Hi,

I just looked over Joel's screen show of the Colorado scribe workshop and was somewaht intrigued by slide image numbers IMG1063 and IMG1064 where the tenon for the wall plate has been set back and adjacent to the jowl rather than set in from the outside face of the wallpost as per "normal" tying joint practice.

I would appreciate if someone could cast some light on this mystical joinery decision ?

Joel,

Could you post your above referenced photo here please to help inflame the discussion.

Regards

Ken Hume

Do you mean this tenon, Ken?
http://picasaweb.google.com/109129016758...345540523790098

Not the place I usually put it, a wide plate, too. Could it have to do with how it was scribed?

Hi Tim,

Correct picture.

I am not quite sure how the draw on the pegs is going to work with a tenon with such a large offset from the reference (outside) face ? Usually the tenon is set back the same distance as the thickness of the tenon.

What am I missing ?

Regards

Ken Hume

The housing for the tie on the plate (outside face) seems like it could be a problem with the plate/ post mortise. Move it to the other side, pick an arbitrary number like 1 1/2 from the other tenon. Make a flush cut peg.

Don't know how a tying joint is "supposed" to be. Just looking at the pictures. Looks like they picked accordingly.

Wall raising, fly the ties, fly the rest. Its late though.

Hi Mo,

I am having some difficulty following your line of argument.

Could you try again please.

Regards

Ken Hume

Hey Ken,

You know I was saying the same thing when I read what I wrote this morning. Absolutely makes no sense whatsoever.

Mo - I got it the first time. Compared to historical joinery the photos may not fit - but I don't see off hand what is so odd about it. Easier to treat that slot between the tenon and shoulder as a 1 1/2" mortise than having to clear out 4-6" in between - removing that material is much easier to the outside.

Ken, historically speaking, how often do you see such a large plate as used in the geo project? It is not carrying any great load, no more that the purlins, actually less. Therefore, it cold have been narrower and the tenon would have been closer to the reference face allowing the draw bore to work well and a smaller section removed where Mike points out the efficiency of its location.

Hi Tim,

On very early [12-1400's] well made buildings one can come across wall plates that are really quite large [8" x 8"] but later on [15 - 1600's] these tend to become 7" wide by 5 - 6" deep. Since a degree of taper exists on every tree the plates are liable to change (reduce) in section as one progresses along the length of the plate. Since nearly all English frames have common rafter roofs the wall plate is subject to outward thrust and bending and so I suppose the width will tend to matter more than then depth especially if internmediate posts / wall studs and braces help to support the plate. The post to plate tenon (in oak) would usually be 1.25" x 4" nominal set back 1.25 from outside reference face. If 1.5" tenons are used then these should be set back 1.5" and in big barn buildings where 2" tenons are employed then set back 2".

Mike,

Thank you for that interpretation of Mo's comments which I now think that I get as well. I follow your logic about the ease of cutting the joint but given the outward thrust on the wall plate there is likely to be an increased risk of splitting the cheek of the wall plate. If boxed heart timbers are used to make the plate (likely due to length considerations) then one has to keep in mind that the reference faces are the top and outside face with any waney edge and hence sapwood being relegated to the interior. Sapwood is notoriously unreliable being the first part of the tree to be affected by decay and bug attack and so placing the tenon in this region of the wall plate is more likely to result in a shorter lifespan.

Even when these joints are configured and cut in an optimal traditional fashion there are a significant number of modes of failure that are known to ocurr over time and so my original enquiry was to try and understand why the designer / carpenter elected to choose this method. You may already have answered the question.

Issues concerning cross grain shrinkage, slope of grain, wane, sapwood and peg draw up are all quite a challenge to balance and the positioning of the tenon plays a significant part in helping to optimise these considerations and will hence likely affect the lifespan of the joint.

Regards

Ken Hume

Ken - am I missing something - when I look at the frame design I essentially see trusses sitting on posts. The plate in this case seems to be holding together a line of posts and will hold up the roof sheathing. No commons on there to even hint at outward thrust anywhere on the plates. And I'm assuming all outward forces are resolved within the truss itself - which requires the joint at the foot of the truss to be properly designed.

Historically would there not have been a ridge (or purlins) from truss to truss to hold up commons? And if so - it seems a waste to build essentially a truss @ the bent lines and not make use of it to eliminate or significantly mitigate thrust.