How to Cut Birdsmouth

A birdsmouth joint is a unique way to cut into a board and it to fit in with another board as well. This is often the design that I see with the boards in the roof, though there are other ways to use this. To make this cut, there are a few steps like the ones I discuss below that can help.

What is a Birdsmouth Joint?

The birdsmouth joint is just a small triangular shaped cut that is out of the base of a roofing joist. It is often used because it will allow the joist to sit right on top of the wall plate when making the roof. It is given this name because it sometimes looks like the mouth of a bird when you view it from the side.

How to Cut the Birdsmouth Joint

Now that I have looked at what this kind of cut is all about, it is time to get to work making the actual joint for ourselves.

I am going to look at doing this like you want the cut for a roof rafter since this is the most common way to use it.

I will also make three cuts to help make sure it is done well.

Calculate the Pitch

The pitch of the roof is going to be calculated out as a ratio. It will tell me how many inches, or centimeters, the roof is going to rise up vertically over a given distance in inches as well. I can take a look at how to do this:

1. Measure from the wall plate to the ridge. If this is measured in a level to be 12 inches, write that down.
2. Measure the angle: I am going to use 90 degrees for this.
3. Measure the point of the roof: I will use this to be 6 inches high to make it simple.

With these measurements in mind, I would have a pitch of 6:12. Or I can simplify that down to 1:2.

If you are not able to measure out the roof that you are using, you can get the construction blueprints for it and then use those to figure out the ratio that you would want to use.

Measure the Birdsmouth

These numbers are going to help me to figure out how to get all of this done with my first cut. I will be able to use some of the following steps to help me mark the ridge or the plumb cut with the help of a framing square.

I am going to use this with the pitch ratio of 7:12, you can change it based on the pitch ratio you determined before.

1. Lay the timber on the floor: Have it flat on the floor as level with the work surface as possible. Add the framing square onto it with the longer edge being horizontally inline with the timber and the shorter edge pointing vertically.
2. Position the shorter edge: You want to make sure that the 7 is level with the top of the timber. Do not let this move around as you do the work.
3. Position the body edge: You want to add the 12 is level with the top edge of the timber.
4. Use this to make marks: this will help you to define the ridge cut so that it will be at the exact angle that you need with the numbers I talked about before.

With these measurements done, it is time to cut into the timber. You can choose to work with a cordless or a handsaw. The goal is to make sure that this is square and you are good to get started.

Determine Rafter Length to the Birdsmouth Location

Now that I have the ridge cut made, I need to go through and mark out the birdsmouth too. To help me get this done, I will have to calculate out the exact length of the timber to make the mark with the help of the Pythagoras theorem I used in school.

You will need to have an idea of the exact width of the building, going from the outside wall to outside wall. You will also need to know the exact height too. This can usually be found in the blueprints as well.

With these numbers, you can then figure out what the length of the building is from the center to the top. With these numbers in place, you can use the following calculations to help:

1. Divide the width: You need to divide the width that you have of the building by two. So if your building is at 72 inches, you will need to divide it by 2 so you end up with 36 inches.
2. Subtract: You can subtract half the width of the ridge beam. In my example, I will have 0.5 inches.
3. Multiply: Take the figure from above and multiply it from the square. Do the same for the height to the top of the ridge and then add them together. The width squared + height squared = square root of hypotenuse.
4. Finish it off: You need to find the square of the figure that you got in the last step. This will be the length of the rafter from the ridge center over to the wall plate that you will cut the birdsmouth.

Make the Joint

Now that I have had a chance to mark out the rafter length and I have been able to mark out the plumb line, it is time to mark and cut out the birdsmouth.

Before you cut into anything, keep in mind that you do not want this cut more than 1/3 of the total depth of the timber you would like to use. So, if you want to use timbers that are 2×6, you should not cut more than 2 inches into it.

Learn how to cut a 2×4 with your circular saw.

With those rules in mind, it is time to mark out the cut. There are different ways to do this, but I will use the two stage method below:

1. Lay the timber down: You want to lay this flat so that the depth is facing up, while keeping it as horizontal as possible. You can measure the total depth before dividing that number by 3. This will tell you the maximum depth for the cut.
2. Measure and mark: You will need to measure and mark the two points on either side of the vertical line to show the birdsmouth, measuring up the base and then draw a line between these two.
3. Look for the intersection: There will be a point where the two lines should intersect. This is the point that will show the heel cut and seat meeting to form the joint.
4. Mark the underside: This final step is to mark the edge underneath at the right angle to help it sit flat and fit into the timber that you want. The framing square is one of the best ways to do this and get it all to be even.

Make the Tail Cut

The final thing that you can do here is to mark the line for the tail cut. This is just going to make an additional plumb cut that is a bit further down to give the overhang that you need. This is normally somewhere between 6″ – 12″, but it will depend on the plans that you have in place.

The maximum overhang that you should allow is two feet because anything more is going to ask you to have more support to keep it in place. A smaller amount is going to be the best for this.

I am going to use an example that has the overhang at 10 inches. This will mean I need to use the same kind of marking as before, with the 7 and 12 marks still on the bottom edge as before until the 10 on the upper part is inline with the plumb line that marks the birdsmouth.

With this all set up, you can draw the new line down the tongue edge to mark the tail cut.

Now that it is all marked out, you need to use a saw to help make all of the cuts. You can use a circular saw, but a miter saw will be more accurate. Take your time to make sure that the cuts get put in the right location for your needs.

Read up on the differences in a circular saw and miter saw.

This may take a little bit of time to get adjusted to and do well, but once you make a few cuts, you will be able to get all of the steps in just a few minutes.

Summary

When you are done, you will have a birdsmouth cut that will be able to handle all of the support that you need, either in the roof or in another location. With some care and attention with the marking, you will be able to get it all done quickly and have it look professional in no time.