how to bird mouth a rafter

In this DIY guide you will learn how to install a roofing joist or rafter that includes how to work out the pitch of a roof, cut a ridge or plumb cut at the correct angle, work out the correct length of joist you need and the exact location for a birds mouth joint, how to correctly cut a birds mouth joint and finally how to cut a tail cut to finish off the end of a roofing joist.

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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.

Why is the Birdsmouth Essential?The design of the birdsmouth ensures that the weight of the roof is evenly distributed over the walls. This even distribution prevents undue stress on any single point of the roof structure, ensuring longevity.

  • Toenails: These are hammered straight into the wall plate through the birdsmouth.
  • Metal Rafter Tie Down Plates: These plates provide an extra degree of security for individuals seeking it.

What is a Birdsmouth joint or Cut?

A tiny triangular cutout at the base of a roofing joist, known as the Birdsmouth Joint or Birds Beak Cut, enables the joist to rest comfortably atop the wall plate, or the top of the wall prior to the beginning of the roof.

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.Birdsmouth joints cut into rafters

Birdsmouth joints cut into roofing rafters and mounted in place on top of the wall plate

A rafter or roofing joist would only balance on top of the wall plate without this joint, which would make repairs challenging in addition to providing no strong connection between the roof and the supporting walls below.

This joint’s addition enables the rafter’s top to rest firmly on top of the plate’s horizontal section, while the wall plate’s vertical exterior side edge is butted up against the rafter’s vertical front edge.

This enables a strong anchoring as well as the effective transfer of any forces from the roof to the supporting walls and ultimately to the foundation.

A process called “toenailing” is typically used to ensure a secure fix for the rafter to the wall plate. This involves driving nails into each side of the rafter at an angle and into the wood that makes up the top of the wall plate.

Toenailing, sometimes known as skew or side-nailing, is a process that makes sure two timbers are firmly fastened to one another to form a strong joint.

Use of joist hangers, rafter tie-down straps, or any other appropriate steel-engineered fasteners designed especially for this task are also options.

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”.Birdsmouth joint notched onto wallplate showing seat and heel cuts

Birdsmouth Joint Notched onto Wallplate showing seat and heel cuts

The birdsmouth joint should only be used when building a traditional cut roof. Since roof trusses are prefabricated to engineer specifications, any cuts made to them could seriously compromise their structural integrity, this is an important thing to keep in mind when installing roof trusses.

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.Birdsmouth joint as used in roof

Birdsmouth Joint as used in the construction of traditional roofs

Close up of a birdsmouth joint

Close up of a Birdsmouth Joint

Making sure the cut is accurate is one of the most crucial things to keep in mind when cutting this kind of joint.

This means that the birdsmouth’s seat cut should rest atop the wallplate timber after the joint has been cut and the rafter has been installed, with no portion of it protruding over the wood itself.

If there is an overhang, it represents an unsupported weak point where the wood could split easily along its grain.

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”.Birdsmouth joint resting on wallplate and also notched into floor

Birdsmouth joint resting on wallplate and also notched into floor

If you were to fix the rafter securely to the wall plate so that it doesn’t move, or if you were to cut angled notches in the wall plate timber itself so the rafters have a flat surface to butt up against, then why exactly do you need to cut a birdsmouth joint?

As with the previous question, the answers to all of these revolve around gravity and the proper distribution of the load produced by the roof.

Because wood, even hardwoods, has a crush point, if a rafter above concentrates a lot of weight on a small area, like the corner of a wall plate timber, the rafter will physically crush the top timber because all of the weight is being concentrated 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.Gravity acting on rafter without birdsmouth joint

Diagonal gravitational force applied to rafter without birdsmouth joint

Gravity acting on rafter with birdsmouth joint

Downward gravitational force applied to rafter with birdsmouth joint

Referring back to the previous scenario, gravitational force is transferred diagonally rather than vertically in all cases. When this happens, the angled object doesn’t crash into the supporting structure below it; instead, it just slides away.

As you can see, adding a “seat” causes gravity to work in your favor by directing any force downward.

Birdsmouth joints are used in the construction of fencing, especially knee rail style fences, in addition to being cut into rafters for roofing.

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.Birdsmouth joint cut for fence post

Birdsmouth joint cut for fence post – courtesy of

This will almost always be a 45° cut, but it will vary depending on how big and how square the wood is that will be used for the arris rail (also called the hand rail).


What is the depth of a 2×4 rafter birdsmouth?

the depth should be one third the width of the rafter , most cut the depth to 1.5″ or two inches on a two by four , the length from the top of the cut to the edge of the rafter is the width of the rafter or 3.5″ ..