Laying out a timber

Introduction to Square Rule Joinery

Understanding square rule joinery is a multi-step process that begins with understanding the goal, the obstacles to reaching that goal, the concept that overcomes these obstacles, and finally the implementation that makes the concept a reality. You can’t simply begin cutting joinery without first knowing how and perhaps more importantly why the steps are important. This series is meant as an overview to help conceptualize the process and you may find that going through the steps multiple times sheds even more light on what is a simple but powerful concept.

Series Index
  1. Overview: defining the obstacles
  2. Conceptualization: Key Concepts in Square Rule Joinery
  3. Conceptualization: The Perfect Timber
  4. Implementation: Choosing Reference Faces
  5. Implementation: Orienting Timber (Coming Soon !)
  6. Implementation: Locating Joints (Coming Soon !)
  7. Implementation: Joinery Layout (Coming Soon !)
  8. Reviewing the Rules (Coming Soon !)


The goal is the same the means are different

As is the case with conventional construction the essential goal of building a timber frame is to craft something that is structurally sound; i.e. square, level and plumb. And while the goals are the same, the means by which a timber framer arrives at this goal is slightly different than in conventional construction. Whereas the conventional carpenter can mark cuts, plumb, level and square the building as he goes, this is inefficient and impractical (if not impossible) when working with heavy timber. Imagine suspending a 2 ton beam while you mark where a joint should be located and you get the idea. Yes, during a raising the prudent carpenter will verify that the assembled timber frame is indeed plumb, level and square as the raising proceeds, but the work of cutting the joinery is already done.

Instead of building on-site, in-situ, the modern timber framer works out the location of the joinery working with frame drawings and employing basic geometry and math. (Note: A smattering of trigonometry is used when dealing with more advance timber framing techniques, i.e. compound joinery). All of this work is done on sawhorses, sometimes on "ponies", in a shop typically months in advance of when the frame is raised. In the case of mill rule joinery the application of math and geometry are likely enough to cut an accurate frame, but when when cutting rough or even round timber more advanced techniques are required.

The Timber Frame Dilemma

Not all timbers are created sawn equal
Rough sawn larch timber with lap joinery
Rough sawn timber with lap joinery

Traditionally, timber frames were built with rough-cut timber and presently that is still the case due to significant material and labor savings. Rough timber is either purchased green from a sawmill or converted by hand from a log (e.g. hewn or handsawn). Generally speaking, commercial rough sawn timbers have a standard deviation of plus or minus a quarter inch, though defects or sawing errors that results in deviations more than 1/2 inch is not unheard of. Sawing errors and/or the effects of drying can also result in timbers not being entirely rectangular or square; these timbers are referred to as out-of-square or irregular timbers. So with timbers of varying dimension, squareness and quality, how do you accomplish the above said goals; to build a frame that is square, level and plumb; not to mention how do you build a frame that is accurate and integrates with modern building systems?

At the heart of the matter is how to accurately layout and cut joinery on two different members when their exact dimensions are not known. Why not just measure each and layout accordingly? Well you could map each timbers dimensions and geometry and transfer that information between the two, which is the essence of scribe rule joinery, but there is an easier way.

an idealized timber …. resides inside each imperfect timber…..

An Abstract Solution

In brief, with square rule joinery you layout timbers and cut joinery in reference to an idealized timber that resides inside an imperfect timber. During one presentation on timber frame layout, an observer commented that she thought this was a "wonderful philosophy". More than philosophy this is the essence of the craft. To take living and imperfect materials and shape them into something that embraces the natural beauty within and at the very least strives for perfection. But I digress…..

The solution boils down to this. Don’t cut what is before you, but rather shape the joinery to what it should be. In essence you are working toward an abstraction, a concept of the ideal frame but the concept is actually quite practical, i.e. less work. Once you master the principle concepts, then the rest will fall into place and dealing with all the irregularities, deviations and unique characteristics of working natural rough timber becomes easy. Once the concept is understood then the implementation can begin and you will shape the joinery and consequently the frame to match the concept.

On to part two …. the essential concepts…. Next