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.

That is how I see it Mike.

Ken, as I flip through a English Historic Carpentry, Hewett, I see a variety of roof systems, most prominent, principal rafter with purlins and rather minor common rafters. Are these common rafters really directing thrust to the walls? Or are they heavy strapping?

In same book I see almost no supporting ridge beams, Mike.

Hi Mike,

I feel that a Rumsfeldian moment is about to descend upon me. Either "I know what I don't know", or worse still, "I don't know what I don't know".

I have scrolled through all the screen show photos again to find that you are correct in stating that this roof does not have common rafters and instead relies on common purlins to bring back all the roof loads to the cross frames [bents]. That said, joint design is not all about handling static loads one also has to take into account dynamic (wind) loads and this particular frame (appears) to be equipped with two devices to counter transverse wind loads - the tying joint and the cross braces. When under transverse wind action the braces on the leeward side of the building will tend to try to lever up the tie beam - this is countered by the teazle tenon peg. The end of the tiebeam is also put into tension by brace action and this is supposed to be countered by the dovetail tenon in the wall plate but due to cross grain shrinkage these do not work very well in this respect. That leaves the two post tenons to counter the tie beam tension by resisting same in vertical shear. There is also bending moment generated at the wall post and this is resisted by the tying joint which acts as a moment connection. "Usually" the post tenon is mounted outboard and therefore one can rely upon almost the full depth of the post and jowl in this respect. In this particular case the effective post depth between the two tenons has been greatly reduced by bringing the wall plate tenon inboard and thus the post is now more liable to split down its length ouboard of the post tenon. There do not appear to be any intermediate posts or studs in the cross frames underneath the tie beam to take up the load should a failure be experienced at the tying joint and hence it appears (to me) that this frame is potentially more vulnerable to disproportionate collapse than if the post tenon had been mounted outboard in the more traditional fashion.

Historically it is rare to find a ridge in a common rafter roof with 2 major exceptions - king post and cruck. Purlins are not generally provided to duct rafter loads back to the cross frames (bents) but are present in conjunction with windbraces to stop racking of the roof. Later rafter butt purlin roofs where the rafters are interrupted by the purlin will duct part of the roof load back to the cross frames via the purlins. Common rafters send nearly all of their load to the wall plate. Really early roofs have neither ridge, purlins or wind braces and rely instead upon a mass of lath and the interlocking effect of tiles and / or thatch to stiffen the roof. There are always exceptions to every rule.

Given that this is a Guild sanctioned project that received big billing on the home page one would hope that someone involved with this project might be able to explain to us the design rationale that lay behind this fairly significant joinery design decision.

Regards

Ken Hume P.Eng.

Hi Tim & Mike,

I dug out an old photo of Cecil Hewett that I took of him in the Barley barn at Cressing Temple, Essex back in 1993.

The Master at Work - 1993

You will see that an aisle post has split down its length due to a tying joint trying to resist the out thrust of the common rafters.

The folder that Cecil is holding contains the orginal illustrations included in English Historic Carpentry and a few moments after taking the photo he dropped the folder and the drawings blew away in this rather drafty barn. A mad scamble ensued by all present to help recover same. When examining Cecil's books and drawings have you seen any illustrations that feature failures ?

Cecil worked mainly in Essex or in the wider area of East Anglia and London and thus most of his illustrations tend to reflect this region. Many of the buildings that he illustrated could not really be described as run of the mill buildings and therefore it would be easy to forgive a person remote from this insight to believe that what one sees in this book is widespread in practice, for example, Cecil does not include any cruck frames so does that mean that these do not exist ?

Regards

Ken Hume

Ken, if I am not mistaken the dovetail on the bottom of the tie beam ends is not a dovetail but a cog, allowing it now to work as you suggested it should.

It can be seen right here, http://picasaweb.google.com/109129016758...744278862278274

So, where is the book with all the failures?

I think you meant to say: