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Need Help With Rafter Foot/Post Joint

Started by Dakota, June 11, 2010, 09:56:33 AM

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Dakota

Well, I am finally ready to get started on a frame I will put up this August.  One joint that I am not sure how to cut is the rafter foot/post joint like this:





I have picked most of my joints for their simplisiticy since this is my first timber frame but I'm just not sure how to go about cutting this one.  I'll be using ponderosa beams that are 8"x10".  If someone could describe the process of cutting this joint it would greatly be appreciated.  Here is a picture of a model of the frame.  Thanks





Dakota

Dave Rinker

Jim_Rogers

First of all, drop the idea of putting a peg in that joint. There is no relish left on that tenon and you'll just blow it out completely weakening the tenon.

Next, although you have created this joint in your model, it is not a joint that I recommend at all. Sorry to say, it is one of the worst joints I've ever seen, even if I cut one when I was at Fox Maple when I took his course.

This is not a traditional timber framing joint. It is a modern timber framing joint, and did I mention it's not a good joint, too.

Well, anyway, I don't want to bad mouth that joint any more, but I try to call them as I see them.

In all of the historic frames that have been surveyed all around New England there has never been a rafter found that touches a post. All rafters always sit on a plate. But again, that is the traditional way, that joint is the modern way.

If you want to cut that joint you would cut the plumb cut on the end of the rafter. That is the dimension showing as 1" on your drawing, but when the timber is full length you cut this all the way across your timber. This is so when you make your seat cut it is shorter then the full with of the rafter. The shorter the cut the straighter it will be and it's important to make this cut straight.
The seat cut is the cut that will also make the bottom of the tenon and the bottom of the birds mouth cut. In your drawing it is the bottom of the 3 1/8" dimension.

Here I have extended the lines and numbered them so that you can see what I'm saying:



So cut #1 is the red line, cut #2 is the blue line. Then you cut lines #3 & 4 but only to the depth of the side of the tenon, on both sides of the rafter, (which is hard to line up on both sides).

After you have chopped out the waste with a chisel so that you have a long tenon, you cut line #5 the pink line to make the tenon shorter. Oh yea and you'll have to re-cut line #4 this time to remove the extra part of the tenon, between line #1 and line #5.

I wish you would reconsider this joint and not use it and use a better joint.

I can recommend one for you. But it will depend on your posts and whether or not you have cut them already.

Jim Rogers 
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Mad Professor

It's going to be even uglier than Jim mentioned with the tenon from the tie (what would usually be a top plate) coming in just below the top of the post.

Dakota

Jim,
That picture and drawing was scanned right out of Steve Chappell's book "A Timber Framer's Workshop".   Sorry, I should have mentioned that.

I have cut no posts yet, so I am open to a better joint.  I have all the timber framing books, so if you could refer me to a good joint or show me one, I would appreciate it.

Dakota
Dave Rinker

Raphael

I'd say Mad Professor touched on your solution, raise the ties up on top of the posts and join your rafters to them.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

routestep

Does Chappell talk about raising the rafter onto the post, does the rafter pair go up and set onto the gable end posts using a crane? Just curious. It looks unstable, and then you unhook the crane.

I agree with the first post, don't use this approach. Either go with Raphael's suggestion or a step lapped into the plate.

Dakota

routestep,

No, he does not go into that but I thought that looked like a weak link and was going to lift the rafters into position with a crane but now I am rethinking and waiting for Jim's suggested joint.
Dakota
Dave Rinker

swampfox

That rafter should always be in compression right?  How would the tenon blow-out?

The model you show has significant posts carrying the load in the bents.   In my mind resolving the thrust of the rafter and thereby negating the tension on the tie.

If you did decide to go with that joint, and are working green wood, make sure to leave a gap between the level cut and the post.  If you didn't, shrinkage could pick the post of its bearing shoulder and cause major problems.

I agree with that it looks like a bad joint.  It could be better if you move the bearing shoulder down the post enough to "back" the post.  In this arrangement your tenon would run through to the roof plane, and as long as it is braced well in the wall plane....  It would also resolve having two bearing surfaces and the problem with shrinkage.

I think you would raise the whole bent with strongbacks.

Jim_Rogers

Quote from: routestep on June 12, 2010, 08:00:35 AM
Does Chappell talk about raising the rafter onto the post, does the rafter pair go up and set onto the gable end posts using a crane? Just curious.

Bents are raised with rafters attached, sometimes by crane, sometimes by hand, and yes with stiff-back planks/timbers attached to this joint as it is the weakest point in the bent (while raising).



QuoteHow would the tenon blow-out?

When peg holes are drilled there must be enough relish between the peg hole and the end of the tenon to hold the peg to the tenon. And there are code rules as to how many peg diameter's are needed between the peg hole and the end of the tenon. This joint does not meet those rules as there is very little relish between the peg hole and the end of the tenon.

So anything, and I mean anything that stresses that joint is going to blow out that tenon relish. It could be the raising, it could the wind pushing on the unfinished frame, the wind blowing on a half finished frame, the foundation settling, setting on some sips on the roof, heavy snow load, again any stress on that joint and the relish pops and there is nothing holding it together. Granted it maybe in compression but if a peg is needed then this peg isn't doing a thing to hold that joint together.

Quote....make sure you leave a gap between the level cut and the post. If you didn't, shrinkage could pick the post off it's bearing shoulder and cause major problems.

One problem it would create is blowing out the relish on the tenon, enough said on that subject, for sure.

QuoteIt would be better if you move the bearing shoulder down the post enough to the "back" of the post.

Exactly.....

QuoteIt would also resolve having two bearing surfaces and the problem with the shrinkage.

Well partly, there will still be two bearing surfaces but now they are vertical not horizontal. You would have only one horizontal surface.

The joint that I'm talking about using is like this:



I can give you an exploded view of this joint once you tell me the sizes of the rafter, post, and roof pitch.

Jim Rogers

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

And this joint isn't the best joint either. The cutting away of the post to create the mortise for the rafter tenon can effect the post, sometimes not in the best way:



As you can see, with this Oak frame, the mortise has opened up some and had an effect on the post top, it has dried it out and it has split.

This pictured joint has two pegs in the rafter tenon and they are using stiff-backs to protect the joint while raising.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

If you do decide to rethink this frame design, you could, as mentioned, extend the plates over the posts, making it a one long piece or two shorter pieces scarfed together, and set the rafters down onto the inside of the plate with a joint like this:



Or some other standard rafter to plate joint.
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Dakota

Jim,"
Because my frame will be ponderosa pine, I like the plates over the posts idea.  The posts will be 8"x8" and the rafters are 8"x10".   How would you suggest I connect the rafter foot to the plate?
Dakota
Dave Rinker

Mad Professor

There are some very nice tried and true joints that LOCK the plate/post/principle rafter together, might be overkill but a superior way to try what you are doing.  Assembly will be trickier.

It's posted and downloadable on the TFG site.  Old historical joints that Babcock/Sobon found in their restorations.







Jim_Rogers

Quote from: Mad Professor on June 12, 2010, 05:49:05 PM
There are some very nice tried and true joints that LOCK the plate/post/principle rafter together, might be overkill but a superior way to try what you are doing.  Assembly will be trickier.

It's posted and downloadable on the TFG site.  Old historical joints that Babcock/Sobon found in their restorations.

This is true and a copy of the book sits on my desk and I use it all the time when I'm trying to figure out which joint would work best for the frame.

Let me give it some more thought and I'll look at some of the options available and get back to you, maybe tomorrow or Monday.

Jim
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension


Dakota

Thanks Jim,
I'll be looking forward to your suggestions.  Just keep in mind that this is my first frame and I'm trying to keep my joints as straight forward (easy) as possible.  I have plenty of practice stock and two months to practice my joints.
Dakota
Dave Rinker

Dave Shepard

If that model frame is what you are building, it looks like all you have to do is make a continuous plate, and put a tenon on the top of the posts. Then you can birds mouth or step lap the rafter, unless I'm missing something. Now, you will be raising wall frames, then raising the rafters on top of them, instead of raising whole transverse sections of the building.
Wood-Mizer LT40HDD51-WR Wireless, Kubota L48, Honda Rincon 650, TJ208 G-S, and a 60"LogRite!

Dakota

Dave,
I think you are right and I think doing like you suggest will alleviate my fear of breaking the bent, transitioning from horizontal to vertical.
Dakota
Dave Rinker

Raphael

Totally off the subject of rafter feet, I was wondering how you plan on enclosing this frame?

It's a good thing to think about at this stage as it can effect frame proportions and the position of timbers.
If you wrap your timber frame and infill your stick frame your outside walls won't be flush unless the posts etc. are stepped in to compensate.

I used panels on my house, and have a rather large pile of panel cut offs to process through some day.
If I'd started from scratch I'd have altered Jack Sobon's design slightly to make it (and my additions) more panel friendly.
... he was middle aged,
and the truth hit him like a man with no parachute.
--Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

Alexis

Jim, in the picture you showed of the mortise that has opened, was the frame raised this way or they changed the post?

Alexis

Dakota

Raphael,
Here is a more complete picture of the model.  As you can see, there is stick framing around the frame for the windows and walls.  I am actually quite a ways along with the cabin.  I built the stick frame portion last summer and will build the frame this August.





Here is a picture of the stick built part of the cabin.




Dakota
Dave Rinker

Jim_Rogers

Quote from: Alexis on June 13, 2010, 09:29:12 PM
Jim, in the picture you showed of the mortise that has opened, was the frame raised this way or they changed the post?

Raised as shown, that was raising day when I took that photo, I was a spectator....
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Mad Professor

Quote from: Jim_Rogers on June 12, 2010, 06:19:47 PM
Quote from: Mad Professor on June 12, 2010, 05:49:05 PM
There are some very nice tried and true joints that LOCK the plate/post/principle rafter together, might be overkill but a superior way to try what you are doing.  Assembly will be trickier.

It's posted and downloadable on the TFG site.  Old historical joints that Babcock/Sobon found in their restorations.

This is true and a copy of the book sits on my desk and I use it all the time when I'm trying to figure out which joint would work best for the frame.

Let me give it some more thought and I'll look at some of the options available and get back to you, maybe tomorrow or Monday.

Jim

The joint I'm thinking of uses a gunstock or rectangular post.  The plate drops onto a large mortise/tenon on the outside/pegged.  A tie beam runs across the top of the bent, locked in with another tenon/pegged, and also a partial lap that locks the plate in place.  The principle rafter fits onto this tie via a mortise, with the tenon from the rafter pegged in place another tenon.

No chance of any spreading the plates with this joint, built like a tank.

I hope this makes some sense, it's a joint from TFG, of a 28' X 36' barn described by Sobon.

I think Mr Babcock described this as the "secret joinery", as it was hard to assemble/disassemble

Jim_Rogers

Sounds like an English tying joint to me....
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

Quote from: Dakota on June 12, 2010, 04:58:33 PM
Jim,"
Because my frame will be ponderosa pine, I like the plates over the posts idea.  The posts will be 8"x8" and the rafters are 8"x10".   How would you suggest I connect the rafter foot to the plate?
Dakota

How big is the plate?
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

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