1. INTRODUCTION

India has a long and rich history of use of timber across various regions of the country. Historic timber structures, as defined in the Wood Committee’s Principles (ICOMOS, 1999)[1] are all types of buildings or constructions wholly or partially in timber which are of cultural significance or which form constituent parts of a historic area. Because of the lightness of the material and ductility of the connections, wooden structures are more efficient than other construction material systems; this is one of the reasons for their diffusion all over the world (Semplici & Tampone, 2006).

The short and robust variety of timber found in India is generally sourced from deciduous trees found in all regions of the country except the north, northeast and Himalayan ranges where a softer variety of timber is found[2]. In areas where access to timber was limited, its usage has been restricted to decorative elements, reserved for the buildings of royalty and rich merchant classes (Rao, 2018). The trees used for construction grade timber include different kinds of Teak, Rosewood, Sal, Sandalwood, Sheesham, Oak, Pine, Jack, Irul, Mahua, Deodar, Mahogany, Mango, Neem, Palm as well as a host of locally available varieties (Rao, 2018).

Historic timber structures were often constructed of old growth indigenous timber and the scarcity of quality timber and lack of tradesmen with traditional carpentry skills pose challenges to timber heritage conservation in India (Chan, 2011). In old days, trees were cut and taken down to the stream in the form of long lumber and put together to the rafts and transported to the markets. This transportation on the water took a lot of time, but during this time, the surplus sap and resin was drained out, making the timber best quality for use (Ito, 2006). Conservation of existing timber structures is, therefore, critical to ensure that such irreplaceable heritage fabric and the associated skills are not lost; while it is desirable to conserve such structures, the sourcing of matching timber for repairs is challenging and needs to be minimized to preserve our natural resources (Chan, 2011).

The International Wood Committee’s Principles states that the preservation of historic timber structures starts in the forest and thus it establishes a link between cultural heritage preservation and the conservation and sustainable use of forest resources (ICOMOS, 1999). Institutions responsible for the preservation and conservation of historic structures and sites should establish or encourage the establishment of timber reserves appropriate for such work (Ito, 2006).

 

2. CONSTRUCTION QUALITIES OF TIMBER

Factors that influence the properties and longevity of timber include the growing condition of the tree, the way the wood is cut, the seasoning method, its use and surface finishes (e.g. axed vs. sawn surface, the application of protective coatings) (Chan, 2011). Due to the anisotropic nature (transverse, radial and tangential directions of wood) of the material, the properties and behavior of wood not only vary greatly between different species but also within the same species (Chan, 2011). No two pieces of wood are the same, and it is due to this unique quality that makes timber such a versatile and aesthetically interesting material (Porteous & Kermani, 2013).

The following are some properties that qualify wood as a versatile building material:

• Wood has great strength to weight relationship.
• Timber has relatively good compression, tension, bending and torsion strengths.
• It can provide long spans with single pieces or can be built up with systems of bits.
• It can provide a total structural mechanism or can be part of a combined system.
• It is relatively easy to mend or add to the timber structure.
• It can be manipulated with precision cutting for structure and ornamentation.
• Wood can be driven into the ground for displacement and skin friction piles.
• It can be nailed and glued as per requirement.
• Timber considerably dampens the effects of impacts, vibrations and noises and does not easily chip and small scale denting can be ‘ironed-out’.
• Wood has longevity comparable with stone and brick and generally better than soil.

 

3. STRUCTURAL USE OF TIMBER

Timber, in India, has been extensively used in roofing, especially in the timber rich southern parts of the Peninsula and is still much in use in the west coastal regions, right from Kerala[3] to Konkan (Rao, 2018)[4]. The dhajji technique[5] prevalent in the Himalayan region uses evenly spaced timber framing around stone or mud brick. This technique demonstrated exceptional resistance to seismic activity during the 2005 earthquake in Kashmir in comparison to modern materials[6] (Schacher & Qaisar, 2009).

Wood is the only one of the common traditional building materials that consistently comes in long lengths – promoting its use for monolithic posts, plates, beams, braces, struts and planks. When shaped and jointed or glued the lengths together form frames and trusses (Hughes, 2006). The structural use of timber is most often seen in the form of long sections of wood as beams or columns or as tie members at the foundation level. The greatest degree of variation seen in timber construction in India is with respect to joinery techniques that depend largely on the type of timber used and the degree of craftsmanship available (Rao, 2018).

 

4. DECAY OF TIMBER

Wood as an organic material may decay and degrade rapidly compared to stone[7], brick or other major historic building materials, as a result of moisture, fungi, insect attack or fire.

• Wood, being an organic material, is actively affected by relative humidity, which causes it to expand and contract. These result in organic decay of the timber caused by fungi resulting in dry rot, wet rot, heart rot, brown rot, insects like beetles and termites, and marine borers (Rao, 2018).
• Defects in wooden structural members, whether at the joints or elsewhere, are more critical to a structure’s load-bearing capacity than other primary building materials (Larsen & Marstein, 2016).
• Timber decay is usually caused by a common set of conditions (e.g. moisture, insects, ultraviolet [UV]), but decay manifests itself in different ways due to variable factors such as timber species and site conditions (Chan, 2011).
• Other risks such as natural disasters, fire, vandalism and theft are also factors of deterioration.
• Building extensions or modifications that block off ventilation could result in the rapid deterioration of original building fabric (Chan, 2011).
• Issues such as changes in use, redundancy of buildings due to socio-economic developments, the use of substitute modern materials in repairs, and appropriate interpretation/ reconstruction approaches also affect the structural integrity of timber structures (Chan, 2011).
• Often roof is the most distressed part of any heritage building in India and depleting roof maintenance skills add to this broad issue. Decay is more pronounced and difficult to address in abandoned historic buildings with no human activity. Extreme fluctuations in temperature and moisture lead to alterations in the moisture and oil content within timber, further accelerating its decay (Rao, 2018).

 

5. ECOLOGICAL APPROACH OF TIMBER PRESERVATION

Whenever historic timber structures are repaired, some decayed and damaged parts must be replaced by new timbers. Nowadays, the quantity of replacement timbers is minimized as much as possible, but it is indispensable to replace timbers to some extent. In order to secure future supply of replacement timbers, saving forests is of urgent importance. Therefore, protecting cultural property buildings and saving forest reserves are closely linked (Ito, 2006). The process of distribution of wood has also been completely changed to modern style making it one of the reasons why big and long timbers are hard to procure. Another difficulty is that it takes much time and efforts to cultivate trees. In order to get one big and fine tree, forest owners must bring up many similar trees around it, and in order to make such group of trees it is necessary to plant much more young saplings (Ito, 2006).

One should look at the possibility of preserving or repairing[8] a timber structure using techniques and construction technology which correspond to those used originally. This also includes the use of the traditional dressing tools or machinery (Larsen & Marstein, 2016). For example, the Principles of International Wood Committee advise when a part of a member is replaced, traditional woodwork joints should be used to splice the new part to the existing part, if this is appropriate and compatible with structural requirements (ICOMOS, 1999).

An ecological approach to timber conservation is one which has its roots in respect for the crafts and craftsmanship is required for the preservation of timber structures. Larsen and Marstein recommend that preservation work or repair of historic timber structures should be duplicated, as far as this is possible, the choices that previous generation made, out of respect for their insight, wisdom and knowledge (Larsen & Marstein, 2016). This means that:

• When replacement of a member in a timber structure is necessary, replacement timber of the same species of wood and of a similar quality should be used;
• Tools and techniques identical or similar to those used by previous generations of craftsmen to convert and dress the timber and to assemble the various members should be used.

The reason for this approach is that it is the insight, wisdom and knowledge of previous generations as represented in the structure handed down to us, that constitute the authentic historic document. Only in cases where the original choices have not proved to be durable or sustainable should we opt for modern solutions (Larsen & Marstein, 2016).

 

6. REPAIR OF TIMBER STRUCTURES

New building materials have gradually replaced wood where the material has demanded labor intensive maintenance, such as on roofs and foundations (Wik, 2006). Timber structures prove that complex systems of ‘wooden pieces’ work well and that our forebears fully understood many engineering principles, and were indeed exponents of sophisticated construction (Hughes, 2006). Generally speaking, historic buildings were constructed with timbers of excellent quality. It means that timbers with very fine and straight grains are necessary for replacement (Ito, 2006).

Traditional methods of repair of timber buildings presuppose (Larsen & Marstein, 2016):

• The availability of timber of the same species, grading and scantling as the old, decayed timber which is to be replaced;
• That forest owners are interested in keeping forest reserves with old-growth, ‘mature’ trees;
• The availability of craftsmen, in particular carpenters who are familiar with the handling of traditional tools; and
• The availability of traditional tools.

Physical defects in timber can be verified using a set of Non Destructive Tests (NDTs) using an experienced specialist. Globally used non-destructive tests for timber include (Rao, 2018):

• Visual inspection and species identification
• Pin driving and screw withdrawal method
• Stress wave and ultrasound methods
• Ultrasonic Pulse Velocity (UPV)
• Resistance drilling
• Digital radioscopy

 

Most factors that cause wood to deteriorate can be effectively controlled or eliminated. The fundamental principle is to minimize contact with water, as this would eliminate fungal attack and significantly minimize opportunities for insect infestation. This is commonly achieved by applying surface treatment such as paint, oil and tar (Chan, 2011). Incorporating eaves and drip lines in buildings to protect vertical timber surfaces, keeping timber components off the ground, applying appropriate surface treatments, ensuring good site drainage and ventilation are some of the key considerations in building with timber (Chan, 2011).

The guiding principle is to “do as much as necessary but as little as possible”[9]. Non-invasive, reversible techniques are favored (Chan, 2011).

1. 1. Repair of the local failures (of members, units and connections) is the first step to be taken.
   2. Second step is to ensure the stability of the structure by means of shoring, loosening the connections and reducing the displacements.
   3. If the timber is partly damaged or decayed, as in the end of the joists, beams, columns, struts, purlins, they can be reused on site by metal capping or even providing new timber using different types of joinery available for taking different types of loading patterns, such as, compression, tension, sheer, bending or torsion. Replacement of timber is best done with recycled timber from dismantled buildings as it would be the best selected timber at that time and assuredly well-seasoned too (Rao, 2018).
   4. The integration of the bracings or the application of a systematic bracing is generally fundamental to prevent further displacements and deformations.
   5. Lastly, the replaced timber has to be treated against potential termite attack and other agents of decay before being placed in the building (Rao, 2018)

Various circumstances might arise during the repair which affects the success of conservation work. Change of ownership may diminish the sense of attachment to and responsibility for the property; a change of function may result in misuse, or the ageing of the residents could mean that maintenance and repairs become neglected. In consideration of such possibilities, routine maintenance work should be conscientiously enforced (Shimizu, 2004).

[1] ICOMOS (International Council on Monuments and Sites) is an international non-governmental organization of professionals, dedicated to the conservation of the world's historic monuments and sites.