How to Cut a Birdsmouth Joint for Fixing Roof Rafters and Joists

Unless you are a seasoned carpenter or a keen DIY’er, you will probably never have come across a birdsmouth joint before so will be unfamiliar with how this particular woodworking joint is used in roof construction and also in building fences.

If you are indeed looking to construct a pitched roof of some kind for a self build project home or even a garden shed or outbuilding you are going to need to cut all of your rafters or roofing joists to fit. The best and strongest way to do this is by using a Birdsmouth joint.

What is a Birdsmouth joint or Cut?

The Birdsmouth Joint (also called the birds beak cut or birds beak joint) is a small triangular shaped cut out at the base of a roofing joist that allows the joist to sit nicely on top of the wall plate (top of the wall before the roof starts).

As I’m sure you have gathered, the name “Birdsmouth Joint” refers to the fact that the joint itself looks a little like a birdsmouth when viewed from the side.

If is wasn’t for this joint, a rafter or roofing joist would only balance on top of the wall plate, making fixing difficult not to mention the fact that there wouldn’t be any solid connection between the roof and the supporting walls below.

The addition of this joint allows the top of the rafter to sit solidly down on top of the horizontal area of the plate, whilst the vertical front edge butts up to the vertical exterior side edge of the wall plate.

This both allows for a solid fixing and also for any forces to be successfully transferred from the roof, down into the supporting walls and then down to the foundation.

To ensure a secure fix for the rafter to the wall plate a fixing process known as “Toenailing” is normally used, whereby nails are driven in to each side of the rafter at an angle and down in to the timber that forms the top of the wall plate.

The act of toenailing (or sometimes it’s also called skew or side-nailing) ensures that both timbers are securely fixed together to form a solid joint.

It’s also possible to use joist hangers, rafter tie down straps or any other suitable steel engineered fixings that are made specifically for this job.

In terms of the cut itself, both the horizontal and vertical cuts that form the birdsmouth joint have their own names; the horizontal cut is known as the “seat cut” and the vertical cut is called the “heel cut”.

One important point to note in terms of roof construction and this type of joint is that the birdsmouth is only used when constructing a traditional cut roof, it is not to be used in the installation of roof trusses due to the fact that they are pre-fabricated to engineers specifications and any cuts could serious affect their structural integrity.

How and Where is a Birdsmouth Cut Used?

As we have briefly mentioned above, the birdsmouth joint is mainly used in forming traditional cut roofs to ensure that the rafters or joists that form the roof sit securely on top of the supporting wall plate.

One of the most important points to consider when cutting this type of joint is to make sure it’s cut correctly.

By this we mean that once the joint is cut and the rafter has been fitted, the seat cut of the birdsmouth should sit on top of the wallplate timber and none of it should overhang the timber itself.

If there is an overhang then this will be an unsupported weak point that could easily split along the grain of the timber.

However, if the seat sits nicely on the supporting wall timber then the downward force (or load) from the roof will be spread nice and evenly across the wall plate and the supporting wall(s). This also avoids any “crush points”.

You may be thinking; why do you really need to cut a birdsmouth joint? Can’t you just fix the rafter securely to the wall plate so it doesn’t move? Or, couldn’t you just cut angled notches in the wall plate timber itself so the rafters have a flat surface to butt up against?

The answer to all of these questions, as with the one above, again centres around gravity and the correct spread of the load generated by the roof.

If a large amount of weight is concentrated down on a small area such as the corner of a wall plate timber from a rafter above, as timber (even hardwoods) has a crush point, the rafter will physically crush the top timber due to the fact that all the weight is being concentrated down on a very small surface area.

Additionally, it’s not just the roof that’s bearing weight down, gravity also plays a major part. As gravity essentially wants to push anything and everything down to ground level, this gravitational force needs to be transferred down into an upright structure e.g. your supporting walls so that it can then be transferred down to the supporting foundations below the supporting wall.

Moving back to the scenario above, in all situations, gravitational force is being transferred at a diagonal angle as opposed to vertically. When this occurs, the angled item simply slips away rather than being forced down onto the supporting structure below it.

So as you can see, the act of adding a “seat” gets gravity on your side through ensuring any force is applied downwards.

Aside from cutting birdsmouth joints into rafters for roofing, they do also come into play in the construction of fencing, particularly knee rail style fences.

The hand rail or arris rail can be fixed to the top of a fence post in one of two ways – screwed straight and flat on top of the post itself or it can be turned to a 45° angle so that it resembles a “diamond” shape and then it can be sunk into a birdsmouth cut into the top of the post.

In pretty much all cases, this will be a 45° cut but will depend on the size of the timber being used for the arris rail (or hand rail) and whether it’s square.

How to Cut a Birdsmouth Joint

Now that we have looked at exactly what a birdsmouth is and when and where it is used, it’s time to get to the nitty-gritty and actually look at how we cut one.

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How to Create a Birdsmouth Joint

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How to Create a Birdsmouth Joint

When it comes to cutting a birdsmouth joint, we are going to look at doing this in the context of cutting and installing a roof rafter or joist as this is where it’s most commonly used, so in this scenario, the joint itself is only a part of the process.

In terms of cutting a birdsmouth joint, we are going to need to make three separate cuts – the ridge or plumb cut where the rafter meets the ridge board, the birdsmouth where it meets the wall plate and the tail cut or overhang.

To ensure that the birdsmouth is cut in exactly the right place, we first need to make the ridge cut as this will become our point of reference for working out the position of the birdsmouth, but before we can make the ridge cut, we need to work out the pitch of the roof.

So, to summarise, the full process is as follows:

• Calculate pitch of roof
• Use roof pitch to cut ridge cut
• Use ridge cut as point of reference to form birdsmouth
• Make tail cut

For the purposes of this project we are going to be working with a simple gable roof e.g. pitched roof, with a central ridge.

Calculating the Pitch of a Roof and Making Ridge Cut

This is the first stage of the process. If you have never done this before, it can appear to be a little involved, but it is certainly necessary.

In most cases, the pitch of a roof is stated and calculated as a ratio. It’s essentially the number of inches (or centimeters) the roof rises up vertically over a given distance (again, either inches or centimeters).

As an example of calculating pitch, starting from the wall plate and working towards the centre or ridge, if you measured in a level 12 inches (or 30cms) and then measured directly up at 90° and at that point the roof was 6 inches (or 15cms) high that would be a pitch of 6:12 (or 6 in 12) and can also be simplified as 1:2.

So, with this in mind, for every horizontal 12 inches of run (the distance from wall plate to the ridge centre) the roof rises 6 inches vertically.

This is all well and good if we actually have an existing roof to measure against, but as we are forming the roof from scratch we haven’t actually got anything to measure against.

With this in mind, it’s time to draw a scaled drawing so that you can work it out from there.

To do this, jot down the actual dimensions of the roof you are constructing and then work out a scale using a ruler.

For our example we are going to be working with a structure that’s 6 feet wide (outside of wall to outside of wall) with a pitch height of 2 feet (wall plate to top of ridge).

Using the scale drawing, we then measure in from the wall plate side towards the centre a total of 12 inches, in this case using our scale which is 1cm = 1 foot.

We then make a mark at the 1cm point and then measure directly up at 90° until we hit the roof. In this case, the measurement is 0.7cms or 7 inches.

So, now we have our pitch ratio of 7:12 or as a fraction 7/12.

Armed with this info, we can now mark our ridge or plumb cut using a roofing square or framing square as it can sometimes be called.

Start by placing your timber flat on the floor or a level work surface and then place the framing square flat on to it with the longer edge, or “body” as it’s called sitting horizontally inline with rafter timber and the shorter edge or “tongue” pointing vertically up from the timber at the end that you are going to cut your ridge cut.

Looking at the framing square you will see a scale notation on both edges, we are going to be using the outer edge.

Position the shorter tongue edge so that the 7 is level with the top edge of the timber and make sure that it remains at this point and does not move. Next, move the body edge so that the 12 is then level with the top edge of the timber.

With the square set in position you can then mark along the tongue edge to define ridge cut that will then be set at the exact angle for the pitch you are working with.

The final job is to now actually cut the timber. For this you can use a handsaw or circular saw. Just make sure it’s square.

Calculate Rafter or Joist Length to Birdsmouth Location

With your ridge cut or plumb cut made you are now all set to mark out your birdsmouth.

Before we can do this we are going to need to calculate the exact length along the timber to make our marks using trigonometry and Pythagoras theorem.

To do this we are going to need to know the exact width of the building from the outside wall to outside wall and also the exact height from the top of the wall plate to the top of the ridge beam.

Once you know these numbers you can then figure out the length from the centre of the ridge to the top of the wall plate which is everything you need to know in order to accurately cut your birdsmouth joint.

Using the measurements, you will need to perform the following calculation:

• Divide the width of building by 2 e.g. 72 inches wide divided by 2 = 36 inches
• Subtract half the width of the ridge board/beam – in this case, 0.5 inches
• Multiply the figure from above by it’s square and do the same for the height to the top of the ridge and then add them together e.g. width squared + height squared = square root of hypotenuse
• Finally, find the square of the figure you arrived at above and this will be the length of your rafter from ridge centre to wall plate where your birdsmouth will be cut

To make this a little clearer using the dimensions from the example image above:

• 72 / 2 = 36 (half width in inches)
• 36 – 0.5 = 35.5 (subtract half width of ridge board)
• 35.5 x 35.5 = 1260.25 (half width squared in inches)
• 21 x 21 = 441 (height squared in inches)
• 1260.5 + 441 = 1701.5 (half width squared + height squared in inches)
• Square root of 1701.5 = 41.2492 inches
• Length of rafter = 41.2492 inches (or 3.4374 feet)

In the example above we have arrived at a figure of 3.4374 feet (or 41.2492 inches). We can then round up to the nearest few decimals, giving us a final figure of 3.44 feet for our rafter. Just to confirm once again, this distance is from the ridge board to the outer edge of the wall plate.

You will also notice that we have subtracted half the width of the ridge board. As we are calculating from the dead centre of the ridge, when installed, the rafters will sit on the ridge board that will effectively offset the centre by half it’s width on both sides, so we need to compensate for this by deducting half it’s width.

As many still use feet and inches as their preferred form of measurement, we have performed the example above using this. You can use metres and centimeters simply by substituting them in place of feet and inches etc.

If you require a tail or overhang at the end of your rafter you will need to ensure that the length of timber you use accounts for this

We can now mark these points both top and bottom on our timber, using the ridge cut as our starting point. Joint the top and bottom points together to give us a vertical plumb line (as seen in the image above).

If the above seems a little complex there are quite a few online calculators available that will perform this job for you.

Making a Birdsmouth Joint

Now that we have marked our rafter length and also marked our plumb line that defines the position of our birdsmouth, the next stage is to mark and cut our birdsmouth.

Before we start marking or cutting anything you have to be aware of that fact that your birdsmouth should be no more than a 1/3 (one third) the total depth of the timber you are using. For example, if you are using 6 x 2 timbers, the birdsmouth should be no greater than 2 inches deep.

With the rules covered, it’s time to mark our cut. There are several different ways of doing this but we are going to use a two stage method.

Firstly, lay the timber flat so that it’s depth is facing upwards and it’s as horizontally level as possible. Measure it’s total depth and then divide by 3.

This will now tell you the total depth that the cut can be at.

Using this number, measure and mark two points either side of your vertical line that depicts where the birdsmouth is to go, measuring up form the base of the timber and then draw a line between them.

At some point along this line it will intersect with the plumb line that states the Birdsmouth’s location. This point marks the location where the seat and heels cuts meet at &90deg; to form the birdsmouth joint.

The final job now is to join this mark to the underside edge of the rafter at the correct angle so that it sits fully flat and level down on the wall plate timber.

The best way to do this is to use your framing square by flipping the square over so that the body side is facing upwards and the tongue edge is to the left.

Set the square so that the 7 (the rise figure calculated above in the section where we worked out the roofs pitch) on the tongue edge is level with the base edge of the rafter and then position the body edge so that the 12 is level with the same bottom edge as the 7.

This will now give you the correct location to draw your line from the depth mark that we just made out to the base edge. This will now form the remaining cut lines for our birdsmouth joint.

Making the Tail Cut

The final job is to mark a line for your tail cut. Marking the tail cut is pretty much just making an additional plumb or ridge cut a bit further down the timber from the plumb line depicting your birdsmouth to provide a little overhang. This is normally around 6 – 12 inches but will depend on your personal preference or what is stated on your plans etc.

One thing to be aware of is that the maximum overhang for a roof is about 2 feet, anything past this will most probably require additional forms of support as it will begin to loose its structural integrity.

In this example we are going to leave an overhang of 10 inches, so with your framing square in the same location as it was in the step above, slide it to the left, making sure that the 7 and 12 marks remain on the bottom edge as they did above, until the 10 on the upper body edge is inline with plumb line that marks the edge of the birdsmouth.

With the square now set, draw a plumb line down the tongue edge, from top to bottom on the timber to mark your overhang or tail cut.

With everything now marked out, the final job is to make the cuts using a suitable saw.

If you are fairly new to this form of, lets face it, quite technical carpentry then cutting a birdsmouth joint can seem quite a daunting job, but as long as you take your time and ensure that you have calculated your angles correctly, marked everything precisely and cut accurately then constructing a traditional cut roof is well within the abilities of anyone.