What is Birdsmouth Joint?

October 19, 2022

Raffle birdsmouth joint on the roof.

You may have heard of the birdsmouth joint as the bird’s beak cut as well. While the name is interesting, many beginner woodworkers have a hard time pinpointing what exactly a birdsmouth joint is. What does it even do? 

A birdsmouth joint interlock with roof rafters to supporting walls. It’s a type of woodworking joint that functions for roofs and top plates specifically! 

Okay, that’s all well and good. But why is it called a birdsmouth joint – or even a bird’s beak cut? 

Well, making this joint creates a pretty familiar image. In the end, you end up with a workable joint that resembles a bird’s mouth! The triangular shape looks like a sharp beak. 

Now, how do you construct a birdsmouth joint? How does it work? And when and where do you apply it? We answer each and every question below! 

How to Create A Birdsmouth Joint 

Correct cuts are a must for birdsmouth joints. But before we get into the whys and the hows, let’s explain how a birdsmouth joint comes to be. 

Birdsmouth joints are made of two cuts: the seat cut, and the heel or plumb cut. Basically, you slice into the rafter and make a heel or plumb cut for the supporting wall. The bird’s mouth shape is formed! 

Structurally, you should be careful about extensions. Don’t let them be unsupported! If you do, this section will simply split along… Effectively rendering the wood grain useless. For added strength and security, you can use nails to fasten the joint from the side and the bottom. That way, you know your project is secure! 

While birdsmouth joints are usually done for roofs, you can also apply this Joint into garden sheds and the like. It’s quite a versatile joint!

How Does a Birdsmouth Joint Work?

We’ve mentioned the two cuts necessary and the nails you can use for more structural integrity. Let’s get into more detail! 

First thing we have to mention: you can’t just use different depths according to your preferences. There are building codes in your local area that you must follow! Before diving into a project, make sure you check regulations to see if you can abide. 

However, you can do the depth up to ⅓ inch of the rafter. That maintains the strength and security without compromising anything!

Though relatively small and unseen in many wood pieces, they’re more important than you realize. The only way any roofing can balance on top of a wall is through the birdsmouth Joint. This is especially the case if there’s no connection between any of the supporting walls underneath the roof! 

Upon adding the birdsmouth joint, the rafter can finally have a solid foundation on the wall plate. The vertical front is solid against the side edges as well!

Aside from the structural integrity – which protects your house in general! – you can also fix up any errors easier. It strengthens your supporting walls and foundations as well. 

How Can a Birdsmouth Joint Be Fixed? 

Secure fixes are a must! The proper way is to toenail timbers and wall plates. Using this technique, drive nails into rafter sides – and use angles. If you’re not familiar with toenailing, you’ve probably come across the term as skew-nailing or side-nailing. Either way, these techniques fix timbers to make solid joints. 

Not a fan of nails, or are they simply not available where you are? You can look around for joist hangers! Steel fixings are preferable. 

Before you go check if your roofs have birdsmouth joints, get basic knowledge of your house first. Usually, birdsmouth joints are only used in traditionally cut roofs! If you have roof trusses, it’s probably not a necessity. It can compromise their integrity if you insist on birdsmouth joints. 

Here are some steps you can take for a birdsmouth joint: 

1. Identify the pitch of the roof 

2. Calculate the pitch of the roof – it’s how many inches or centimeters the roof rises over a given distance 

3. Make a ridge cut by measuring the ratios

4. Use a framing joint for measurements

5. Cut the timber!

6. Mark and measure the birdsmouth you need to do. You can use online calculators as well. 

7. Cut out the birdsmouth Joint and make sure everything is precise. 

8. Mark the tail cut and finish it off. 

9. Congratulations! You have a birdsmouth joint. 

What Are The Benefits of a Birdsmouth Rafter Cut? 

We have a solid connection first and foremost! The stability is unparalleled. You don’t have to debate and balance rafters from the edges. Instead, your work is easier and you can simply hook everything onto wall plates! 

Roof assembly, overall, is easier. It’s also strong and distributes weight a lot better than other techniques! The stability and solid connection allows any stress to distribute evenly – making the structure more durable. 

What Are The Uses & Applications of Birdsmouth Joint? 

Why use a birdsmouth joint in the first place? There are two purposes that you’ll find worthwhile:


Traditionally cut roofs no longer need to be worried about all the time. Instead, the birdsmouth joints come in to secure supporting wall plates and ensure rafters and joints are balanced! 

With precise, secure fits, no overhang should be seen. That way, you eliminate all possible weak points. You don’t want the joint to split along the grain!

No Crush Points 

When cut correctly, gravity has nothing on birdsmouth joints! Once everything is seated nicely onto the timber, the weight from the room spreads evenly throughout the entire building. The wall plates, supporting walls, and foundations are supported, without any crush points.

Applications of Birdsmouth Joints

First off, you know that they’re used for traditionally cut roofs! Aside from roofs, you can also make use of these joints for sheds and fences. Screw them on flat and straight, or at angle – preferably at 45 degrees

You’ll know you’re successful by the shape. Make a bird’s beak and let it sink down!

Birdsmouth joints are usually not visible, but they help every single part of your house stay stable. There wouldn’t be a building standing without foundation support! In summary, just remember: 

  • Birdsmouth joints are joints supporting rafters and the supporting walls. If you want to use a birdsmouth joint or want to check if your roof has one, it’s best to check what type of roof you’ve got. Usually, birdsmouth joints are only for traditionally cut roofs. 
  • Take note that birdsmouth joints don’t have to be just for roofs alone! You can use it for sheds and fences as well. However, when making birdsmouth joints, you have to consider taking a lot of time for measurements and making sure you’ve got all the right tools on hand. It’s best to make woodworking efficient!
  • Secure birdsmouth joints mean less fixes for you in the future. Plus, if anything does go wrong, at least the roof is secure enough for you to do it properly! 


Birdsmouth joints are useful! While they aren’t that well-known, they have their purpose and function in woodworking. Make sure you know how to do them and when they can be used today. It’s never too late to learn. However, many carpenters opt for some other easy alternatives to birdsmouth joining method.

All they need are correct cuts, precise measurements, and a lot of patience. If you plan to take on roofing projects, either hobby-wise or professionally, then it’s time to learn all about birdsmouth joints. 

Thankfully, we compiled this article for you. We hope it was useful! Whenever you have any doubts, remember to refer back to this article. Let us know your thoughts!

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