I have recently become very interested in building a timber frame home and have been doing a lot of reading about joinery techniques. I have one question which I can not find addressed to any extent in the literature I have. Working with green timber, when a girt is joined to a post with a housed tenon, the girt will shrink in height and a gap will be left in the mortise above and below the girt. The lower gap is what concerns me as the girt would seem to be supported only by the pegs until the space disappears somehow- possibly by splitting of the tenon through the middle of the peg hole and settling of the girt by gravity until it bottoms out on the housing. Is this what happens? I am concerned about the slop caused by this shrinkage and wonder if it is a serious enough problem to be concerned about. I welcome any input regarding my thinking here and would appreciate any tips for minimizing the problem. Thank you, Gene

Gene,

I also am curious as to why this is not addressed in any literature - and have come to the conlcusion that it more than likely doesn't affect the structure. I have moved 4 barns in the past year that have been standing quite rigidly for 125+ years. The evidence shows that even with all the pegs pulled out, the structure is incredible. I cannot remember very many timbers hangin on pegs - although I am sure it happens. One thing to do to help in your question would be to cut all your joinery, and when everything is ready to raise, drill all your peg holes, being careful to layout from the bottom of all mortices and tenons. The theory here is that if the material sits around for awhile, you are compensating for the shrinkage by waiting until the last possible moment to drill your holes.

Mike

I guess my question would be what shape does the mortise take on after drying,does the tolerance on the cheeks get tighter or looser.Also a drilled hole in green wood does it get tighter on the peg over time or looser.

If you are worried about the gap between the girt and the housing, change your joinery design from a tenon with a peg to a wedged thru dovetail tenon. And plan your wedge to be driven in from the inside. Then if the girt shrinks and the gap grows you can pound the wedge in from above the tenon to lower the girt onto the housing. Also or another method would be to draw bore the tenon with it off set more to the bottom. Then make long tapered pegs and if the gap appears pound the peg in more to lower the girt onto the shoulder. (I’m not sure if this would work.) Another method for compensating for the shrinkage is to cut a hollow in the middle at the bottom of the girt at the shoulder.
In Steve Chappell book “A timber Framer’s Workshop” starting on page 144 he talks about shrinkage of lumber and beams. And how to calculate the amount it will shrink and how to change your joinery in the designing state to compensate for the shrinkage. I reviewed it and the bottom line is you’re going to get some shrinkage no matter what you do. But you can lower the amount you see by changing your joinery. He mostly talks about how it will look not how it will work or effect the strength of the joint. If you don’t have this book you could get it and review this section and hopefully it will be helpful.
At a timber framer guild project last summer, the instructor had me hollowing out housings on tie beams where braces and queen’s posts attached to compensate for shrinkage. My point is that this hollowing out is a standard procedure to compensate for shrinkage. This barn was made of both white pine and hemlock they were fairly green timbers. And the whole frame was draw bored.
As to pegs, most of the projects I’ve worked on used dry pegs and green timbers. So the moisture in the green timbers was sucked up by the dry pegs that made them swell up and get very tight. We were told not to stop pounding them in (half way for example) and go for lunch, and come back and expect to finish pounding one in. It wouldn’t move after that amount of time. In all my reading and viewing pictures of joints and pegs I’ve never seen any pegs lose or falling out of holes. Most pegs are cut with a drawknife and have edges to grip the hole; this was to help prevent the peg from rolling out if the building racked in the wind. Although I have seen and used turned pegs but those were the dry ones, mentioned.Good luck

Now you've done it, opened that proverbial can of worms.

This goes to draw boring, when joining green timber the common practice is to set up a slight double draw, pulling both in and slightly down

Mike, DRP, J Rogers, and Will. Thanks for your commments and helpful suggestions. I picked up a copy of Steve Chappell's book and found very good explanations of my concerns. I was really impressed by the practical detailed nature of this book and find it the best reference out of a half dozen I already had. Thanks for recommending it. Gene