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General Rules for Joinery Design

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Jim_Rogers:
Recently a timber framer suggested I post these "General Rules" for joinery design, that I learned about recently at a Joinery Decisions workshop class.

Here they are:


There are 8 general rules to timber framing for joinery design.
The "Golden Rule": Design joints to do the same structural task assigned to the loaded timber without putting the capacity of the receiving timber at risk.
1). Cut the joints and arrange the fastenings so as to not weaken the pieces of the timber that they connect as little as possible.
2). Place each abutting surface in a joint as nearly perpendicular as possible to the pressure with it has to transmit.
3). Proportion the area of each surface to the pressure with it has to bear, so that the timber may be safe against injury under the heaviest load which occurs in practice and form and fit every pair of such surfaces accurately in order to distribute that stress uniformly.
4). Proportion the fastenings so that they are stronger than the loads that are anticipated in the pieces that they connect.
5). Place the fastenings in each piece of timber so that there shall be sufficient resistance to the giving way of the joint by the fastenings shearing or crushing their way through the timber.
6). Select the simplest forms of joints, and obtain the smallest number of abutments.
7). Both the tenon and the mortise should be shaped to be parallel with the grain of their respective members.


Most traditional layout was done with the layout tool at hand, which was the framing square. And as it has two parts, the body (the 2" wide part), and the tongue (the 1 1/2" wide part), most layout was one of these two dimensions.
Understanding and using layout faces or reference faces helps us to correctly layout, cut and join timbers so that every piece of the frame lines up to the others and it all goes together correctly.
Each timber has a reference/layout face and that face depends on what the "general frames rules" are, these rules were established by the master framer or frame designer.
But some standards usually always apply. For example, all gable end bents have to have joints flush with the outside of the building. All roof joints have to be flush with the top plane of the roof slope. All wall joints have to be flush with the outside of the walls, all floor joints have to be flush with the top surface of the timbers supporting the floor. These are general timber framing layout rules that apply to most if not all frames.
Some of the interior bents and other interior parts can have different layout faces and rules depending on the master framers plan and none of them are wrong, if it all goes together right.


Other standard rules for joinery decisions are that tenon size should be one quarter of the timber thickness. That means a 8x8 should have a 2" thick tenon and a 6x6 should have a 1 1/2" thick tenon.

Pegs should be one half the tenon thickness. So a 2" tenon should have a 1" peg, and a 1 1/2" tenon should have a 3/4" peg.
Now sometimes a 1" peg will take away too much wood and therefore a smaller peg may need to be used as long as it's strength is high enough as to not fail during it's load carrying capacity.


I hope this is helpful to you.

logmason:
What is a bent? The link above this post, glossary, will not open on this computer, and my dictionary is no help.

Jim Haslip:
Good Info Jim... thanks

Joel:
Jim...

As usual, an informative post from you. Thank you!

Joel

Jim_Rogers:
The glossary, I believe, is in pdf format. This requires a pdf reader program. Most commonly used is acrobat reader. You can go to their website and download a free reader and then you'll be able to view the glossary. And you can print it for future reading and use when not "online".
I would suggest you get a reader program. I'm sure you can find one at www.adobe.com/acrobat, but if not let me know and I'll find the correct website address.

A "bent" is an assembly of timbers. Like, for example, a gable wall (with or without rafters). It is usually assembled flat on the deck and then raised up to a standing position. The raising up can be by hands, crane, ropes and tackle, forklift, or other methods.

Jim Rogers

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