Need Help With Rafter Foot/Post Joint

[Dakota]

Plates will be 8x10 and the pitch 12/12.  I got Jack Sobon's books out and see that he recomends a housed bird's mouth seat exactly like you drew.  As you correctly drew, the end of this seat is open on the ends of the plate.  How would you recommend I secure the rafter foot in the  seat?  As the outside of the frame is covered by a stick frame for the windows and insulation, I could inlay a hurricane strap that ran from the top end portion of the rafter over the rafter end and down onto the post, or secure it with a trunnel.
Dakota

[Jim_Rogers]

Ok, so I went into my drawing program and found the frame design that I got this picture from:



And I changed the sizes of the plate to 8x10 the post was already 8x8 and I changed the size of the rafter to 8x10 keeping the top surface of rafter at the same elevation.

And I modified the seat cut on the back side of the plate for the new thicker rafter.

This lowered the seat cut and create a bad situation:



This is shown on the right hand side labeled first position.

What is bad about this is that the birds mouth cut in the rafter is just about centered. Cutting the rafter in this position is bad and won't meet code, should it have to.

In order to make it better and be safer and meet code, I moved the plate down so that the birds mouth cut will be in the lower 1/3 or less of the rafter.
This is shown on the view on the left, labeled correct position.

This keeps the rafters at the same height as they were and as we are adding the plate over the top of the posts, the plate can be adjusted to make it work out right. The post can be cut correctly to support the plate and everything should work out right, hopefully.

I'll post more about this post to plate joint, later.

Jim Rogers

[Jim_Rogers]

Ok, so I went back into my drawing program and added the tenon to the top of the post as I would design and cut it.
I had to make some assumptions as to what to do.

Following some standard "general frame rules" I decided to make the joint 1/2" under the timber size, this creates a housing on the bottom of the plate (the opposite of the plate's reference face). And using some other standards, such as 2" off the face and 2" thick.
I create the tenon, housing and bored a peg hole.

In order to show this joint I then copied the plate and moved it out a little and rolled it over 90° so we could see the bottom to see the mortise and housing.

And I copied the post, moved it out a little and spun it around 90°, so we could see the reduction on the opposite reference face.

This is what I got:



The mortise is 2" off the plate's adjacent face and then 2" wide, it is 5 1/2" long and 5" deep. It is also 2" off the end of the plate. Sometimes when people chop this mortise they wait and do not cut the end of the plate off until the mortise is done. This is to prevent the "relish" from blowing out between the mortise and the end of the timber. This short 2" wide 2" log section is cross grain and it is very easy to pop out when you're not expecting it.

The peg hole is centered on the tenon and is 1 1/2" off the shoulder of the post. And is 1" in diameter.

This is how I would make this joint.

Jim

[Dakota]

That is a really nice illistration and design of the joint.  Should I assume by the drawing that the rafter is held in position on the plate by compression and no need of any kind of peg to hold it in position?
Thank you so much for your effort and drawing.
Dakota

[Jim_Rogers]

I would secure the rafter to the plate with a long sip panel type screw or two... just to be sure.

[Dakota]

Thanks Jim,
You've answered all my questions.  I really appreciate it.  I'm sure I'll have more before this project is complete.  Love those drawings, they make it so easy to see what you are talking about.
Dakota

[Hilltop366]

Just wondering if the structure would be better or worse or no different to put the top plate back down and raise the ones on the bents to the tops of the post and put a birds mouth on the end of the rafter to these instead?

Another idea/question, on the pic of the model it looks like the beam that is running from post to post on the end bent is a through run with the center posts under and resting on top of it, I was thinking that unless the builder has a source of long enough beams it is going to have to be scarfed together from two pieces or more. What I would like to know is if it would be better (eaiser to get, cut and handle timbers, stronger bent for raising, less movement in high winds) to run the post straight through to the top tie beam and house mortise & tenon the horz beam to the center posts?

(After thought, another possible option) If the centre post ran through to the rafters?

Please excuse my long descriptions and questions but it has been quite a while since I have done anything with timber framing.

[Jim_Rogers]

Just wondering if the structure would be better or worse or no different to put the top plate back down and raise the ones on the bents to the tops of the post and put a birds mouth on the end of the rafter to these instead?

If you're talking about raising the tie beam to the top of the post and then attaching the rafter to the top of the tie beam with a birds mouth cut, that could be done if the clearance through the building for the tie beam doesn't effect the interior areas of the building. This is way was done to create an English tying joint connection, at the post.
It can form a truss this way.

Reviewing the model this "raising of the tie" to do what you have suggested will effect the interior areas of the building and probably is something he doesn't want to do.

Another idea/question, on the pic of the model it looks like the beam that is running from post to post on the end bent is a through run with the center posts under and resting on top of it, I was thinking that unless the builder has a source of long enough beams it is going to have to be scarfed together from two pieces or more. What I would like to know is if it would be better (easier to get, cut and handle timbers, stronger bent for raising, less movement in high winds) to run the post straight through to the top tie beam and house mortise & tenon the horz beam to the center posts?

When someone is designing a frame, or asks me to design a frame for them, one of the first questions I ask is: "what is the longest piece of timber you can get?" This will determine if the tie is an interrupted tie or a full tie. And whether or not the posts are full height up to the collar beam or two shorter ones at the tie. Sometimes this information make the decision for you.

(After thought, another possible option) If the center post ran through to the rafters?

There are three ways to handle this situation. The center posts can run up to the collar beam as shown. The center posts can run up to the rafters and the collar beam can join the rafters very near this post as well. Or the collar beam can be between the posts holding them apart, then called a straining beam. Remember each name is given because of the position the timber holds in the frame.

Supporting the collar beam which supports the rafters prevent them from sagging towards the middle of the building, in this case, is a good thing and it somewhat reduces the outward thrust of the rafters at the plate level.

Good question.

Jim Rogers

[Hilltop366]

Yep I think it all makes sense to me Jim. Thanks for your time.

It always amazes me how many ways there are to make one frame.

As I look more at the pictures of model and the part of the building that it already built there are limited options to finish the timber frame part and have the roof and tie beams line up.

What I did on my house when I built it ( it is about two thirds stick frame and one third timber with stick frame wraped around the TF) was to lower the roof line for the TF part as to not have the problem of trying to get both lined up just right this way any small error in calculating all the different layers of the roof like the pine ceiling, insulation & exterior boards will not show up, plus I like the look of the step in the roof. The down side is the extra time and materials in having the roof end twice.

Dakota you are lucky to have this great source of info while building, I was't able to find a source that I could ask questions to when building 10 years ago, just read some books over and over and went from there, the more I read on here the more mistakes I find in mine but it's not too bad I don't think it will be falling down any time soon.

Hilltop

[Dakota]

Hilltop,
This is a great Forum.  Jim has helped me a bunch with the tools he's sold me, and good advise as I close in on this project.

My tie beams will be cut full length, with no scarf joints.

It's hard to see on the picture of the model, but the two buildings are different heights.  Jim suggested a few years ago that it would be much easier to connect the two buildings if the roofs were at different heights.

Dakota