Timber Framers Guild Forums General General Forum Questions Tolerances

Here's one for the experts out there. Allow me to set the scene. As far as you can tell you've done all that you reasonably could to have your frame match the perfect model that you have raised in your head hundreds of times. You have layed out your joints slowly and methodically, measured 3,4 no... 10 times before cutting. You've test fit and draw bored all the pieces that you could manage and hoped for the best with the monster timbers. Come raising day the frame goes up with ease, just a trim here and a shave there and everything fits nice and tight. Hero yet again. But wait!! What's this I see? A gap! Right there where the post joins the rafter. It must be 1/4" wide! Holy crap everybody standing around, the client included must be staring at that glaring gap also. I mean how could they not notice, it's flashing neon red and screaming "Look at me, look at meeee!!" Not knowing for sure if I even need to address the, ahem, uh...gap but feeling obliged to at least comment I offer up a rambling and less than convincing lecture on roof loads and frame settling and solar flares and how, in time, say 2 or 3 million years that gap will close up.
So, what I want to know is from all you guys with so many frames under your belts you can't remember them all is this. What kind of tolerances do you guys expect in your joints? Do I just suck at this or is it simply unreasonable to expect precision hair-line joinery all the time in a moving, checking, twisting material like wood? The frames in the glossy mags from what I can tell are perfect in every way. Not even a chipped edge?! (My hunch is that the framers tool of choice in those cases was a mouse but what do I know)I thank you for any feedback. Ray

Ray:
Without seeing or knowing what type of rafter to post connection you made it hard to say what happened.
You should do, if you didn't, a frame fit up, at the shop or site, first before the frame is raised.
This frame fit up should have been done on the ground or on horses so that you can inspect and measure the frame to insure it is the right size, square and true.
At the frame fit up this gap should have showed up and then you can identify the problem and figure the best solution to it.

Hope this helps....

Jim Rogers

gaps happen.
don't sweat it, if you are laying out and cutting carefully, the gaps will dissapear over time... a 1/4 gap will not affect the ability of the frame to stand strong, in my opinion. Also, gaps always look worst to the builder, others never seem to mind them as much....

There are so many variables. It sounds as if you are a concientious craftsman and attempt to perform your work perfectly, although wood and humans are rarely perfect.
A gap is difficult to live with, but it can become a reality if the material shrinks excessively, etc.
I personally do not employ the post to rafter connection, but I could imagine the post shrinking away a good bit.
My experience with looking through Timber Homes Illistrated is quite the opposite of yours. I find gaps, poorly executed joinery, etc all the time.
I personaly would have to find out why the gap occured and take steps to remedy it in the future.
I dont know if it is true but I have heard that when a Japanese Tea House carpenter makes a mistake, they leave it. It represents the days work. No more/no less.

I had a similar gap on a rafter-post connection and, as I recall, we did do a fit-up. We tried to pull the joint together (in-situ), and with all kinds of come-alongs and commanders could not get it together. The peg holes were drilled with a slight draw but there was no way to get the peg started. Unless....a really off-center, long taper on the peg. It worked and went in like it was planned that way. The joint tightend up beautifully.
Is it possible that a draw bore was accidentally drilled the wrong way (a PUSH bore) on your frame? It certainly wouldn't the first time we've seen that!

Ray, how do you dimension and lay out your rafters?
I use a compter or calculator program to generate a list of angles similar to the Hawkindale Angles. Each angle is rigidly defined in terms of its location on the roof. Since angles are ratios they describe the geometry of the intersecting components of a hip/valley roof on any scale.
Beginning with the angles and gable spans the rafters and joints may be dimensioned using trigonometry. I generally work with log builders, sometimes there’s the odd timber frame or some stick framing. Given the irregularities of logs and variations in dressed timber up to half an inch it’s best to work from the theoretical centerlines of the roof members. Lengths, mortises and tenons are solved on-site.
Dimensioning is generally done with a programmable calculator for speed and accuracy but any calculator with trig functions will suffice. The lengths (inches) and angles (degrees) are expressed to five decimal places. Final results are rounded of to the nearest 1/32 inch. Tenons are cut “outside” the line leaving them a hair long, mortises are made a bit shallow by cutting “outside” the line.
This has always worked well in my little corner of the world. The calculations can be done quickly, the results are deadly accurate, layout and cutting takes place on the safety of the ground. An entire complex hip or valley roof can be produced in the shop. We usually end up touching up on-site to remove a bit of excess wood and the assembly generally requires a little help with a sledge. No gaps.

Thanks guys for responding. I guess I wasn,t all that clear with my question. Basically I'm not as concerned with the rafter/post joinery as I am with total frame workmanship. The frame in question is your basic "Benson-esque" high-posted cape with a 4' knee wall, 12-12 pitch so simple 45 deg. everywhere. I know why the gap occured (queen post a tad too long) but this particular bent was raised in two sections to allow me to install drop-from-above intermediate plates.
Ya Jimmy I hear you about test fitting and I do it 90% of the time but I'll tell ya, I mill my own sticks like you do and you know as well as me that a 20' white pine 8x10 weighs slightly less than the moon so I tend to forgo moving the big ones too much.
So far my frames have allowed me to snap full size lines on the shop floor and use those to come up with measurements of the more math reliant pieces like rafter beams and collars. I could I suppose resort to SOA-COA-TOA (or is it COA-SOA TOA?) but I just can't seem to trust trig enough when it comes time to put saw to timber (sorry Tom!)
Thanks for the encouraging words Mark and Collin. Yours are kind of what I was wanting to hear I guess. It occured to me later on that aside from being in old barns and a visit to Jack Sobon's place a couple of years ago that I had never really seen anyone else's work first hand. This got me wondering how my work stacked up to others out there. Do they have gaps also? I agree with you Mark that most people don't really notice the flaws and are usually thrilled with the frame or don't really give a crap one way or another. I, or WE as framers on the other hand do give a crap about the mistakes, in fact a mistake like my "gap" drives me nuts but I must admit I'm starting to get over it.
Mojo...thanks brother. I tell ya, at the time I was looking for anybody to blame. I was going to blame this one guy who nobody seemed to know but he offered me a beer so I figured he wasn't such a bad dude. I did think it would work if I hung my jacket over the joint but I figured someone just might move the jacket some day and discover my flaw.
Thanks to all. Feel better already.

Here, here! I'll drink to that!
(I think it's SOH-CAH-TOA yer lookin' for. Maybe that's the problem)

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

Hi Thomas,

Thank you for sharing your experiences with us. This is probably one of the most valuable contributions ever made on these pages.

I think that almost every timber framer who has ever tried to build a timber frame must have had similar experiences.

You now "know what you don't know" and I suspect are now ready to research, learn more, experiment and in so doing develop a capability to teach others about how to deal with these type of issues.

I feel another Jack Sobon coming on.

Regards

Ken Hume