdsmouth cut calculator turns building width (w), heel cut depth (ch), overhang distance (o), roof pitch angle (θ), ridge board thickness (tridge board), roof pitch x 12, tan and cos into minimum ridge board depth (dmin ridge board), minimum rafter depth (dmin rafter) and total rafter length (lrafter), so you can order the right amount of material the first time.
Material estimates are where projects go over budget. Getting the quantities right before the delivery arrives is worth more than any discount on the order.
How to Measure for the Birdsmouth Cut
Measure everything in the same unit and take the largest dimension you can reasonably use rather than the average. Formwork, ground conditions and cutting waste all push the real figure above the theoretical one.
Where a plan calls for a specific thickness, check it against local building regulations before you order. The calculator reflects the relationship, not the code.
Allowing for Waste and Errors
Order a little more than the calculation suggests. The gap between the theoretical quantity and what actually arrives on site is normally waste, spillage, over-excavation and uneven grading, and all of it comes out of the same budget.
Write the figures down and compare them with what you actually used on site. That is how you build an estimate you can trust for the next job.
A Worked Example
Working through an example, with building width (w) set to 10, heel cut depth (ch) set to 5, overhang distance (o) set to 2, the calculator returns rafter segment rise (y) of -0, and rafter segment run (x) of 0, and rafter segment (lsegment) of 0. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out minimum ridge board depth (dmin ridge board)?
Enter building width (w), heel cut depth (ch), overhang distance (o), roof pitch angle (θ), ridge board thickness (tridge board), roof pitch x 12, tan and cos. The calculator then works through the relationship and reports minimum ridge board depth (dmin ridge board), minimum rafter depth (dmin rafter) and total rafter length (lrafter).
What does the birdsmouth cut actually calculate?
It takes building width (w), heel cut depth (ch), overhang distance (o), roof pitch angle (θ), ridge board thickness (tridge board), roof pitch x 12, tan and cos and converts them into minimum ridge board depth (dmin ridge board), minimum rafter depth (dmin rafter) and total rafter length (lrafter) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter building width (w)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.