OBSERVATIONS
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IN THIS ISSUE
One of the delights of what we do is that we're always learning, and dealing with new information. A while ago we went to a workshop on hewn timber conducted by Tom Paske, an artisan-consultant from western Massachusetts, who offered some information we thought might be worth passing on: Two primary tools were used to hew timber to size: the broadax and the adz. The working methods associated with the two tools differed, and not surprisingly, they were used for different purposes. The broadax was used to hew larger timbers. It was a large tool, and worked fast, but left a rough surface. To hew a timber, guidelines were first struck the length of the rough log, and guide wedges cut between the lines down to the intended face of the finished timber. The log was then positioned so that the face to be cut was vertical. Standing beside the timber, the axman then used his broadax used to hew off the unwanted material. A broadaxed member is typically rough and ragged appearing. The bottoms of the guide wedges typically are visible as v-shaped cuts running across the grain.
The adz was used to hew smaller timbers, and also to smooth the finish of larger broadaxed members. When worked with an adz, the timber was set with the face to be cut facing up. The adzeman straddled the piece as he worked, and swung the adz between his legs like a sharp croquet mallet. The surface of an adzed member looks as though it had been roughly planed using short strokes. It is a principle of structural engineering that a wooden beam is strongest for a given amount of material when it is rectangular, and set with the long dimension vertical, but most hewn timber is roughly square in cross-section. This apparent wastefulness seems odd. Some scholars (obviously theoretical types) had speculated that early builders hadn't yet discovered this principle, but a more convincing reason quickly presents itself when you remember that these large timbers were not saw-cut, but rather were hewn, by chopping away the unwanted parts of the tree trunk. A square member provides the most wood for the least hewing. Hence, square members.
Most framing members that are not approximately square in cross section have been sawn, rather than hewn. Some early buildings, however, do have rectangular hewn beams. Typically, members were first hewn square, and then were split down the middle to make two more-or-less equally-sized members.
It is usually easy to tell when this has been done, even if you are not skilled in reading cut marks --the member will be roughly twice as tall as it is wide, and the two vertical faces will look different, since different methods were used to form them. Back
to the top We recently performed a comprehensive assessment of an old wooden meeting house dating from around the time of the Revolutionary War. One of the church's concerns was the exterior side walls, which undulated in and out like waves on a vertical sea. Inside, we noted that the window sills bowed upward. Something was obviously not right. Our research turned up an interesting story: The church was built as a meetinghouse. The interior was lit by two courses of two-sash windows, one above the balcony that encircled three sides of the space, and one course below. As was typical of the practice of the period, the windows were set between (and attached to) pairs of studs.
Some time in the eighteen-thirties, the building was modernized. The balcony was removed, the pulpit and main entrance were relocated from the long sides of the building to its ends, and most significantly for our story, the old windows were replaced with large modern three-sash units. In order to do this, it was necessary to make large openings in the wall, cutting studs, balcony-level girts, and diagonal braces in the process. These changes did not disturb the main posts, and so did not threaten the building's main structure, but once the girts and studs had been severed, continuity was lost between the posts. The wall became more flexible, and over time was able to bow in and out, resulting in its wavy appearance. Not only that, the windows were supported on by their sills, which rested on the cut-off studs. Since the windows overhung the studs, all their weight (and the weight the construction that bore on them) was taken by the sills. Window sills aren't designed to carry such loads, and over the past hundred sixty years their ends slowly settled, causing their centers to bow upward.
These problems could have been prevented if, when the building was modernized, the builder had not cut corners: He should have opened the entire wall, top to bottom, and reframed the spaces between the main posts to maintain continuity and support the windows. Instead, he took a shortcut, and simply cut window-sized openings in the outer wall, and set the windows into them. Fortunately, the interior plaster showed little or no distress, indicating that the movement was not an immediate concern. We counseled monitoring the condition of the plaster for signs of additional movement, but otherwise taking no immediate action. Back
to the top SCRIBE RULE JOINTS, SQUARE RULE JOINTS The joints of the hewn timber frames of wooden buildings constructed from the earliest days until well into the nineteenth century were cut according to two very different methods, commonly known as the scribe rule and the square rule. For centuries, the joints of building frames were formed by carefully positioning the two members to be joined, and marking, or scribing, the outline of edge of the member in which the mortise (slot) was to be cut on the member that was to be tenoned (tabbed) into it. Then, when the tenon was cut, its back was formed to match exactly the contour of the other member. As can be imagined, this procedure, known as "scribe-rule framing" required a great deal of careful positioning and test fitting to produce good joints. Each joint was unique, so every member had to be fitted to its proper place in the frame. The idea of making some pieces separately from the frame, and inserting them when it was erected, was inconceivable.
Around the time of the War for Independence, a new way of framing was developed, the so-called "square rule." Instead of laboriously scribing and cutting each piece to its mate, square-rule framing allowed all joints to be cut to a constant plane as though the member were uniform in size, regardless of its actual shape. The economies were immediate. Instead of having to cut and fit each member to a unique situation, they could be cut to specified dimensions, for great savings in time and labor. Furthermore, since it was no longer necessary to fit each part to its mate, it became possible to make many smaller members--such as diagonal braces--interchangeable with each other. They could even be made off-site with an excellent likelihood of fitting once they were set into the frame. This seemingly magical feat was accomplished by recessing the joints into the receiving (mortised) member so that they all lay on a common plane. The appearance of scribe-rule and square-rule framed joints differs significantly.
Copyright 1998-2008 Allen C. Hill |
