The Lewis Conservation Center will be made up of five connected timber frames, a “Big House, Little House, Back House, Barn” where the Big House is a reproduction of the 1722 East Derry Meetinghouse. One frame, the Gallery, re-uses the Green Barn, a scribe rule frame from the 1740s. The Education frame has the same exterior dimensions as the Gallery, but the interior has an open plan and a king post truss system. Similarly,the roof of the Meditation frame is supported by a king post truss, but with studded gable ends, and additional office space. The fifth frame, the Porch, is a relatively small frame, exposed to the weather, and constructed entirely in white oak. This is a big project, with a tight timeline. The frames are also mostly reproductions, built from new wood, which created opportunities to increase efficiency. Throughout the LewCon frame we used one efficient technique that we rarely find in old barns and churches. It’s called housing.
The Green Barn determined the patina and surface pattern of the rest of the frames. The frame was hewn with adze and broadaxe, which gives the timbers a surface texture like hammered copper. They’ve patinaed to a silvery brown. The frame was cut with scribe-ruled, which is the method PTF must use for most in-kind repairs. In cutting a scribe rule, the framer arranges the timbers on the ground, and traces their intersections one onto the other. At each intersection, the framer uses a marriage mark to identify the joint. Each piece is specifically fitted to its mate, and has only one home.
Contemporary square rule frames are much more promiscuous. Square rule relies on the concept that within each imperfect timber, there is a perfectly square and straight timber of consistent width and thickness. For instance within a timber that measures approximately 6 3/4″ wide and 6 7/8″ thick, we have a perfect timber measuring 6″ x 6″. We align this imaginary perfect timber with the two straightest faces of our real timber, and label these faces our “reference faces.” At each joint we measure 6″ from the reference face, and cut a parallel plane on the opposite face exactly 6″ from our reference, and exactly as wide as the ideal adjoining timber. On the non-reference face, a square cutaway reveals the face of the ideal timber. Into that new face, we cut the mortise. On the tenon end of adjoining timber, we perform the same trick, reducing the end so that its shoulders fit within the shallow canyon. The cutaway is called “housing.” Housing in a post, for instance, creates a ledge that relieves some of the weight from a girt tenon. Housing can significantly increase the stability of a joint, reduces twisting, and results in a neater looking joint by hiding the end-grain cut on the tenon’s shoulder. Since each piece is reduced to its ideal thickness, multiples are interchangeable, any brace can fit into any brace mortise. No one’s wearing a marriage mark. The result doesn’t look like a scribe rule frame, but sometimes it doesn’t need to.
Ironically, using square rule, a contemporary joinery method, makes hewing, a historic surfacing method, much more efficient. The client wanted all of the exposed framing to have the “hammered copper” look of the ancient Green Barn. Hewing from trees the enormous Meetinghouse frame was not an option. But without housing, hewing after cutting joinery can result in gaps between the joined faces. Housing preserves the joint, while the hewer scallops the faces surrounding it.
The 1722 East Derry Meetinghouse sports a parallel rafter chord king post truss. It has two pairs of ascending struts, and one pair of descending braces. The tie beam, or bottom chord is cambered, or curved, so that the center of the tie is an inch higher than the eaves. The tie beam is 44′ 6″ long and has mortises to accept four rafters, two struts, two braces, and a king post. Achieving 7 tight joints over a very shallow curve is challenging. Instead of calculating and trimming the end cuts of the rafters, struts, braces and post, the crew accommodated the curve by varying the depths of the housing. In the event of loose joinery, slightly shaving housing across the long-grain is much easier than shaving end-grain. The rafters were so thick that the depth of their housing increased 1/8″ from the rafter toe to rafter heel. The housing for the king post was exactly 1″ deeper than the housing for the upper rafters. Essentially, the crew adjusted housing depths to create an “ideal” straight tie beam out of a cambered bottom chord.
The crew cut all this housing using a router with a custom aluminum router base. The base is 2′ long, with handles, which gives the operator plenty of bearing on the timber. The finished frame fit together beautifully, with minimal futzing. We’ll continue to scribe rule our historic frames, but hope for more opportunities to square rule our new frames and near-reproductions.
More photos, below: