Squaring up the timber, whether by hand or with a sawmill, is of course only the first step in building a timber-framed barn. The next step the master builder took was to begin the process of "laying out" the joinery. Since timber framing is a system of building with wooden timbers held together with wooden pegs or connections (joinery), nearly every piece of wood in the frame needs to have mortices and tenons laid out and cut into it. The care and accuracy with which this work was done was not only the mark of a fine craftsman, but an important part of insuring all the hundreds of parts of the timber frame would fit together properly on raising day. To the lay person this appears an amazing feat indeed, but generations of timber framers have refined and passed on the knowledge that makes it all a system that could be easily repeated from one barn frame to the next.

Laying out timbers has evolved as framing systems and conversion methods have changed, and learning how to read the information left behind by the timber framers who built Ohio’s barns can be quite enjoyable and enlightening. The information comes in various forms that begin to make sense when one learns to think like a barn builder. This might appear to be a challenge, but the methods used by early timber framers were both logical and direct. Once you begin to understand the logic, the evidence left by the builder is not difficult to interpret.

In the late I8th and early 19th Centuries building a large wooden barn was of course, very challenging. The builder’s approach to meeting this challenge was to make things work as simply as possible. There were no tape measures, power tools or even drawings, so the builder had to employ methods and patterns that worked with primitive tools, were easy to understand and communicate, and still produced joinery cut to very exacting tolerances. The process wasn’t made any easier by the fact that the hewn timbers were often irregular in both size and shape. To overcome these defects the builder used a system that involved stacking the timbers to be cut one on the other in such a way that if you looked straight down on top of them it would look like the section of the frame he was trying to build. The frame, of course, would be stood up after it was pegged together. During the stacking process he would carefully level each timber in both directions so that he could use a "plumb" and a divider to transfer the information he needed to lay out the mortices and tenons from one timber to the next. In so doing he could "scribe" each joint to fit exactly, even if the timbers were odd sizes and shapes. This process was known as "scribe rule" layout.

The individual interested in studying Ohio’s historic timber barns need not learn the intricacies of scribe rule, but should be able to understand why barns built in this way are easy to identify. This method of cutting timbers makes each piece unique, meaning it can only fit in the location for which it was scribed. Thus, it was necessary for the timber framer to mark the timbers with carpenter or "marriage" marks so that the timbers could be set aside before the raising, and be correctly located again or "married" during construction. These marks most often look like Roman numerals, but may include arcs and triangles to help the framer find the piece in three dimensions. The marriage marks are usually cut in pairs at the locations of the joints, but are sometimes actually scribed across the connection so they only are complete when the frame is assembled. Typically they are cut with chisels or a race knife which had a blade formed to a "V" shape at the tip.

Scribe rule barns are not very common in Ohio, but do represent some of the earliest barns. The reason for their rarity is a change in the way barns were laid out that began around 1800 in New England, and spread to Ohio by the 1820’s. This new system called "square rule" layout, did not require stacking the timbers in mock assemblies to align the joinery. Instead the carpenter would snap "chalk lines" on the faces of the hewn timbers that indicated where a smaller timber would be placed inside it. In this way he could work all of the timbers to a known size by removing the wood at the joints that was beyond the chalk line. The fact that each timber only had to be handled once meant much less work was involved, so the new system quickly replaced the old. Another advantage to square rule was that the standard frame patterns it produced, used pieces that were identical or "standardized" and the use of carpenter’s marks was quickly eliminated.

Identification of square rule framed barns is straight forward since the timbers have housings at the connections. Housing is the term timber framers used to describe the area of wood removed from the morticed timber back to the chalk line. This area is typically sized to match the tenoned timber. It also becomes obvious, once a square rule barn is discovered, that there are no marriage marks. The standardized patterns used in the square rule system meant common parts like braces and wall girts were often interchangeable and larger parts could be easily identified by the joinery they contained. Finding a square rule barn means it cannot be earlier than 1800 and is probably newer than 1820.

One last change occurred in the way timber barns were built in Ohio right around the turn of the 20th Century. Although this system was not identified or named in early carpenter’s manuals, this author has chosen to identify it as "mill rule" layout. The premise of this system of layout is the assumption that the timbers supplied by the sawmill are all exactly the right size and squarely cut. In fact by the turn of the twentieth century, sawmills were very precise, barn framing patterns had changed and hewing timbers was no longer necessary. Ohio’s road system had improved considerably so transporting timbers relatively long distances was quite practical.

At first glance, mill rule and scribe rule barns look similar. They have simple mortice and tenon joints with no housings. In Ohio, however, all scribe rule barns were built with hewn timber and mill rule barns are built completely of circular sawn timber. Mill rule barns are also most often quite tall because beginning around 1880 the hay track began to be used for loading hay into the mows. This meant that much taller hay stacks could be created and barns were constructed from then until the end of timber frame barn construction, around WWI, to accommodate these tall stacks. Identifying a barn as mill rule means it was built in the very late 19th or early 20th Century.