Craftsmen's Practice of Timber Framing Methods
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CRAFTSMEN’S PRACTICE OF TIMBER FRAMING METHODS SURVEY AND ANALYSIS OF HISTORICAL AND MODERN TIMBER FRAMING METHODS FOR ADJUSTING IRREGULARLY SIZED TIMBER LASSEN, Ulrik Hjort, Ph.D. Student University of Gothenburg, Sweden, [email protected] SUMMARY In this paper I have surveyed and classifi ed the different transferring methods used for adjusting irregularly sized timber. In contraty to the research that exists in this fi eld that has mostly focused on buildings and construction types, I have tried to classify and explain the methods that I have identifi ed keeping the focus on what actually happens in the practical situation. It can be seen as an attempt to re-establish the craftsman’s know-how in hands on restoration and reconstruction. The empirical material constists of my own carpentry experience and of participation in practical restoration and reconstruction projects in Danish, Norwegian, French, English, Japanese and American timber-frame traditions. I have also used historical and contemporary literature and handbooks for carpenters that involve methods of scribing irregularly sized timber. The preliminary classifi cation of the transferring methods is followed by a visual description of each identifi ed procedure and a short analysis of effi ciency, workplace arrangements, context of tradition, tools and markings for the different methods. Despite advanced power tools, CAD-drawings and prefabrication, every experienced timber framer has undoubtedly found himself in a situation when the transferring methods are effi cient and therefore needed. Even today there will always be situations that require a craftsman’s practice of timber framing methods. INTRODUCTION Contrary to Swedish log-building traditions, building with timber-framed structures is today not considered as a part of the Swedish historical know-how. However, the built heritage of Sweden consists of a large number of timber-framed buildings, and the fact is that we today know very little about these constructions and the building methods used. During the last 150 years standard sized materials and precise measures have replaced the necessity for carpenters to master the traditional so- called “scribing” methods of adjusting one asymmetrical material to another. The aim of this paper is to analyse and organize different transferring methods used in timber-framed structures. The survey constitutes a step in my thesis, in which I want to trace, describe and maybe even revive the historical Swedish timber framing techniques. There are different terms for methods used when connecting two irregular timbers in a timber frame construction. Almost every building tradition has a term for the principles of these methods: “scribing” in English, “tilridsning, opsnøring and afbinding” in Danish, “piquage” in French, “aufreissung, austragung and aufscnüren” in German, “dragning” and ”verkplan” in Swedish and ”sumitsuke” in Japanese. Stability, effi ciency and tight fi tting are criteria of quality in this context which are achieved by neutralizing the irregularities of the timbers or by adjusting one timber to the other. These qualities can be achived in different ways, and there are almost as many different solutions as there are different types of constructions. Square Rule and Scribe Rule (fi g. 1) are two terms used in the English speaking world of timber framers. These two methods are considered almost as contrasts and have in the last decades caused a polarisation within the trade of timber framing. Already in 1836 Edward Shaw found that the Square Rule was less labour intensive and therefore more effi cient than the Scribe Rule [1]. However Shaw only describes two methods within the ”scribing family” called ”Double Cutting” and ”Tumbling”. Will Beemer describes different methods of the scribing family: mapping, tumbling, double cutting, plumb bobbing, bubble scribing [2]. He points out that a skilled timber framer should master more than one of these techniques since they all have advantages in different situations. Rubert Newman shortly describes what he considers as the four most important layout techniques for modern timber framing: Scribe Rule, Square Rule, Mapping and Mill Rule [3]. At a presentation at a timber frame conference in England, Joe Thompson introduces another way of classifying the different methods. Instead of using the term, scribe rule, he puts focus on the transferring process. He suggests a classifi cation into four groups: Plumb Bobbing, Double Cutting, Line Transfer and Square Rule, but he also uses Square Rule: A smaller, perfect timber is envisioned within a rough outer timber; joints are cut with housings to this inner timber. Pieces can be interchangable. [5] Mill Rule: A layout system for perfect timber dimensions using the face of the timber as reference. [4] Mapping: The irregularities of one timber are identifi ed, measured and transferred to the other timber [2]. Scribe Rule: General term for all methods where each timber is custom-mated to its neighbours. The process requires setting out all the timbers for a given assembly in a framing yard or on a fl oor, positioned relatively as they will ultimately rest in the building. [5] Double Cutting: Mortise and tenon are cut, preassembled and scribed using the 2 foot mark as reference point. [2] Line Transfer: The face of one timber is transferred onto the other and parallel projected to the position mark [6] Tumbling: The tenon timber is put on the side for marking the position of the shoulders. The face of the mortise timber is extended onto the tenon timber by moving the tenon timber to the position mark [2]. Bubble Scribing: A pair of dividers with cross levels transfers the surface of one timber to the other by keeping the points of the dividers in a fi xed distance vertical above each other [2]. Plumb Bobbing: The Plumb line is used as a reference line vertical to the horizontal timber. The distance between the timber and the plumb line is transferred from one piece to the other [2]. Fig. 1: Nine English terms have been identifi ed in this research (fi g. 1), but there are also important terms in other languages and traditions that also have to be considered, and their signifi cations are to be found in local litterature and handbooks. The methods have different origins but as the conditions often have been more or less the same, it is possible to equalize and thereby organize them theoretically. the terms: vertical and horizontal transfer [4]. The use of the term ”transfer” has inspired me to try to collect and organize the different terms and methods according to the practical situation of a timber framer despite tradition and semantics of words. In this study I will survey and evaluate the methods for adjusting one timber to another in timber frame constructions. The terms and defi nitions are here grouped, as mentioned, in relation to the practical situation and the point of departure is taken from a timber framer’s point of view. Schematic drawings are used to visualize and describe the procedures. The suggested classifi cation will not justify all the qualities of each method, as the methods are results of specifi c construction problems. The survey of the paper consists of four main chapters. First, I present the fundamental principles; the layout drawings, the grid of reference planes in the construction and the use of reference lines on the timber. This provides the basis for understanding the practical situations that are common for all the identifi ed methods. Second, I present the three groups of transferring methods: 1) Vertical Transfer 2) Horizontal Transfer 3) Distance Transfer a) Reference line (fi g. 4) a) Double Cutting (fi g. 10) a) Mapping (fi g. 12) b) Line Transfer (fi g. 6) b) Line Transfer (fi g. 11) b) Square Rule (fi g. 13) c) Transfer of vertical points (fi g. 7) c) Transfer by profi le board (fi g. 16) These are divided in relation to the practical situation, whereafter they have been divided into two or three subgroups according to the transferring principle. Finally I discuss the value and necessity of this practical knowledge in timber-framing of today. THE FUNDAMENTAL PRINCIPLES To understand the difference between the groups of building methods there are some fundamental principles such as reference planes and lines, layout-drawings and workplace arrangements that have to be clarifi ed. In all constructions and working methods used today, there is a need for reference planes. These perfectly fl at planes which bisect the building create a constant theoretical grid that can be seen both on the layout-drawing and in the fi nished construction. Reference planes are found on e.g. the outside of exterior walls, the top of fl oor joists and in the main cross frames [2]. The reference lines describing these planes are most often placed inside or on the timber revealing the position of the timber in relation to the reference plane. Most parts in a timber frame construction will refer to at least one reference plane. These reference planes and lines make it possible to neutralize or to transfer shapes and irregularities in the timber. When building an exterior wall, the vertical plane that is fl ush with the outside is used as a reference plane. In log house constructions the building process takes place in the vertical reference plane, by stacking one timber on top of the other adjusting the new timber to the one already in the wall. As all timbers are in a horizontal position this technique works well. As a contrast, building a timber framed wall in the vertical plane is quite elaborate. To be able to mark and cut the timbers during the building process, it would be necessary continuously to stabilize the posts. A way of avoiding these problems in timber framed structures is to make the vertical reference plane horizontal.