I’m going to be very honest here — I will not be making any in-depth attempt to describe timber frame joinery layout here on this blog as part of my documentation for building our timber frame and straw bale house. I don’t feel fully qualified, and besides, you can find some pretty good documentation elsewhere. Personally, I’m in the camp of really needing to see and do layout to actually understand it. Reading about it, and having someone describe it to me makes my brain slowly turn off. Call me visual.
However, I can say a few things about the two systems we employed when building our timber frame. They are Square Rule and Scribe Rule layout. They are two singular approaches to achieving the same basic effect — making two different pieces of wood relate to each other, and join in a logical way.
What is Square Rule Layout?
Square rule is an American method of layout. Tom Cundiff says that with the “discovery” of abundant big, straight timbers in colonial America, carpenters were able to work with some of the finest available wood available anywhere in the world. Compare this situation to what people were familiar with back in Europe, where builders had to learn how to adapt to using crooked trees and irregular timbers, because many of the biggest and straightest timbers were already long gone.
I like this description of Square Rule from the Heartwood School:
The term “Square Rule” was coined by Edward Shaw in his 1830′s book Civil Architecture to describe this traditional system which began in the U.S. around 1800. It’s based on the idea that within every irregular, rough sawn (or hewn) timber there lies a slightly smaller perfect timber (see drawing). For example, a 7×7 post may actually measure 7 1/4″ by 6 3/4″; it would be laid out as if it were a perfect 6 1/2″ by 6 1/2″. All joinery is cut to this inner timber, which usually shares two adjacent square faces with the outer irregular timber.
Get it? Every timber, no matter that it may be off from its “true” dimensions (rarely will you get an actual 8×8, unless you pay a lot of money, maybe), has an invisible, perfect timber contained within it. Brilliant, huh? This is the most commonly used system in the US to this day. Everything can be laid out with a carpenter’s square, which amazingly, is not what they would use on the other side of the ocean…
What is Scribe Rule Layout?
Scribe rule is, in a word, fascinating. There is an amazing history there, of which I only know a tiny fraction, but imagine carpenters in older times, building entire cathedrals without referring to any measurements, or any tape measures, and using only a pair of dividers and a plumb bob to do all of the layout. No math, no reading of measurements at all. Seriously. Think of Scribe Rule as truly custom fitting timbers to each other by literally mapping the shapes onto adjoining pieces.
Here’s a great description from New Heritage Woodworking:
Each timber is custom mated to its adjoining timber and vice-versa. Exact dimensions as well as localized characteristics are transferred (copied or scribed) onto each other. The process requires laying entire frame assembles (ie bents or wall sections) in a shop or yard. Timbers are an exactly match and positioned and typically require specialized tagging/marking systems to track their future assembly. Scribes, Plumb bobs and specialized transfer tools are often used to layout the joinery.
With Scribe Rule, you can more easily lay out round wood and irregular timbers. There is a lot more hefting and moving of material with this system, as you are tracing and transferring marks to individual pieces.
I am really just scratching the surface here, but I encourage you to read up on this stuff if you are interested, or better yet, take a workshop to get a much fuller appreciation for the beauty of joinery layout. I have a lot of respect for this part of the timber framing process, and the people than can fully imagine how things fit together, before two timbers ever touch. It’s the most important part of timber framing.
Here is one more link: Square Rule at DIY Timber Framing