Great topic! As a newbie - raising my first large frame, I've been wrestling with this for the last several weeks. I logged timber and cut my frame over the past 24 months, and now that we're raising it, I'm getting a real education in tolerances!
I'd say every timberframe book I've read is somewhat misleading as to the final joinery tolerances that you can reasonably achieve. Of course they include only pictures of their best joints - and I have many joints that look like those, but not all of my joints look like those pictures! I was careful to plane all of the timbers to exact sizes (+- 1/16), and pull dimensions from references faces where it seemed necessary. But over the course of time, my 8x8's that were cut early in the process shrank to 7.75x7.75's (while my recently cut timbers are still true 8x8's!). I have 24' oak 10x10's with over 1.5 inches of bow in them! They were cut true at the saw mill, and stored with all the advised precautions, but daggone it's hard to find a 24' white oak where the heart of the timber doesn't wander just a little bit! The 45 degree angles on the faces of my braces are now more like 43 degrees due to the non-isotropic drying/shrinking properties of wood.
Miraculously, most of my joints look OK (tight - wood to wood) and most of the bow can be pulled out of the wanky timbers when placed within the frame, but I'm finding there's a trade-off between "squareness of frame" and "tightness of joint". I mean while we're on the subject of joinery tolerances, does anyone want to comment on how square the resulting frame is?!
I decided (somewhat arbitrarily) to drawbore all of my braces and just drill everything else while assembled. What I've learned is that if you were to drawbore your entire frame, and house all of your braces, it would be virtually impossible to adjust the squareness of your frame. OK, you could assemble and disassemble everything multiple times before drilling the holes, and you could cut every brace to a unique length (that you can only discover during a test assembly), but who has the patience and strength to do that? (my house has over 100 braces.) My braces are not housed, so I have been able to adjust the "squareness of the frame" by kerfing the shoulders of the braces that seem too long once assembled. But I've yet to find a remedy for a brace that's too short. (or seems too short because the timbers it joins have shrunk or bowed).
My frame is 2 1/2 stories (and to use Ray's terminolgoy... "benson-esque"), so the posts are 20 feet tall. While raising and pegging the frame, I tried to leave as many joints unpegged as possible (the bents were fully pegged - so I'm talking mainly about the girt joints that connect the bents) - hoping to adjust the squareness of the frame before final pegging. I put homemade plumb bobs on every post that wasn't rigidly pegged in to the final assembly. What I've found is that many of my posts are within 1/4" of plumb from top to bottom, but some are 5/8" out of plumb from top to bottom and I can do no better... without allowing some of the joints to gap open. I'd rather have a tight joint and a slightly crooked frame than to purposely open up a joint.... so my frame is a little bit crooked and I'll live with it. That it is only 5/8" of an inch crooked in 20 feet seems to me miraculaous, given how much shrinking and bowing has been going on in my timbers. But the whole house is not leaning 5/8", just a post here and there - with each bent I raise, it seems to be averaging out, not compounding. I asked a builder for one of the biggest timberframe companies how to deal with out-of-square-frames when applying SIPs, and he said they usually just rack the frame before applying the SIPs. Egads I thought... I tried racking my frame with my backhoe - the top of the frame gave an inch or so in 20 feet, but when I released the pressure, it sprang right back into its unsquare self.
Yes, some of my rafter to post connections have almost 1/4" of gap in them (because I chose a joint that has two parallel load bearing faces and due to shrinkage, only one of the faces is now bearing the load... if that makes any sense!) I shimmed some of those gaps to ease my shameful conscience.
How do some of the big companies do it? Well, I was invited "via a marketing postcard" to a raising by a big timberframe company. I watched them raise part of the frame that morning, and gee, those braces were just dropping in to place. It looked so easy. When they turned the crane off and took a lunch break, I went over and touched some of the braces (as yet to be pegged, but fully in the assembly). The braces had at least 1/4" play in them (up-down, left-right). I mean _none_ of the braces were relying on a "load bearing face" if you will. I suppose the plan was after the raising to pull all of the braces into an aethetically, non gapping position, and peg them. The company rep came over after he saw me wiggle a brace. I didn't say a thing, but he proudly volunteered that the braces in their frames weren't really necessary because the other joinery cut by their hundeggar was _so_ precise. A different timberframe company I talked to bragged that _all_ of their joinery (cut on their hundeggar I think) was "fully housed". I was impressed at the time. I have a few "fully housed" joints in my frame, and what I've realized is that if you have a fully housed joint, you can hide a _lot_ of joinery sin in that housing. Aha.
Now that I'm well into this frame, I've gone back to the timberframe magazines and looked at the joinery. Because I know what to look for, I can find every one of the sins that I've committed in my own frame... right there in glossy print in other people's frames. You just have to know what to look for. But as Mark said, when anyone besides a timberframer views the frame, I think they'll be impressed with the final product. I'm building a frame for myself, so I can and will literally live with the gaps... but I don't think I'm ready to build for a client... (so deep inside I still do admire the big timberframe companies who are able to pull it off.)
-Thomas
