粤北明清木构建筑营造技艺研究

Introduction

  • 近年来,华南理工大学的程建军教授的《岭南古代大式殿堂建筑构架研究》涉及到粤北地区几处殿堂的研究,大范围的研究工作尚未展开。本文研究的内容在建筑类型上主要包括学宫、会馆、寺院、祠堂、衙署、大型民居、园林中的主要建筑以及建筑群中的次要建筑,是与人民生活密切相关的一类建筑。由于粤北地区位置的特殊性,传统建筑大量而广泛的存在于乡土社会中,其建筑形制、等级以及设计水平更容易受到地域、功能、习俗、业主、匠师等不同主客观因素影响制约。
  • 于粤北传统建筑的广泛性、多样性,建筑大木构架中往往保存较多的原生性的特点,将会更直接的反应出早期岭南木构架的传承关系,粤北地处赣粤古道,尤其南雄是古代进入广东的必经之道,木构架设计法则将体现起承转合的影响力。粤北地区传统建筑在建筑构架的表现上为更多的灵活性、多样性、复杂性与开放性,对其发展演变的脉络的梳理、设计规律的总结与设计方法的探索将有助于对岭南殿堂建筑大木构架的研究,同时也是岭南传统建筑研究的有益补充。
  • 以梁思成、刘敦桢等为代表的第一代建筑史学家就对中国古代建筑大木构架进行了系统深入的研究,以二人为主要研究者的营造学社展开调查、测绘,研究了大量古建筑实例,发表数十份科学调查报告和图纸,为大木构架研究积累了详实可靠的例证;梁思成细致研究了《清工程做法》、《营造法式》两部古代典籍,为后人研究大木构架奠定了坚实基础。先后完成的著作主要有《清式营造则例》(1934 年)、《营造法式注释》(卷上)(1980 年)、《中国古代建筑史》(刘敦桢 1980 年)、《中国古代木结构建筑技术(战国—北宋)》(陈明达 1990 年)、《中国建筑类型与结构》(刘致平 2000 年)、《应县木塔》(陈明达 2001 年)、《中国住宅概说》(刘敦桢 2004 年)、《中国建筑史》(梁思成 2011 年)等。

国内对于古代建筑大木构架的研究

  • 一批古建筑学者从各自不同角度对中国古建筑大木构架进行详细、深入研究,取得一批丰硕的研究成果,其中影响力较大的有:陈明达先生《营造法式大木作制度研究》一书,结合现在保存实例对《营造法式》中规定的大木作制度进行详细阐释;杨鸿勋先生《建筑考古学论文集》,将考古学引入古建筑研究领域中;潘谷西先生的文章《营造法式初探》(一、二、三、四)中提出《营造法式》与江南建筑有着密切关系,对法式中殿堂、厅堂、余屋的用料、构造、建筑式样的区别进行详细阐释;并将古代建筑的长、宽、高从大木制度的角度剖析为建筑类别、正面间数、间广、檐柱高度、屋深、屋顶样式、铺座、材等八个方面,化繁为简,为古建筑构架研究提供方向性的指引。徐伯安先生的文章《营造法式斗栱形制解疑探微》、结合实例对大木构架中斗栱形制、布局原则进行清晰、详尽的解释;张十庆先生专著《中日古代建筑大木技术的源流与变迁》、《营造法式变造用材制度探析》(一、二)、《营造法式研究札记—论以中为法的模数构成》、《营造法式的技术源流与江南建筑的关联探析》、《营造法式栱长构成及其意义解析》、《古代建筑的尺度构成探析》、《从建构思维看古代建筑结构的类型与演化》等文章中立足于江南地区从建筑学、文化学、社会学、类型学、宗教学等多学科、多角度对《营造法式》中大木做法从源流、尺度、用材、技术等角度进行分析;《中国江南禅宗寺院建筑》文中对木构架有较为深刻的认识,研究的侧重点放在江南地区传统建筑,大量的案例研究,文中可借鉴的研究方法较多。潘谷西、何建中著《<营造法式>解读》,作者充分尊重原书的基本理论,用现代的语言及图示对《营造法式》进行解析,见解独到、分析精确、一语中的。
  • 蔡军、张健著《<工程做法则例>中大木设计体系》,将日本建筑史学领域的研究方法和思路,来对中国传统建筑进行解读。在整理、研究《工程做法则例》中古典建筑设计的模数化体系时,运用日本的“木割”理论,能够图表化、形象化地清晰表达古典建筑书籍中繁杂的文字,系统化解读其设计技法。郭华瑜《明代官式建筑大木作》,对大量明代官式建筑遗构进行了考察、测绘,并对比分析了明代官式做法与宋、元、清各代官式建筑做法的异同之处,对明代大木构架的传承和发展进行了分析与总结,大木构架的类型有殿堂结构形式、厅堂结构形式、柱梁结构形式、楼阁结构形式;并分析了大木构架的平面构成、剖面、立面构成,进而分析了屋顶的做法、斗栱的类型等。

国内关于传统营造技艺的研究

  • 了解古代建筑制度和技术,主要从梁思成、刘敦桢等《<营造法式>注释》(梁思成)、《清式营造则例》(梁思成)、《营造法原》(清 姚承祖)等历代建筑技术专书,1983 年文化部文物保护护科研究所主编的《中国古建筑修缮技术》是对传统古建修建做出了详细的指导;1991 年马炳坚先生的《中国古建筑木作营造技术》着眼点在于对于北京官式做法及北方地方传统建筑做法,从宏观的角度,较为通俗详尽。2001 年出版的《中国古代建筑史》(第一~五卷),以历史时间为线索,分时期对于中国传统建做出较为详尽的梳理,是中国古建筑技术宏观发展史的巨著;2003 年潘谷西先生主编的《中国建筑史》等,都涉及传统建筑的营造技艺。
  • 井庆升《清式大木作操作工艺》重在做法上,详细记录了大木的构件尺度,及安装方法,记录了我国清代的许多大木作操作工艺。刘大可《中国古建筑瓦石营法》以明、清官式建筑的做法为主线,主要介绍了古建筑土作、瓦作和石作的传统营造方法和法式,包括地基、台基、墙体、屋顶及地面等部位的样式变化、构造关系、比例尺度、规矩做法以及建筑材料等方面的知识。
  • 近年来,各建筑高校建筑学专业博士生也进行了传统建筑木构架营造技艺方面的研究。乔迅翔的《宋代建筑营造技术基础研究》,是对于宋代传统建筑营造技术的问题研究。其认为建筑的基本要素包括工和料。工,即劳动者,包括工匠和役夫;料,即劳动对象;在整个官方营造体系中,由营造机构来进行统辖管理。文中阐述了宋代营造机构的发展沿革及其构成,并且对于宋代工匠、役夫的成分、地位等展开讨论,在建筑的工程管理中如何发挥营造团队的作用,并研究相应的管理运作程序和和管理制度,关注营造工序中的每一个环节,从设计、选址、备工备料、到施工营建,力求还原当时营造的轮廓,对于测量、起重与运输等工程数学的具体技术问题也有展开讨论。后面对于《营造法式》中的功限、料例等条文进行了着重探讨。
  • 中国艺术研究院马全宝博士的《江南木构架营造技艺比较研究》,文中讨论了江南木构作为我国传统建筑体系中的重要组成部分,以木材为主要结构材料的建筑体系,传统建筑的营造技艺经过不断发展、完善,成为一个完整科学的技术体系,是东方传统营造水平的代表。江南殿庭构架规模较大,面宽有二至九间,进深达六至十二界,规模形式较高。利用比较研究的方法,通过比较江南周边地区以及北方和中原地区的传统木构架,对江南木构架建筑的历史发展变化进行了探讨,指出作为我国南方重要的营造体系的江南木构架,代表了当时先进的建筑技术水平和技术特征,体现出及江南地区木构营造技艺的地域多样性。
  • 杂志期刊文章有的龙非了的《论中国古建筑之系统及营造工程》、孙大章的《民居建筑的插梁架浅论》、张十庆的《古代建筑的尺度构成探析(一、二、三)》、李浈《官尺·营造尺·鲁班尺—古代建筑实践中用尺制度初探》、张玉瑜的《大木怕安—传统大木作上架技艺研究》、王世仁《明清时期的民间木构建筑技术》等都涉及到了传统建筑营造技术与过程的相关知识。

国内对于岭南大木构架研究现状

  • 一直以来,华南理工大学、东南大学、华侨大学等高校和台湾地区的很多专家学者都倾注大量的心血对岭南地区传统建筑构架与设计方法孜孜以求的进行探索研究,也取得了丰硕的成果。上个世纪 40 年代以后,以华南理工大学建筑学院的龙庆忠教授为代表的一批学者教授,数十年来对许多岭南重要的古建筑进行了大量的测绘和研究工作,发表多篇学术价值很高的论文和专著。如龙庆忠先生的“中国古建筑在结构上的伟大成就”、“南海神庙”、“瑰玮奇特、天南奇观的容县古经略台—真武阁”等系列论文,对岭南古建筑的构架和设计方法上作了考据和论证;陆元鼎、魏彦均教授的《广东民居》对广东民居及祠堂的布局、形制及构架和装饰进行了系统的研究论述;邓其生教授对岭南园林进行详细勘察,对岭南园林的布局格体、设计手法以及园林建筑的形体、体量进行深入探讨研究;吴庆洲教授的“粤西古建筑瑰宝”、“肇庆梅庵”和“德庆悦城龙母庙”的研究对岭南早期的大木式建筑的形制做了详尽的分析;程建军教授多年来一直对岭南古建筑的大木构架进行大量细致的研究,并发表了多篇论文和专著,如“南海神庙大殿复原研究”、“广州光孝寺大雄宝殿大木构架研究”、“广府式殿堂大木结构技术研究”、 “粤东福佬系大木式构架研究”、 “压白尺法初探”和《岭南古代大式殿堂建筑构架研究》对岭南大式殿堂建筑构架和设计方法进行了系统性的总结,提出很多权威的见解等。
  • 李哲扬老师致力于粤东传统建筑大木构架的研究和学习,其论文《潮汕传统建筑大木构架研究》、《潮汕传统大木构架建构方式考察》对粤东潮汕传统建筑的大木构架历史形态、发展规律和建构方式进行了深入挖掘,阐述了潮汕传统建筑体系是广义闽南建筑系统中的一个子系统,同时具有潮汕地区的自身自然地理及发展历史特点,它也是一个相当独立的、具有鲜明个性的区域建筑体系。该文就是针对潮汕传统建筑体系中大木构架的形态式样、构成尺度、设计匠法等多方面进行深入分析研究的一篇论文。同属岭南系统中的一部分,该论文对于本文的写作有很大的参考价值。论文研究的对象包括传统建筑实体与工匠技艺,作者进行了充分的前期调研工作,并通过匠师访谈、摄影、测绘等方式,获得了大量的第一手资料,首次披露建筑“水布”做法等内容,论文中,对影响潮汕地区传统建筑体系产生与发展的历史、地理因素进行了综合的分析,并简要地回顾了该区传统建筑的发展历史。对部分潮汕传统建筑名词进行了收集、整理、图解等的工作。其中选取了六个突出的殿堂实例,作为整体构架设计的分析对象,着重研究分析了它们的尺度构成设计特点;其余的众多实例则为研究该区传统大木构架的时代特征提供了实物支持。文中对于潮汕大木构架设计法的探讨,主要着力于尺度、用尺、尺法等的探讨,还对潮汕传统建筑中所蕴涵沉淀的古制源流进行了考证分析。
  • 东南大学张玉瑜博士的论文《实践中的营造智慧—福建传统大木匠师技艺抢救性研究》,致力于福建传统营造技艺的抢救性研究,通过现场调研查访,对大木构架设计的主导者—匠师进行系统研究,对福建地区传统建造体系中的木匠技术和大木作技术进行记录、解构与分析研究,另外也对木作雕刻、油作、漆作等进行分析。来揭示左右大木构架设计过程中派系师承、设计尺法、风俗禁忌等影响因素,开拓了大木构架研究的新思路。
  • 台湾著名学者李乾朗先生对台湾传统建筑匠艺进行研究整理,对台湾地区传统建筑设计手法进行系统总结。认为对于中国传统建筑营造而言,中国建筑具有顽强的延续性;继承多于发展;中国建筑具有强烈的普遍性;虽地方与官式、地方之间存有差异,但汉地建筑同属一个建筑文化和技术体系;营造技术体系是实践基础上的操作系统,有其特定规律,如简明性、方便性、习惯性等。

国内对于粤北大木构架研究现状

  • 粤北地区木构架目前的研究少有问津。近年来,程建军教授主持了粤北地区几处学宫的修缮项目,并多次带领博硕研究生深入粤北地区,对古建筑考察研究。
  • 程建军《岭南古代大式殿堂建筑构架研究》该文讲述了岭南古代建筑的地理与历史文化背景,岭南大式殿堂建筑的概念与类型,岭南大式殿堂建筑木构架形式分析,岭南大式殿堂建筑木构架形式再分析,广府大式殿堂建筑木构架的时代特征与加工工艺,岭南大式殿堂建筑木构架中的古制,粤东福佬系大式殿堂大木构架名词与设计方法,岭南古建筑与日本古建筑的关系。文中涉及到粤北地区的韶州府学宫大成殿,并对其进行了简要分析。
  • 民居系列丛书《广东民居》中对粤北地区民居及祠堂的木构架进行了部分研究。广东工业大学的朱雪梅教授《粤北地区传统村落形态和建筑文化研究》关于粤北地区的传统民居构架类型有部分研究,还有一些学者对于粤北的构架有零星的研究工作,更大范围的研究工作尚未展开。
  • 岭南建筑经典丛书岭南古村落系列《走进古村落》粤北卷,从村落聚居入手,谈及村落文化在地域上表现出的水乡文化、山居文化、海洋文化的特点,又因移民南迁及向海外拓展的缘故,同时表现出移民文化和侨乡文化等多样性特征,内容涉及到粤北的12 个古村落,主要从文化、装饰角度来讨论民风、民俗,关于传统建筑营造技术,木构架的相关理论几乎没有提及。

国外相关研究

  • 研究中国传统的木构技术,推而广之到国际范围,那么主要有关联的还是东亚地区,日本、韩国、朝鲜等地区,尤其是日本,在文化上与中国具有同源性,从唐代吸收借鉴中国的传统技术,而后发展自成体系,研究成果比较丰富。
  • “他山之石,可以攻玉”,中国和日本两国建筑文化之间有特定的源流关系,日本建筑界对于大木构架研究的方法、成果值得我们借鉴、学习。日本学者对于建筑理论的研究不仅表现在总体上的全面广泛,而且尤其在建筑细部上,深入而具体。日本建筑史研究作为一门独立的学问,已经取得了很多成果,研究方法也渐趋成熟。浅野清通过遗构修理所作的考古实证性研究,著作《唐招提寺金堂复原考》、《法隆寺建筑综观》、《奈良时代建筑の研究》、《东大寺华法堂的现状及其复原的考察》;关口欣也以遗构为主要研究对象,对所有现存的中世禅宗寺院,从平面、构架、柱高、斗栱、装饰细部等诸方面展开研究,力图寻求其形制,构成及其发展演变的规律,汇编成专集《禅宗建筑的研究》等。关野贞先生试图将一个新的科学方法应用于建筑考古学研究上,提出判定建筑年代时可以根据营造尺的性质进行尺度判断,并在其广泛应用于后期日本建筑史的研究上。
  • 学者竹岛卓一的《营造法式の研究》全书共三卷。对于总制、泥作、砖作、彩画作、窑作等进行了系统的体系研究。并将《营造法式》各卷均译成日文。河田克博等重点研究了日本近世建筑书中唐样建筑的设计技法。
  • 日本铃木充教授对于中国的《营造法原》进行了分类研究,分别从解题与台基、平房楼房、提栈、厅堂总论、厅堂及其材料等五个部分展开讨论,是对《营造法原》比较系统的研究,也为更多日本学者了解江南营造做法提供资料。
  • 欧美。由于中西方文化背景差异较大,欧美对于中国古典建筑的研究较少,以翻译介绍为主,少有专题研究。Else Glahn 著《Chinese Building Standards in the 12th Century》(1981)将《营造法式》译为英文,增加了《营造法式》的国际知名度,该书以翻译为主,注解为辅,少有研究。关于中国古典建筑研究有 Nancy Shatzman Steinhardt 的专著《Liao Architecture》和期刊论文 The Tang Architectural Icon and the Politics of Chinese Architectural History(2004)。

 

Timber Decay in Buildings

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From the book Timber Decay in Buildings: The Conservation Approach to Treatment, by Brian Ridout, 2013

Preface

  • There are two camps: remedial timber treatment industry vs. environmental control (drying etc.)
  • The epithet ‘preservative’, although doubtless a powerful marketing device, is an unfortunate generic name to apply to biocides for use on timber, because it implies that decay will inevitably occur unless the timber is given some form of treatment. Yet timber is easily preserved by a dry environment. 
  • Current legislation requires that precautionary treatments be justified
  • Some organisations: BRE & British Wood Preserving and Damp Proofing Association

Chapter 1: Origins and Durability of Building Timber

  • Wood is not the uniform material it appears to be

Between Softwood and Hardwood

  • The terms ‘softwood’ and ‘hardwood’ are used extensively within the timber trade, and frequently lead to confusion. Softwood refers to the conifers, the needle-leafed or cone-bearing trees (for example pine and cedar), some of which provide quite hard timber. Hardwood is used to describe timber from the so-called broad-leafed trees (for example oak and mahogany) and includes species whose wood is in fact very soft. Nonetheless, the distinction between softwood from cone-bearing trees and hardwood from broad-leafed trees remains extremely useful.
  • The softwood trees are botanically known as gymnosperms (from the Greek: naked-seeded) because the seeds develop exposed on the surface of a cone scale. The gymnosperms living today are the representatives of a group that extends back in time for more than 300 million years (Sporne, 1965). Modern softwood trees are mostly restricted to regions where the climate is harsh and the soil is poor in nutrients. Their ability to survive in these areas derives to a large extent from an ability to restrict water loss, by the possession of a water-conducting system controlled by valves, and by narrow waxy needles that restrict vapour loss.
  • The hardwood trees are known botanically as angiosperms, or hidden-seeded plants, because the seeds develop enclosed in an ovary which eventually becomes a seed capsule. They are a more recent addition to the flora, and do not appear in the fossil record until about 100 million years ago (Sporne, 1974). The majority of the hardwoods have broad leaves to maximize light absorption, and an open water conducting system. They tend to favour environments where conditions are suitable for prolonged vigorous growth, although many are able to tolerate poor soils and harsh weather.
  • These differences in efficiency of water transportation and habitat have a considerable effect on the structure of their timbers.
  • At the molecular level, softwood trees and hardwood trees are similar, even though the exact make-up of the lignin and hemicellulose components may differ. At the structural level, however, there are differences that relate to environment and growth form. Softwood trees have leaves and wood structure that reduce water loss in harsh environments (Rundel and Yoder, 1998). But these environments, particularly the colder environments, impose restraints on form. Damage from high winds and snowfall is easier to avoid if the tree has a straight trunk and an open-branched structure (Figure 1.6): wind resistance is reduced and snow falls off as the branches bend down under its weight. The tree can maintain sufficient growth by maximizing conduction within the tracheids during the early vigorous growing season, when the tracheid walls are thin (earlywood cells), and maximizing strength by thickening the tracheid walls during the latter part of the year (latewood cells). The two types of tissue together produce a more or less distinctive annual ring. Storage requirements are low because the tree can maintain a longer growing period by minimizing water loss, and the soil is of poor quality.
  • Hardwood trees have a rather different lifestyle. They favour milder climates where water and nutrients are more plentiful. These conditions mean that the tree can sustain a bulky growth form if competition will allow, producing a wide crown of leaves for photosynthesis. If growth is affected by direct competition from other trees, as in an oak woodland, then long straight trunks will be produced, pushing the leafy head towards the light (Figure 1.7). In contrast, parkland growth allows plenty of space, and produces oaks with a lower and more spreading form because light is readily available (Figure 1.8). In economic terms, the forest growth produces a greater volume of better quality timber. Modern forestry techniques aim to maximize growth by giving space, while providing enough competition to maintain quality. A thicker stem and wider crown require stronger wood, and the larger species tend to have denser timber and a high volume of structural fibres. Tree roots cannot function at soil temperatures below 4 ° C so water vapour lost from the leaves during cold periods cannot be replaced from the soil. Hardwood trees, with thin-cuticled broad leaves that maximize photosynthesis, must either drop their leaves when the ground is cold, and become dormant, or risk desiccation and the loss of leaves (and the nutrients they contain) as a result of frost damage. If leaf fall is part of the tree’s natural strategy then nutrients can be withdrawn, and the leaf sealed from the stem. The leaf then dies, changes colour, and falls off. Loss of leaves in winter also reduces wind resistance and snow deposition, thus reducing mechanical stress to the tree during adverse weather.
  • Substantial storage facilities are required, and many hardwood trees have wide rays which add considerably to the decorative qualities of timber such as oak by producing a patterned grain. When the favourable season returns the stored material must be rapidly mobilized and photosynthesis maximized. This is aided by an open and direct water-conducting system.

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  • The start of heartwood production varies. In European redwood in southern Norway it has been shown to commence around the pith after 20 years (Uusvaara, 1974) but the starting point apparently depends upon latitude. Bruun and Wilberg (1964) demonstrated heartwood production after 30– 40 years in Finnish redwood, whereas Hägglund (1935) showed that it commenced after about 25 years in southern Sweden and 70 years in northern Sweden.
  • (In the living tree) The woody tissue of the tree is protected from attack by the living sapwood, which is mostly too wet to be susceptible to colonization by pathogens (Hudson, 1986), and is also able to respond in a variety of ways to infection. Sap rots do sometimes occur in living trees if the sapwood is damaged, but the decay is usually localized.
  • If a wound occurs in the sapwood zone of a living tree then the tissue can respond by compartmentalizing the damage and any consequent infection (Shigo, 1983). The wound is healed by the stimulation of growth hormone production, which in turn stimulates the production of callus tissue over the surface. Damaged and diseased sapwood is isolated by the production and deposition of tyloses, gums, resins and other toxic materials in wall-like zones which box in the injury (Shain, 1979). If the wound penetrates to the inert heartwood, then the barrier will be incomplete and decay may occur.
  • It is the dead heartwood in the living tree which is vulnerable to a greater or lesser degree, depending on its natural durability. This difference in durability between the sapwood and heartwood is, as described presently, reversed when the tree dies. Most fungi appear to enter the heartwood zone via small dead branches, sapwood wounds, broken tops and roots. Decay may be categorized as a butt rot if it is at the base of the tree, or a heart rot if it is further up the stem. The molecular and structural organization of wood is not the only influence on durability. Age and extractives also have a bearing on durability and resistance to decay.youngvsmature.JPG
  • The period of juvenile growth varies in length and the density/ strength of the timber increases away from the centre. Eventually, the cells reach a maximum length with nearly vertical microfibrils in the S2 layer, and the tree may be considered to be mature. At about this time (usually about 10– 20 growth rings from the centre) the cycle of earlywood and latewood is fully established in those species where this occurs (Krahmer, 1986).
  • A tree may live for a very long time, but eventually vigorous growth slows and the tree becomes senile. Increase in trunk height ceases first; increase in diameter may continue for many years. Growth rings become narrow, and latewood production declines as lignin production decreases, so that the timber produced is brittle. Accompanying these changes are changes within the heartwood, which progress outwards from the pith. These changes, which frequently include the breakdown of extractives and the formation of minute compression fractures, increase the timber’s susceptibility to decay. The centre of the tree is usually invaded by fungi which slowly destroy the core, leaving a hollowed trunk.

Sorption of Water by Timber

  • Distortion caused by shrinkage and swelling is generally recoverable to a large extent, provided that no plastic deformation of the cell walls has occurred. Permanent distortion may, however, arise if there is lateral restraint, perhaps caused by panel surrounds or floorboard fixings.
  • The shrinkage of timber from green to air dry was important in traditional timber-framed construction. Joint pegs provide an interesting example. Oak was usually worked more or less green and would therefore shrink. If the joint pegs behaved in a similar fashion then the joints would loosen, but if the pegs were made from dry timber, which would expand as it absorbed moisture, the joints would be tightened.
  • Dimensional changes which are a response to diurnal or seasonal humidity fluctuations are generally called ‘movement’, and are of importance in good quality joinery and carpentry. If timber with a high movement value is used, or timbers with different movement values are mixed, or timber is worked at a substantially higher moisture content than it will achieve in its intended environment, then loose joints and undesirable gaps are likely to occur. Tables that group commercially available timbers by their movement values are published by the Building Research Establishment and other similar organizations. Values are usually quoted for dimensional change from fibre saturation down to 12% moisture content.
  • Sometimes changes in the building environment make undesirable movement inevitable. This type of damage frequently occurs when central heating is installed in rooms with window linings and dado panelling. Heat from radiators placed in front of the windows may cause localized lowering of humidities to 25– 35% and timber moisture contents to 6– 8%. The resulting movement of the timber will be readily visible, and is frequently mistaken for decay. If some concealed decay is present, perhaps as a result of water penetration 200 years earlier, substantial buckling and distortion may occur. This is a major reason why active dry rot is sometimes reported from dry buildings after restoration works.
  • Responses to moisture vary according to the age of the timber.

Decay

  • Decay commences in softwood timber by the breakdown of pit membranes, thus increasing porosity. This process is sometimes encouraged, particularly for species which are difficult to treat with preservatives, by storage in water. Increased porosity can, however, lead to excessive and uneven uptake of preservative, which may cause surface bleeding, and to difficulties with the application of glues and coatings. Decay by brown and white rot fungi will substantially alter moisture uptake. It will be remembered that water molecules are held by the cellulose/ hemicellulose within the timber and it is these structural polymers that are destroyed by brown rots. Brown rots therefore cause a sharp drop in equilibrium moisture contents, particularly during the early stages of decay. White rots remove lignin and the proportion of cellulose exposed increases. Timber decayed by white rots therefore tends to have a higher equilibrium moisture content once about 60% weight loss has been exceeded. Moisture meter readings should not normally be taken from decayed sections of timber.
  • The moisture level in Guangzhou is high – there is high level of air moisture and also seasonal monsoons. How does this affect timber structures there?

Chapter 3: Post-Harvest Changes and Decay

Effects of Moisture Contents

  • It has been shown that it is difficult for most decay organisms to exploit the living tree (see Section 1.5). The nutrient-rich sapwood, the most vulnerable part of the actively growing tree, is too wet for colonization by most organisms, and the living tissue can produce growths that check the spread of pathogens. Decay fungi therefore mostly enter via dead or damaged tissue, particularly branch wounds and roots. The fungus still has to overcome the natural resistance of the tree, and also any gums, resins or phenolic compounds produced at the site of wounds, which inhibit spore germination and compartmentalize fungal growth. It is easier for fungi to attack the juvenile wood at the centre, where the concentration of antiseptic extractives and the moisture content are lower. The most common forms of decay in standing trees are therefore butt or heart rots.
  • Because of its moisture content, sapwood may weigh twice as much when green as it does after oven drying. Heartwood is usually drier in softwoods (and in some hardwoods); in some pines, for example, moisture levels are about 120% of the dry weight in the sapwood and 35% in the heartwood. This situation changes when the tree is felled. The moisture content is slowly reduced as the timber dries, to about 17– 20% in the wood yard and perhaps about 15% in a cool building. In a heated building the moisture content may drop to less than 9%. As the water content drops and the air content increases, a variety of changes, including decay, may occur. Decay remains a possibility until the timber moisture content is too low for the relevant organisms to survive.
  • The progressive moisture loss, from living tree through felled log, conversion and finally incorporation into a building, presents a wide range of conditions which different decay organisms may exploit, although the extractives may remain an insurmountable obstacle in some timber species. Sapwood, which was a resistant material in the living tree, becomes highly susceptible to decay as it dries, because of its elevated nutrient content.
  • Wood decay fungi and insects vary widely in the moisture contents they can tolerate. Most require relatively high timber moisture levels in order to thrive. Some groups can cope with drier conditions, but as moisture content decreases, so does the variety of decay organisms. Moisture levels below about 18% provide a harsh environment for all except a few specialized insects, which probably derive a proportion of their water requirements from the breakdown of cellulose. In temperate climates, most of these specialists are beetles; in warmer regions termites have a major economic impact. In general, fungi in buildings will not damage timber with a moisture content below about 22%, whereas colonies of most wood-boring beetles will not thrive at moisture levels below about 12%. A few may continue their attack at moisture levels as low as 8%. These differences enable the classification of biological hazards (see Section 3.5).
  • It is important to remember that damp timber provides an environment which will ultimately be colonized and decayed by a succession of organisms. The types of organisms that take part in this progression and the speed of attack depend on parameters which include moisture content, temperature and the different chemicals present in the wood. Preservative treatments may inhibit or destroy some organisms, but preservatives are really only changing the environment, and sooner or later a suitably tolerant sequence of colonizers will reach the damp timber and commence its destruction. These colonizers need not themselves be decay organisms. Benign microfungi, for example, may modify the toxins so that decay fungi may develop. Wood is only immune from decay if it is kept dry.
  • Decay organisms may be classified by their ability to degrade the components of the timber, and they will range from cell content feeders in sapwood, to those capable of destroying the entire cell wall of sapwood and heartwood.
  • So basically how moisture works in timber is that the green timber comes with lots of water, then during the conversion process, the timber is dried out to a certain percentage of water content. If the percentage is not low enough, organisms might start to attack it. Preservative treatments might help but really still depends on the drying. These organisms eat away at the wood mass and increase its porosity (decreases density). Timber that is always submerged in water does not have the same issue because due to the absence of air, not many organisms can attack at the timber. (is that true?) So it is really a combination of heat, moisture and air that creates decay in timber. 

Nutrient availability after conversion: the potential for decay

  • Wood is not an easy food source for decay organisms to exploit. Timber conversion may increase the problem for decay organisms because soluble nutrients tend to be carried to the surface as the timber dries and large amounts of this surface zone are lost when the timber is planed and worked.

Damage to timber can be caused by

  • Insects
  • Fungus (moulds and stains or soft rot)
    • Mould and stain fungi (fungi imperfecti) are mostly cell-content feeders that exploit the nutrients in sapwood. They usually do little significant damage (Viitanen and Ritschkoff, 1991b), although the bulk of pigmented hyphae produced by stain fungi may cause the discoloration of damp timber. They do, however, have a primary part to play in the natural cycle of decay by increasing porosity and detoxifying some natural fungicides. Colonization by moulds is facilitated by some types of bacteria that break down the pit membranes within the sapwood (Blanchette et al., 1990). Stain fungi are more independent than other moulds, and travel from cell to cell by boring fine holes in the walls. Viitanen and Ritschkoff (1991b) grew a range of common moulds on redwood and spruce. They concluded that the lowest air relative humidity for growth on sapwood was 80%, and that growth was very slow at this humidity level.
    • Soft rot damage progresses slowly as a surface decay in wet timber, and the wood has a fine surface checking, similar in appearance to brown rot damage, when it dries. The fungi can, however, tolerate a wide range of environmental conditions, and dry timber which is intermittently wetted may eventually be destroyed (Savory, 1955). Savory also showed that the brash surface frequently found on timbers where there were no other indications of fungal decay, could usually be attributed to soft rot fungi. Significant rot damage is more common in timber exposed in soil or aquatic environments than it is in buildings. This restriction has been ascribed to a lack of additional
    • The primary division of decay into white rots and brown rots, according to the colour of the visible damage, was proposed by Hartig in 1874. The fungi that cause white rot (Basidiomycete; Figure 3.3) grow within cell cavities and attack all constituents of the cell wall from the lumen inwards. Some white rot fungi commence by attacking hemicelluloses and lignin, whereas others utilize all cell-wall components at the same rate. Decay commences with the depolymerization of the hemicellulose. The resulting decay appears as a mass of white fibres, and weight loss may eventually exceed 95% (Zabel and Morrell, 1992). White rot decay is predominantly associated with hardwoods (Hudson, 1986). The reason seems to be that most of the white rot fungi have difficulty in attacking the lignin unit which predominates in softwood lignin (see Figure 1.2). White rot fungi in buildings tend to thrive in wetter sites than brown rot fungi; thus they are frequently found in external window sills and below substantial roof leaks. This view is perhaps supported by laboratory decay tests, which showed that white rots required more water than brown rots to achieve an optimal wood-weight loss (Highley and Scheffer, 1970). Examples are oak rot (see Section 8.5.2) and some others of the ‘pore’ fungi (Polyporaceae).
  • Surface degradation caused by mechanical damage
    • The combined effects of light, wind and water movement produce stresses which result in small surface checks and cracks. Surface material loosened in this fashion is eventually lost and erosion takes place. This process is mostly very slow, published estimates ranging from 1 to 7 mm per century (Kühne et al., 1972; Browne, 1960). Erosion varies with the type of tissue: the softer earlywood is likely to be preferentially removed, leaving behind ridges of latewood (Feist and Mraz, 1978). A severe artificial form of this type of damage will occur when softwood is cleaned by grit blasting (see Section 12.3).
    • Both alkaline and acid attack may occur in damp timber if there are partially embedded iron fastenings (Figure 3.9). The resulting electrochemical reaction is driven by differential oxygen availability (Baker, 1974). In essence, the exposed portion of the fastening acts as a cathode while the concealed section acts as an anode. Electron flow from the anode to the cathode produces corrosion products at the anode which may be hydrolyzed to free acid. A progressive and concomitant increase in alkalinity at the cathode produces an increasingly alkaline environment so that surface degradation also occurs. Blue/ black staining frequently spreads from the cathode, particularly in oak, as soluble iron salts react with tannins to form iron tannate.

Acidity and Corrosion of metals by timber

  • Fresh oak tends to contain particularly high levels of acetic acid. This can often cause metal corrosion, so fixings in oak must be corrosion resistant.
  • It is worth noting that kiln drying tends to raise acidity

Drying and wetting: A historical perspective on timber decay within buildings changes in moisture content after felling

  • Timber is not a homogeneous solid like a metal which contracts and expands equally in all directions. It has different internal arrangements along the grain and across it, so that it shrinks and swells unevenly as it loses or gains moisture. Green mature timber shrinks little along its length, more across the radial face and much more across the tangential face. Tangential shrinkage is usually 1.5– 2 times as great as radial shrinkage. Unequal shrinkage causes stresses within the log that lead to splits along the weakest tissue, which is the rays. Sawn timber subjected to uneven drying will warp, the severity and type of distortion being dependent on the manner in which the log was cut.
  • Sawn building timbers therefore have to be dried to a level that does not suit the majority of decay organisms, but in a controlled fashion so that distortions are minimized. This process is known as seasoning, a term that probably derives from the practice of leaving oak logs and ships’ frames for several seasons in order to rid them of saps liable to ‘ferment’ (Bowden, 1815, pp. 85– 93).
  • Nowadays seasoning is usually undertaken in a more controlled fashion and its purpose is to make the timber as stable as possible with regard to both distortion and decay. Seasoned timber is also lighter (therefore less costly to transport), stronger, holds nails better and is easier to machine, paint and glue. It also usually accepts chemical preservatives readily because the wood cells are empty of liquid. Ideally the less durable timbers should be converted as soon after felling as practicable because they are susceptible to a wide variety of decay organisms.
  • If the bark is completely removed from the log before or after felling, then uneven drying and the release of stresses may cause severe permanent shakes to develop from the outside inwards. This reduces the amount of usable timber that can be cut from the log. However, if the bark is left on the log to retard drying, a different set of problems occurs; these include end splitting and staining as well as attack by wood-boring beetles and fungi.
  • Different timbers vary considerably in their anatomical structure and in their physical properties, and there is a wide variation also within any one species. It follows, therefore, that although the drying of all wood, regardless of its species and size, is governed by the same physical laws, there are great differences in the drying conditions that can safely be imposed. Water seasoning is worth a mention here even though it is not a method of drying, because it is an old-established practice. If logs remain waterlogged then starch is removed and with it the risk of powder post beetle damage when the timber dries. Fungi are also inhibited but bacterial attack may increase the porosity of the sapwood. This is not necessarily a disadvantage because it may aid the penetration of preservatives.

Air Drying

  • Air drying (Figure 11.1) is probably the oldest technique available. The timbers must be stacked in a way that allows adequate, even air flow and prevents distortion. Correct stacking is of considerable importance, as the diameter of the felled trees decreases and the risk of distortion, particularly cupping, in the resulting smaller section timber increases (Wengert and Denig, 1995). Planks and other timbers are normally stacked horizontally with thin spacing timbers of uniform size, known as stickers, between them. Stickers are placed so as to form vertical lines throughout the stack, usually at about 300 mm centres, in order to avoid unequal stresses. Stickers have to be kept thoroughly dry in order to avoid staining in susceptible timbers (Wengert, 1990). The rate of drying may be controlled to some extent by the thickness of the stickers and their distance apart. Heavy weights may be placed evenly on top of the stack to minimize distortion. If stacks are too large for adequate ventilation then the air becomes saturated and drying ceases. Maximum dimensions have been given as 4 m wide and 5 m high with a 1 m minimum wide passageway between stacks. A suitable cover is constructed over the top to protect the stack from rain. The edges of the covering should extend beyond the sides of the pile to prevent rainwater from dripping onto the timber (Desch and Dinwoodie, 1981; Rietz and Page, 1975). The cover may be a shed, with or without fan-assisted ventilation (Figure 11.2).
  • If carefully piled, the timber may dry to a moisture content of about 17– 20%. The time taken depends on numerous factors, including type and thickness of timber, season of the year (see Section 12.7), weather, and dry conditions within the yard. The latter is important because if the area around the stack is cluttered and damp then drying will be impaired. Good foundations (i.e. a flat surface that is sufficiently firm to take the weight of the timber) are also required to prevent deflection (Food and Agriculture Organisation, 1986). If conditions are satisfactory, 25 mm-thick redwood should air dry in about 3 months. Hardwoods such as beech and oak of similar dimension may take 7 or 8 months, and it is usual to add one year for each extra 25 mm of thickness (Desch and Dinwoodie, 1981). End shakes are the most common form of defect during drying; they occur because of fast evaporation from end grain. They may be avoided by using a sealant, or by using some form of end covering.

Kiln drying

  • Natural seasoning does not produce dry enough timbers for many purposes and requires a large amount of yard space for a long time. The timber industry in many parts of the world is having to become more efficient and competitive. As the cost of logs and the demand for dry timber rise, storage time and drying degrade have to be kept to a minimum if profits are to be maintained. Kiln drying (Figure 11.3) is therefore frequently more economical, but a range of defects may still occur. There are two conventional types of ventilated kiln. In the first the conditions are uniform and the timber remains stationary within the kiln for a set period of time while being dried by a current of hot, moist air. The second type of kiln is long and conditions become progressively hotter and less humid along its length. The timber is slowly moved from one end to the other as drying proceeds. The second type of kiln is more difficult to control, and is normally only used for softwoods, which can safely withstand faster drying than hardwoods. These kilns may assist in avoiding a high proportion of kiln-induced defects which have been shown to be caused by variations in moisture content in both the timber and the kiln. Defects are mostly produced by localized variations in conditions within the kiln. Figure 11.2 Air drying under cover in a drying shed. Timber is stacked in the kiln in the same manner as for air drying, but great care has to be taken because the risk of defects occurring is far higher under the more severe kiln-drying regime. It is essential to ensure that loads are composed of similar species and sizes. Different species of timber are dried according to schedules that have been developed by trial and error in order to hasten drying and reduce defects. However, the satisfactory kiln drying of timber depends ultimately upon the degree to which the individual pieces of wood are uniform in structure, upon careful monitoring and upon the skill of the kiln operator. It has not been unusual for up to 25% of pieces passing through a kiln to be subsequently downgraded due to kiln-induced defects (Hart, 1990). The cost implications of this may be substantial (data from Southern Pine Lumber Mills). Figure 11.3 Drying kilns at Henry Venables timber yard. Moisture movement from the core of the timber to the surface is a relatively slow process. If drying proceeds too rapidly because the air is too dry, then the outside of the timber may dry and set while the inside remains damp. The result is known as ‘case hardening’ (Figure 11.4), and differential drying when the plank is resawn will cause severe cupping (Stumbo, 1990). If case hardening continues too far then shrinkage of the tissue at the centre will be restrained by the hardened shell. The result is a mass of small shakes and defects at the centre which are invisible from the surface of the timbers. This defect is known as honeycombing (Figure 11.5).
  • If the timber is very green and evaporation of moisture occurs too rapidly, moisture is forced from the timbers faster than air can be drawn in to replace it. In this case the diminishing volume of water causes the cell walls to cave in, especially in the spring wood, and the whole timber shrinks and warps. This is known appropriately as collapse (Figure 11.6). Most of these defects can be corrected by changing the drying schedule, provided that they are detected early by careful monitoring, and that they do not derive from abnormalities in the wood (Lamb, 1990). Collapse may be avoided by predrying. Most modern kilns today dry a charge of softwood every 24 h although some operate on a 12 h drying cycle (McConnell, 1990). Hardwoods must be dried at a much slower rate if defects are to be minimized, and 25 mm thick oak dried from green will take about 5– 6 weeks. This period may be halved if the timber is air dried first. One problem with conventional kilns is that the water collected from the timber may contain up to 3% of organic extractives, some of which may be hazardous (Singer et al., 1995). Waste disposal must therefore be taken into consideration. These condensates may also cause significant corrosion damage to the kiln (Little and Moschler, 1994).

honeycomb shakes.JPG

(Above) Honeycomb shakes are separations of the fibres caused by drying stresses. These occur after case hardening when the interior of the timber dries.

  • Several other methods of drying have been tried with varying success (an excellent overview is provided by Milota and Wengert, 1995). Drying by dehumidification seems likely to become of increasing importance, particularly for drying pretreated timber, and drying with vacuum kilns is becoming an accepted practice in many parts of Europe. Commercial vacuum kilns can produce a vacuum which halves the boiling point of the water within the timber; the moisture can therefore be driven out at a lower temperature than in a conventional kiln. Timber is frequently dried to a moisture content specified for a particular end use, and its moisture content when supplied should preferably be within about 2% of that specified. Dimensional change for every percentage variation in moisture content may be small, but may nevertheless affect gluing because a glue layer may only be 0.23 mm thick (Taylor, 1994). Care must be taken after drying to ensure that the moisture content remains stable, and that it is not altered by inappropriate transportation, on-site storage, or other building works. The average moisture contents that modern softwood timbers attain in dry buildings are generally about 12– 15% in domestic roofs, and perhaps 12– 14% in floor timbers (above ground floor) and joinery. Heating systems may reduce moisture levels to about 6– 9%, when cracking or joint separation may occur, whereas suspended ground floor timbers may have moisture contents in excess of about 18% – high enough to allow insect attack if sapwood is present.
  • Recent amendments to BS4978 and BS5268 (Part 2) 1977 concerning dry stress-graded structural timbers, under 100 mm thick and for interior use, require that they be graded at a maximum average moisture content of 20%. If these timbers are subsequently pretreated with a water-based preservative then they must be redried to the same level. Timbers must then be marked as ‘Dry’ or ‘KD’ (and ‘Wet’ if undried, and therefore for outdoor use). Kiln drying will make a substantial difference to the cost of timber for repairs. Kiln-dried oak costs about three times as much as green (Venebles, 1993)

Capture.JPG

Figure 11.6 Collapse is a kilning defect that occurs in some timbers if a rapid loss of moisture causes cells to deflate.

  • Large-dimension softwoods also present a problem, and sizes over about 100 × 300 × 6000 mm are likely to be green and prone to shrinkage.

Response on Material Matters

Material considerations of project

  • For the project
    • Skin that wraps around urban spaces to denote a new urban territory
    • Opportunities for vertical attachment of timber (flexibility)
    • The skin becomes occupyable at certain spaces – for attachment
    • How does the skin meet historical materials (bricks & timber & oyster)
    • How does the skin meet new materials (concrete & steel)
  • Properties of skin (which is a line)
    • Linear (directional)
    • Enclosing
    • How it meets other walls
    • How it meets ground
    • How does it open
    • How does it extend vertically and horizontally
    • How does a line/skin embody volume
  • Properties of timer
    • Assembly
    • How it meets the ground
    • How it meets the roof
    • How it interacts with water
      • Duration of its exposure to water (constant, periodic)
      • Type of exposure to water (dripping, soaking, splashing)
      • Exposure to type of water (humidity, rain, snow)
    • How it interacts with time
      • How could it be not only in terms of visual difference, but there is a deeper meaning to the material conferred by time – its demise.
    • How it interacts with activity/play
    • What type of timber is it
    • Quality of the timber sourced
    • Where timber is sourced
    • How timber is treated
  • Replacement for the same material that has similar properties to timber
    • Natural material that reacts with the elements
    • Bamboo?

Production

  • Rendered/montage elevation of part of project (100m2 @ 1:20/1:50)
  • Hand-size mini model of project (or part of it) that distils the material/formal character of project
  • 3x precedents:
    • they should be videos – that show the passing of time
  1. one architectural
  1. one natural/found (a forest, an airplane, a potato)
  2. one found image/photo of a phenomena, a situation, a texture… a painting… a print.
  • Photos from fieldwork