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Tenon layout

Started by Clovis, March 29, 2008, 10:21:58 AM

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Clovis

I'm using google sketch up for my timber frame plans. (what a great free program!) When I am laying out my tenons for an outside face I have been using 2" tenons 2" from the face. When I am laying out a tenon for a inner bent location I am still using a 2" offset.

Question, should I be centering all my tenons in my square rule frame or always be using the same face and going 2" off of that?
I'm not afraid of the great outdoors!

alpmeadow

Hi Clovis
Yes I like sketchup for my planning.  The square rule for me (as a beginner) means that I have placed my beam(crown up) to layout my reference face(LRF) and secondary reference face(arris is that edge between).  If your timber is perfect(no defects), square & level, exact width and depth along full length, then tenon can be marked & cut from the edge. In the real world of timber size, twist, and character, then a mature, sober, responsible person who knows that they're doing is needed to check it, or go back to the books(sobon, etc) to adjust layout more carefully. 
My timbers varies from 7 3/4 to 8 in width and depth, so I am using 1" housing and using a string line marked 2" off the apparent edge.
Hope this helps
alpmeadow
Tallis Creek Woodlot, LT40G28,KubotaMX5000

Clovis

I will be using 8x8 and 8x10 timbers, but the frame will be 7x7 and 7x9 in dimension for my square rule layout. All joints will be housed at 1" or 1/2", depending on where in the frame the joint is. I will be going with 2" tenons should they always be centered in the beam Ie: 2.5" from the 7" face or just always 2" from one face?
I'm not afraid of the great outdoors!

alpmeadow

Here is a image of my beam showing the layout with the tenon at 2" off the outside building edge of the building and 4" off the inside building and beam edge(not centered on the beam)
You can see more of my project in this forum at

https://forestryforum.com/board/index.php/topic,30121.0.html

Hope this helps
alpmeadow
Tallis Creek Woodlot, LT40G28,KubotaMX5000

Jim_Rogers

Clovis:
If you're asking about how the tenons (and that's tenons with only two "n's") are laid out on interior bents, they are usually laid out from a reference face of the building.
For example if you have a three bent design then each gable end bent has to be laid out from the outside of the building in, so that all outer joints are flush with the outside of the building which in plan view is a reference plane.
The one interior bent can be laid out one of three ways: one with all the joints flush to the same side as gable end number one, two with all joints flush to the same side as gable end number two, or three with centered tenons. Traditionally they would be to set to the side of one of the gable ends as it is easier to layout and cut.

You asked: "should I be centering all my tenons in my square rule frame or always be using the same face and going 2" off of that?"
and: If you look at a floor plan view of your frame then the outer line around the frame is a reference plane, joints are usually traditionally laid out 2" off the reference side and then 2" thick if you're using 8" thick timber. If you're using 6" thick timber then it's 1 1/2" off and 1 1/2" thick tenons.

Jim Rogers



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

Brad_bb

Jim, it seems you are confirming what he has in that pic?
He shows that 8" timber with a 2" tennon.  Would you want to then place the tenon 3" off the reference face to get it closer to center.  Granted depending on the actual width of the timber it will never be perfectly centered (cause if the timber is 7.75 wide then one shoulder will only be 2.75 and the other will be 3").  I would think that you'd preferrably want to center it approximately from the layout face and not leave such a large offset?
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Jim_Rogers

If you use your standard layout tools such as a framing square (framing "L" ;D) then there isn't any 3" part of it.
Follow traditional methods to make it easy on yourself. 2" off the layout face/reference face and then 2" thick on 8x8 timber, irregardless of the actual thickness of the timber.

If you own a "Big Al" layout tool you can do whatever you want, but it makes things harder if you don't follow the simple layout rules.

When each frame is designed, the designer establishes some "general frame rules" these rules apply to every joint in the frame.

For example:

1) all bents are laid out from the west end except the east most bent.

2) all joints are housed to 1/2" under size of timber.

3) all joints are 2" off layout face and 2" thick.

4) braces are framed to 4" thick.

With these simple four rules you can layout and cut just about any frame.

If the timber sizes are 8x8, such as Jack Sobon's garden shed.

Now if the frame is all 6x6 timbers then the rule #3 is 1 1/2" and 1 1/2" and the braces are 3" thick.

What rule number four means is that if you layout 2x2 and you buy or cut your stock ahead of time, and it shrinks you won't have enough wood to make the tenon 2" thick, and you can't change the offset unless you change the offset of all the tenons and their mortises.
So what you do is you make or order timbers 4 1/8"  thick, layout the tenon 2x2 and size the tenon by hand planing the back side down to 4".
I think I did a whole story about this before.....

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

Clovis

Thanks Jim! Yes I do recall you going over this before and guess what, you will do it again. You have now jogged my memory. It is also good to have the tenons to one face of the frame so that when you are raising it a member can't be put in backwards. Like I said its comming back to me, I guess all that reading paid off.

Now let me see..... thats tenon not tennon right?

Now for lesson #2 today, it's regardless not irregardless thats not even a word!!!!  smiley_whacko

P.S. We need a spell cheque on this web sight. Clovis
I'm not afraid of the great outdoors!

Jim_Rogers

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

Clovis

I guess I should have known better than to go against someone who's as sharp as his chisels! :D
(regardless still sounds better) Clovis
I'm not afraid of the great outdoors!

Housewright

Hi Jim and Clovis;

Two other historic layout methods are to make your reference face the side toward center (toward the threshing floor) or on the side away from center (threshing floor).

Toward center is very common on the earlier barns in my area. Layout away from center and centered on the piece are not common in my experience.

Historically they seemed to make things easy on themselves. They laid out off of the square they had. The same house I just mentioned was laid out 1 7/8" off the edge with a 1 7/8" mortise, and I found a square on the property which has a blade measuring 1 7/8"; possibly the original layout tool.

Jim D.

routestep

I agree with the above about the 1 1/2 or 2 inch off the reference face.

I have one more quirk that was pretty specific to the project I'm working on and my tool limitations. I'm working with nine inch posts and beams and my bigger saw is 10 inches. With a 2 inch layout and 2 inch wide tenons I would end up hand sawing a lot of wood off the non reference side, about 11/4 inches plus or minus about my saw cut. So I moved the tenon over to three inches off the reference face and saved my arm a bit off extra work.

Clovis

Away from the threshing floor makes sense to me. I am going to see what the easy route is to take and then just cut all of my tenons that way. I think I will still be cutting the tenons 2" from the 7" reference face not the 8", 8 1/4", 7 7/8" or what ever other demension the timber is. Most of the timbers will be reclaimed from the barn I am taking down and they are all over the map size wise. From 7 x7 up to 12x 12.
I'm not afraid of the great outdoors!

Mortice

I thought I had read somewhere that the tenons were laid away from the outside of the building to create a consistent surface to be able to attach your enclosure. That was the reasoning I had thought about when I was reading this topic.

Clovis

Yes it seems common to have then laid out from the outside wall, but the barn I just took down had them laid out from the outside (2" with a 2" tenon) and they were spliting from the thrust of the common rafters. Of course this barn is well over a hundred years old and we had a hell of a time bringing her down but they were still spliting at that joint. I think if they were laid out from the inside with the comon rafters the thrust may not have been able to split the post.
I'm not afraid of the great outdoors!

Mortice

Clovis, I see what your saying.

You think that having more "meat" between the tennon and the exterior face would have avoided this problem on the barn. I wonder if those tennons are normaly designed to take those forces or if there wasnt enough bracing in the barn to control these forces?  undersized posts that had too much deflection and created a split by chance? I'm learning all of this so these are just the questions that pop up...

alpmeadow

Hey Clovis
Just to investigate a few more aspects
-is it the tenons or mortises in the posts for braces, that is the problem?.
-if it is tenons, then thicker tenons are called for, if mortises are blown out, consider the following:
-you say that the bottom of the barn posts have rotted and the bottom 24" were shored up at some point.
-I believe that the problem may not have been the load from above but the uneven forces coming from the lack of support as posts rot and the building sags

Also - as you recover the posts and frame, keep eyes open, and mark anything unusual, ie locations showing nails or other hardware, that may have been in place, broken off, or could lurk under the surface
-scanning with metal detector on any reused timbers should be normal practise before resaw
Enjoy the adventure
alpmeadow
Tallis Creek Woodlot, LT40G28,KubotaMX5000

Clovis

Alpmedow. Its the tenons on the top of the posts. It had a continuous top plate on it. It had 2"x10" tenons . Take a look at my photos there are multiple pic's of it. Not one post was rotted at the bottom or had been repaired. They were not all split either, just a few and it could of happened a 100 years ago as it dried out. I just think that if I was planning for a large load at that joint I would put the tenon in the center of the post. (The tie beam had 3" tenons) This old girl would have stood for another 100 years with a little care. I will be using the timbers as found. I want all that aging, it would take me a hundred years to reproduce it. :D
I'm not afraid of the great outdoors!

alpmeadow

You're right with a little care(no moisture) those timberframe barns could be there for another four generations.  It looks look the barn has quite a story to tell, how it changed over the years, and now you will add to its story, in your future home!
Cheers
alpmeadow
Tallis Creek Woodlot, LT40G28,KubotaMX5000

Clovis

I think this barn is headed for it's fourth reincarnation. The timbers were sawn (1) but it had some old top plates (2) that were used for girts (sawn) and it had about 5 hand hewn timbers (3) from another old structure. I will now be using them in my cottage (4). Wait it was moved once so that would be 5!
I'm not afraid of the great outdoors!

routestep

Clovis, I'm having trouble visualizing what you mean by the post's top tenon splitting from rafter thrust. I have/had the concern with my principle rafters shearing off the top tenons due to thrust at the tenons base - where the post transitions back to its full size. I've decided to use struts to take up some of the side load.

I have a few posts and beams that have developed checking cracks as they dry out. Same with braces. I've used sealer to slow down the drying process, but cracks still have developed. These cracks are just about centered on the tenon. Could this be what you are seeing?

I've come up with two reasons (for me) for tenons being stepped over 2 inches (or 11/2) , don't know if either reason has a historical bases:

1. It is a fast visual way to orientate the post/beam
2. Oak pith can have some rot in it and sapwood is not very strong. To keep the rot out of the tenon stay away from the pith - to minimize sapwood stay away from the outside.

Clovis

Routestep; Look at Jims post for the gin pole. That is a continuous top plate they are raising with the gin pole, the same as the barn. That plate tranfers the rafter load (thrust) to the post (tenon). The tenon in Jims post looks to be 2" with a 2" offset from the outside wall. The ones on the barn (not all) were split on the inside of the tenon-post joint (inside of barn). The drying cracks were centered on the post until they got to the tenon and then it jumped over to it.

The posts are close to 10 inches so that leaves 6" of post not doing anything at that joint. I just think that if it was layed out from the inside it might be stronger but I am definatly new to this.
I'm not afraid of the great outdoors!

routestep

OK, I think I understand the failure, a check propagated up to the tenon/post junction. I think Jack Sobon had a list of failures in one of his books. I don't have the book, but looked through it once. That crack you have could have been one of them.

Your idea might  be worth a try, but I've never heard or seen a frame built that way. Is there a down side to an inside tenon? If I was a better woodworker a double tenon might do. I've heard of them being used in beams - cross ties - but not posts. But then the plate would have two mortises. I used a strut.

I wonder if the posts that failed were in an inside bent, they carry the most roof load. A gable end bent would carry a smaller load, maybe only half as much as an inside bent, and the barn's siding could carry some load in the portion of the post above the cross tie beam.

If the post was an inside bent I would speculate the crack allowed the plate to bend out some. In one picture you have there is a canted queen post and what looks like a strut out of 2 by 4 material or some small stuff. I think that piece was taking up load sharing.

moonhill

Clovis, post a picture of this cracked post.  By the photos in the barn post, the post look fine.  I have seen post with cracks you could put your hands in from either side and touch your fingers, this was failure, due to the tie beams being removed via chain saw.  Someone had to go back in and install 2 cables.  What you are describing seem to be drying checks.  There are thousands of barns out there with a tenon lay out 2" off the outside face.  I also don't see any reason not to lay the post tops out like you are saying, with the exception of consistency.  Are you going to make a change in other areas as well, the next thing you know you are standing on your head.  Or at least that is where I find myself when I make those sort of changes.  I try to repeat things through out the frame as much as possible. 

The photos also show the post being fairly close to the top plates, an ideal location to help stop this problem you are describing.  The canted queens also reduce thrust at the plate.  Just checking on this, again would be curious to see a photo.  Tim B.
This is a test, please stand by...

Clovis

Yes they are definately drying cracks but were they forced there? Or do they go to that point naturally. The cracks in the rest of the post were in the middle and then jumped over to the tenon at the top. They were not in failure but would be the first place they would fail if pushed harder. This barn has been moved so who knows its past, and it was on the inside bents as suggested. The north corner of the outside bent was rotten away and you would not have known it the way she still wanted to stand. The siding and all the other parts of the barn on a corner held it together.

Two tenons for a post of this size would seem to be the best layout at this joint. But would they, or would they not be needed if all other variables are executed properly??

Photos are not on the cards at this point, I have them under tarps and all stapled in. I took some other photos put have not looked at them yet to see what I have.

If I decide to move a tenon it will be for a good reason and they will all be the same. The old timers did it a certain way for a reason and I'm not about to mess with that. As said there are a thousand of these things still standing with that joint.
I'm not afraid of the great outdoors!

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