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rafter to post

Started by Alexis, February 18, 2008, 08:05:13 PM

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Alexis

Hello all,

I would like your opinion on rafter directly joined to the post... I know they are used a lot in modern timberframing but... is it good?

Is there some traditional example of this joint? Can they stand the test of time? Do they rely on sips for their strenght or are they strong enough to use with other enclosure?

I like a lot the traditional joinery with full lenght plate, but the sawer around here seems to stop at 20 ft long for timber...so I'm investigating...

thanks a lot

Alexis

Alexis

oops, forgot to mention: my design is a 48x33 two full story colonial. three 16 bay...

witterbound

Seems to me like rafter to post connections are used all the time in bent designed frames, but not in post and plate frames.  Why not just use 20 footers as plates, and use scarf joints to make "one" 48 footer plate?

Jim_Rogers

Quote from: Alexis on February 18, 2008, 08:05:13 PM

Is there some traditional example of this joint?

Alexis, there is no traditional example of this joint because it wasn't done this way.
In traditional timber framing a rafter never touches a post. It sits on a plate. (Edited, or on a tie).

For longer timbers you'll need to make scarfs. For 48' you'll need three timbers to allow for at least 2' overlap for the scarfs. And make sure you place the scarfs correctly so that they are properly supported. Lots of info here about that. Do a search for "scarfs" and you'll see them.

QuoteDo they rely on sips for their strength or are they strong enough to use with other enclosure?
A frame should be designed to stand on it's own. And then sips or other enclosure systems just make it stronger.

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

Alexis

Does the fact that my plate would have 2 scarf joint anihilate the advantage of the plate over modern joint?

Alexis

moonhill

Alexis, what are you planning on using for the enclosure system?  I think that answer may help in your decision as well.  I stay away from the modern methods my self, traditional systems tried and tested.   You could use a 26' hewn stick with a 20' sawn combination, save yourself some hewing.  Look around for another sawyer with a 24' + track.  Maybe use the 20' track guy and have him cut two 26' pieces, where you hew the last 6' by hand.  You will have to adjust your lengths to fit your proper scarf locations, these are only examples to pass on ideas.  I don't think 2 scarfs would be any worse than one just a little more time.   Tim B.
This is a test, please stand by...

Jim_Rogers

Quote from: Alexis on February 19, 2008, 11:20:28 PM
Does the fact that my plate would have 2 scarf joint annihilate the advantage of the plate over modern joint? Alexis

Alexis, a plate doesn't have any tension in it's length. The pressure against a plate is on it's side, from the rafters.

The problem with rafter touching post is that the plate that ties the bents together now is not continuous it is an interrupted plate. Sometimes just called girts or connecting girts.
If the joinery in connecting girts/interrupted plates isn't done correctly then there is not enough strength there and the exterior enclosure system has to hold the building together.

Here is an example of what I mean:



If you look at this post you'll see the top tenon that went into the bottom of a tie beam. And to either side of this, below it, you'll see two mortises for the connecting girts to joint this post, one right and one left. If you could measure these mortises you'd see that they are less than four inches deep, as the post is 7" wide. So what that means is that if the peg, that is to hold these connecting girts to the post, is laid out one and one half inches off the shoulder of the girt and the tenon is less than four inches long the relish to the end of the tenon is very short. And this would probably blow out if any stress was put on the joint. And the peg spacing from then end of the tenon would not meet the standards set to prevent the blowout.

If it were me designing this frame I wouldn't have made two short tenons connecting the post, I would have put a through spline made of oak there to prevent any risk of peg blow out.

My point is that each frame design has to be carefully reviewed. Understanding what the enclosure system will be and what effect that will have on the frame is very important.

Next you have to consider raising methods and order.
Bent raising with rafters attached to posts makes the bents much heavier. Then you have one bent raised and now you have to decide, will I place the girts into the first bent and try and raise the second bent to join with this girts? Or will I raise bent two and then try to lean out one bent to insert the girts? Both have risks involved and sometimes lots more raising equipment is needed.

In a traditional raising you'd raise the bents, bracing each off with boards:



Then using some planks over the ties creating a deck to stand on, you'd put the plates on:



Although this frame, shown above, is just a yard shed 14'x16' it can be done the same way with a larger building. (My brother, in the second picture, and I put this shed together in a few hours one Saturday.)

In traditional timber framing, one set of timbers, such as tie beams, connected the posts at one elevation. And another set of timbers, plates, connected the posts at another elevation. This created an overlapping gird of timbers, if viewed from above, that prevents the frame from racking in two directions, end to end or side to side. Thus making it a very strong free standing frame.

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

shinnlinger

Here is what I did to get around the problem.


I used eve ties to connect each post.  I did this so I wouldnt have to mill 50 footers or deal with scarves (I am NOT and experinced TFer)

The tenons are haunched on the ties and there is a small "fire cut" on the floor girt.  I feel that there is enough wood left on the 8x10 post where all three intersect.

Originally, I was going to reuse an old barn frame on the property, and even poured my foundation to work with it, but it got sketcher as I studied it and then a huge windstorm downed a bunch of huge pines next door and I bought a sawmill so the old frame is still in purgatory, but It does have  hand hewned 1 peice 50 ft plates on sawed posts.
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

Alexis

wow! thanks for the amazing responses...

So I'll go with a scarfed plate... but for the tie joint... is there alternative to the english tying joint or should I not even consider an alternative since you cannot ask anything more of this joint. The only thing that bother me, is that I would have to use a big post... and a joweled post seems like a waster of wood for me... If i need to order a 20ft 10X10 that will be reduced to 8X10 for most of it's lenght...I'am buying wood that I won't use...

thanks

Alexis

shinnlinger

Oh one more thought,

Depending if your are going with a low posted cape or a cathederal or something, scarves can be very cool, but in my case they would have been up 4 ft above the floor in bedrooms, possibly at the back of a closet, so all that work would have been for naught.

But properly done, a highly visible scarf is a thing of beauty.

I went up and look at the eve ties today though and I feel pretty good about them holding up to the test of time....especially with the nail plates backing them up.
Shinnlinger
Woodshop teacher, pasture raised chicken farmer
34 horse kubota L-2850, Turner Band Mill, '84 F-600,
living in self-built/milled timberframe home

moonhill

Alexis, get a copy of Jack Sobons book Build a Classic Timber-Framed House.  It uses a continuous top plate and straight posts.  Tim B.
This is a test, please stand by...

Jim_Rogers

Quote from: Alexis on February 20, 2008, 05:16:05 PM
is there alternative to the English tying joint
Alexis
The answer depends on where you want the tie. There are three locations for the tie. With the English tying joint the tie is above the plate. The next location is "at the plate" and the third location is "below the plate."
The tie location will depend on ceiling heights and clearance needed for your structure.
And the type of joint used depends on the tie's location.

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

Alexis

moonhill,

I have the sobon book, but I was wondering if his tying joint is as strong as a regular one, since there is no post tenon that enter the tie beam

moonhill

Alexis, that joinery is just as strong, maybe stronger.  The way it is configured is different than the english tying joint though.  I haven't cut that joint from scratch before but have rebuilt old ones.  The joint doesn't have a tenon but it does have a dove tail and a cog on the end of the tie.  It is interesting that it is a comon rafter building and there is no thrust on the walls from the rafters, which you would have with a dropped tie.  I think it a straight forward way to go.  Tim B.
This is a test, please stand by...

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