Step-by-step Instructions for Cutting a Birdsmouth

July 17, 2021

Examples of birdsmouth joints.

Birdsmouth joints can be hard to decipher at first! Today, we have a guide on how you can cut out a perfect birdsmouth joint. Heads-up: this is best applied for roof rafters and joists. After all, rafters and joists are where birdsmouth cuts are commonly used.

However, you can reference this article still to make birdsmouth joints for other projects. As much as possible, you should have the following: 

  • Construction plan for the roof 
  • Design – it doesn’t have to be that intricate, but make sure it’s drawn carefully! Prioritize care over style.
  • Measured roof pitches (take note of it!) 
  • The number of rafters you might need for the project – you don’t want to have less! 

Before starting anything, you should have all of these in place. Take the time to figure out the details. 

You’ll need to have three separate cuts, so make sure you know how to do the following: 

  1. Ridge or plumb cut. You need to make it where the rafter connects to the ridge board. 
  2. The actual birdsmouth cut for the wall plate. 
  3. The tail cut. If you’re not familiar with the tail cut, it’s also called the overhang. 

What You May Need: 

It’s time to make a birdsmouth cut! They’re all basic tools, so the birdsmouth cut can keep the process simple and efficient. Before anything else, go to the hardware store or look into your workshop for the following: 

Stair Gauge: Clamp-like ; Usually in a hexagonal shape; Hollow in the middle.

The stair gauge can help out with your rafters. Laying them down won’t be an issue with stair gauges! It’s handy and can hold framing squares tightly. To help with the snug fit, you can also use thumbscrews. After everything, the stair gauge can help with the square glide on the rafter. It’ll flow smoothly and consistently!

Rafter: Before using a rafter, you need to lay out everything first. Know terms like the pitch, run, and step-off before you move onto different cuts! 

The pitch is the rise of the roof, while the run is the distance that the rafter covers. Once all the details are laid down, move onto layouting different cuts. The step-off is the best way to achieve this by making use of a framing square. 

Circular Saw: Circular saws come in handy for birdsmouth joints! For the best possible cuts – all precise and accurate – use a circular saw. It helps as the cut you do for one rafter can be used as a pattern for the rest. It can make workflow quicker as well! 

Ensure that the blade is sharp. Use a set square, too! 

Hand Saw: If you like, you can use a hand saw over a circular saw. Use this tool to finish off the birdsmouth cut! That way, you can make sure everything’s up to your needs.  Now that the cut is done, you can consider cutting corners as well. All of that can be done with a hand saw!

Framing Square: The framing square is a must for lay-offs and proper dimensions. It also provides somewhere you can lay out cutoffs! To make everything successful, use a framing square.  

Clamp: We’ve all struggled with cuts. It’s frustrating when none of them turn out equal! To balance it all out, clamps can help. A pattern can be set with clamps. In the absence of them, you can have a helper hold everything together. 

Marker Pen: You need something to draw with properly! Mark positions and different kinds of measurements with a marker pen instead. It’ll help keep accuracy – and with birdsmouth cuts, that’s a crucial thing.

The 5 Steps Into Making Birdsmouth Cuts 

These are five basic steps, but they can get quite complex. Make sure you follow everything to get the possible birdsmouth joint!

Step 1: Make a Blueprint or a Plan For Your Cut 

A cut wouldn’t be accurate or proper if you don’t have a plan! You don’t want to be wasting your time or labor if the cut will just end up bad. Take a step back and look at the big picture. 

You need a plan. You need to look at how much rafters you’ll need and measure out the pitch – and where it’ll sit! Preliminary measurements are a must. Distances from wall plates, plate ridges, and outside walls must be measured! Take some time off the day and dedicate it to this.

Step 2: It’s Time to Calculate

This is very tricky, but you’ll get the hang of it! We’ll expand more about which calculations you have to make, so take this chance to prepare yourself. Measuring tools, paper, pens, and a calculator is in your favor. 

Step 3: The Pitch Calculation and Ridge Cuts 

Okay, this is a necessary step. Do not skip it! You will need to calculate the pitch of the roof and calculate it as a ratio. 

This ratio includes the inches or centimeters of the height of the roof (how it rises vertically) when given a certain distance. The unit of measurement should be the same. 

Try out measuring the pitch by starting from the wall plate. Measure it towards the middle. Take those distances together and analyze: 

  • If you start from the wall plate and measure up to 12 inches at 90 degrees… 
  • Then the roof is 6 inches high, you get a 6:12 pitch. 
  • Apply some basic math and simplify this. You get 1:2!  

The 12 inches is the run, or the number of inches from wall plate to ridge center. For every 12 inches, the roof stands up to 6 inches. 

Next, measure the distance from the wall top to the ridge plate. You need to multiply this. 

Take the same measurements. Subtract that distance (wall top to ridge plate top) from the distance of one wall to another. Do measurements first then apply the steps! 

Next, divide everything by two. Then multiply it by itself. 

After, add the two numbers and see if you can find the square root. The square root is crucial! Any additional measurements can be done with the square root. And trust us, there will be some additional measurements you have to do!

Step 4: Cutting a Birdsmouth Joint

You’ve got all the measurements and markers down. It’s time to define where you can position the cut you’re going to make. 

Before anything else, however, you need to have some things to keep in mind. First, the total depth of your birdsmouth cut shouldn’t be more than ⅓ of the timber depth! For example, you’re using 6 x 2 timbers. The depth, then, shouldn’t be more than 2 inches. 

Got that down? Mark the cut! We recommend using a two-stage method to keep everything efficient. 

The first step should be in laying the timber flat on the ground. Make sure it’s as level as possible! The depth should face outwards. Measure the depth here with any tool you like, then divide it by 3. You’re essentially getting the total depth. 

Once you have the total depth, measure it out and see two points where the birdsmouth should go. These two points should take up the base. Draw a line between these two points! You’ll find that this line intersects with the plumb line – where you’ll effectively find where you should place the birdsmouth cut. 

Aside from being where the birdsmouth cut is supposed to be, this point is also where the seat and heels cut intersect!

Now, all you need to do is make sure the underside edge is at the correct angle. Flat and level edges are crucial! To achieve this, you can use a framing square. The tongue edge should be to the left and the body facing upwards. 

When using a square, get the measurement for the rise, tongue edge, and base edge. Position it properly on the rafter, draw the depth mark, and make out where you can cut the lines for the birdsmouth joint!

Step 5: Making the Tail Cut

The tail cut is pretty straightforward. Add a plumb or ridge cut into the wood piece! The plumb lines should have a little overhang with the birdsmouth cut. As much as possible, you want it to be around 6-12 inches. However, this measurement is completely arbitrary and you need to get something suited for your personal preference! 

Additionally, your plans may differ as well. Make sure everything is suited just right to your project. Besides the right measurements that can be customized, you don’t want to go over two feet with the overhang! 

Personalized measurements are fine. Just make sure you maintain structural integrity and add other forms of support! For example, if you leave an overhang of 10 inches, you want the 7 to 12 marks to stay on the bottom edge. You want the 10 inches to align with the plumb line as well! 

Set the square! From the tongue edge, draw a plumb line from top to bottom. You can mark your overhang and tail cut like this. 

And guess what? 

Now’s the time to get out your saw. The final job is to do the cut! 

Yes, there are a lot of steps involved before you even begin wood cutting. Sadly, technical carpentry can be daunting! But remember: practice will increase efficiency and accuracy. Don’t be afraid to take your time as well. Don’t rush! Calculate angles, mark everything, and make sure you cut as accurately as possible. 

Congratulations! You’ve got a traditionally-cut roof. It takes a while to master, but this guide is here to help you out. It’s always within your ability! 


Traditionally-cut roofs are always there for you to try. To prepare rafters for hand-cut gables and sheds, it’s time to do a birdsmouth cut! It’s classic, easy, and pretty straightforward. 

We know there can be difficulties. If you’re a beginner, it can be intimidating and complex to approach a birdsmouth cut! But it’s something anyone can do, so just refer to this guide when you’re having a hard time. We’ll guide you to success! 

Remember to be careful. Make plans, check layouts, wall plates, and if it will support a birdsmouth joint in the first place.

We hope you enjoyed this step-by-step guide! Was it helpful? Just come back to this article every now and then whenever you need to make a birdsmouth joint! 

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