Here is an exploded view of a bird’s mouth rafter foot where it meets the plate.
A typical timber frame joint where a rafter meets a plate is called a bird’s mouth, and this is a tremendous isometric rendering of this type of joinery.
Birds Mouth at Timber Frame Rafter Foot
Hi Jim,
I have been looking at traditional rafter joinery for quite a while but something isn’t clear in my mind. In the case of a birds mouth joint, what prevents the rafter from lifting? The reason why I ask this question is that I intend on building a timber frame gazebo and so the wind becomes a big factor.
Thanks a lot !
This joint would have to be secured to the plate for the very reasons you mention, that is uplift by the wind.
In your situation you may want to consult an experienced timber frame engineer to get the correct strength metal fastener to use to hold down the rafter to the plate.
I don’t want to say just put in some long shank screws without know what strength, what length, and how many are needed.
This is what an engineer will tell you based on your location and wind exposure.
Every frame and it’s design has to be carefully reviewed and all the local factors have to be taken into consideration.
Such as, but not limited to: wind load, exposure to the wind (site location) snow load (if any), seismic load (if any) and types of wood intended to be used.
It maybe possible to “just peg it” but that is not my call. Sometimes the area in the plate where the peg will go through may have a tenon from a post underneath. This may limit the area where a peg can be used.
Good luck with your project design.
And it was a good question, thanks for asking.
Jim Rogers
I agree with Jim completely it certainly depends on where you are and the wind loads for your area. Typically I use a structural screw, for example a GRK RSS screw or equiv., that holds the rafter to the plate but you will need to consult a timber frame engineer for the exact screw spacing and embedment length into the plate.
Hope this help.
Brice
Hello Jim,
I am new to the site and a novice to timber framing. I am about to attempt the 12×16 shaker shed in Jack Sobon’s book. I have decided to lengthen the shed but not widen it. I have been looking at the birds mouth rafter seat in the book and I think that I would struggle with this and ultimately ruin the beams. I found the rafter seat that you have on this site and it seems that this would be better suited to my current skill level. I have two questions 1) If my plates are 8×8 how much of the bottom of the rafter should sit on the plate and would it be preferable to house the bottom of the rafter in the plate to prevent side by side movement? 2)Do I have to add an additional inch to each end of the cross beam on my bents if I am using shoulders for the through mortise, or is this factored in in the plans? I hope that I explained this so someone can understand it. Thank you in advance. J P
You asked: ”1a) If my plates are 8×8 how much of the bottom of the rafter should sit on the plate…?”
and: Well, you should NOT have the ”V” part of the birds mouth anywhere near the center of the rafter. I think you have to limit it to the bottom 1/3 of the rafter or the bottom 1/4. This is so that the rafter doesn’t get split by the weight of the roof and any snow or wind load on it.
You asked: ”1b)would it be preferable to house the bottom of the rafter in the plate to prevent side by side movement?”
and: That is one reason for the housing on the inside of the plate. But the other reason is that the plate has to be ”sized” to a consistent size on that surface so that the rafter foot/heal fits it right.
In square rule joinery the top surface of a plate is the reference side. Then the outside of the plate is the adjacent side. The inside of the plate is not considered an accurate face. This face can vary from one end of the plate to the other if the timber is rough sawn. For example one end can be 8 1/8” and the other end can be 7 7/8”. So you need to have a constant section for the rafter to sit on and against. So a housing is cut on the inside to a know dimension. Usually 1/2” under the size of the plate. In this case 7 1/2”. Now if everything is put together correctly the birds mouth on the rafters will drop in properly when you put the frame together
You asked: ”Do I have to add an additional inch to each end of the cross beam on my bents if I am using shoulders for the through mortise, or is this factored in the plans?”
and: The tie beam length is the width of the building. So if it’s a 12’ wide building then the tie beam is 12’ long. The reason why is that it is the through mortise, but also a through tenon. The tenon is nearly flush to the outside of the post. I say nearly flush as the tenon is usually trimmed back 1/8” so that it doesn’t extend beyond the post if the post shrinks a little when it dries out.
I hope this has helped you to understand timber framing this shed design.
I am building my 2nd timber frame this coming winter, and erecting it in Spring 2014. Last time I cut “birds feet” where the triangle cut from the rafter sat on the outside of the plate. I avoided rafter expansion issues with a collar tie.
This time I want to cut birds mouths like you show on this page, but I wonder how to do this and continue the rafter past the wall. Is it not possible to extend the rafter past the wall with this technique, or if you do does it create a weak place in the rafter?
Thanks!
If you want to extend the roof decking past the outside of the plate to shed rain water. You can do it two ways that come to mind.
One is to add a sprocket timber to the side of the rafter.
I’d have to upload a picture or create another drawing to show you how this is done.
The other way is to do a step-lap rafter tail and a step-lap rafter seat in the plate. This will create a overhang.
If you want to see one of these step-lap rafter tails and seats, there should be one in the previously posted rafter details on this site.
You can extend the rafter tail beyond the plate some depending on your load. Recently we did this on a frame in Atlanta, GA. And the tail extended 12″ beyond the plate.
If you need more help post again.
Jim Rogers
hi Jay, can you advise where to get my plans drawn up for my timber framed house?
thank
chris
Chris:
There are lots of timber frame designers around the country. You should try and find one close to where you live so he is familiar with the local regulations.
Where do you live?
Jim Rogers
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