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Report 2 of 4:
Timber
Not Termites - Sri Lanka
Introduction
With the increasing population pressures in developing countries, the amount of natural forest is reducing while the demand for wood and wood products rises. In less affluent societies, timber is by far the most important source of structural members for building, purely on account of cost and local availability. It is also a renewable resource and attempts have been made, albeit often on a small-scale, to establish plantations of commercial species in developing countries.
In the modern world, the policy of the foresters is to plant at least enough land to compensate for extraction and natural wastage. In such an operation, trees are planted in close proximity to each other so that in their early stages they grow in slim, upright form with little development of lateral branches. Since the full potential of the site is concentrated on about 200 trees per hectare forming the final crop (in the case of many hardwoods), thinning is carried out at intervals. The thinning operation often provides trees of a quite reasonable size which can be efficiently used in the round form. In most cases, these timber thinnings, or poles, have a fairly thick outer sapwood layer which is easily penetrated by preservatives, and it is sufficient to provide a continuous and reasonably thick outer layer of protected wood.
Timber poles, being almost circular in cross section, are stronger in bending than rectangular timber of similar grade and section. The round pole possesses a very high proportion of the basic strength of its species because knots have less effect on the strength of naturally round timbers compared with sawn sections. Generally, poles can be very easily erected and produce a rigid framework to support the roof and walls of a building and, if necessary, part of the floor as well. Poles need not be embedded in very deep holes because even soil of low bearing strength could easily be improved around each pole, eliminating the need for an excessive depth of setting.
In pole construction, the poles actually have an inherent ability to resist wind uplift, especially if the roof framing is very securely attached to them. Above all, the design of any pole structure can be simple enough for unskilled persons to construct.
Low-cost housing construction
For centuries, people of the villages of Sri Lanka have built houses using materials that are available locally. However, the lifespan of this type of building can be short and it may need replacing every few years. Low cost houses using the same materials can be given extended lives using the techniques described below. Poles for roofs are treated with preservative and properly jointed. Foundations are made from rubble bound with mortar. Walls are made from cement stabilised soil blocks and roof tiles from micro concrete.
Rubble foundations
The foundations are first set out accurately using string lines and stakes. The depth and width of the foundations will depend on local ground conditions. The fill consists of rubble stabilised with cement mortar but bricks or concrete blocks may also be used, if available. The completed foundations must project at least 100mm above ground level and are finished with a concrete screed.
Cement stabilised blocks
The walls are made from soil blocks stabilised with cement. The main local soil (laterite) is ideal for block manufacture. Soils with too much clay or silt are unsuitable. The soil is first crushed and sieved then water and cement are added. The mix is then compressed in a CINVA ram machine to form the block. Blocks must then be spray cured for at least seven days.
Walls are built using normal block laying techniques and a cement/sand mortar (1:6 mix). Walls should be protected from rain during construction with polythene sheets or similar material.
Preservative treatment
The roof is fabricated from small diameter (50-150mm) timber poles treated with preservative in an open tank using the boron diffusion method. The poles are debarked and treated while still green. The tank is filled with water and the correct quantity of preservative is added. A fire is then lit under the tank to warm the solution. The solution is stirred to ensure thorough mixing and, when the solution has reached 50°C, the poles are placed in the tank. After several hours the fire is extinguished and once the solution has cooled down, the poles are removed from the tank, stacked and wrapped in polythene or a similar material. After fourteen days, the treated poles are ready for use in the roof structure.
Health and safety procedures must be followed at all times and operators should wear appropriate protective clothing.
Micro-concrete tiles
Roofing tiles are made from local sand, aggregate and cement (1:1:2 mix). The materials are mixed with water to the correct consistency. A scoop of the mixture is placed in a frame on the vibrating table and levelled with a trowel. The tile nibs are formed and the vibrated screed is then transferred to the mould. A holding-down wire is inserted into the nib. The moulds are stacked for an initial 24 hour curing period after which the tiles are transferred to a water tank for up to two weeks. The cured tiles are laid on battens at 500mm centres. The nib must be located directly against the batten. Tiles are fixed to the battens by nailing through the holding down wire.
Fabrication
The roof consists of a ridge purlin, wallplates and rafters. Wallplates are bedded in 12-25mm of mortar. They overhang the gable wall by 600mm and are joined using nailed half laps. The ridge purlin is supported off struts at internal wall positions. The ridge purlins overhang the gable wall by 600mm and are bedded in mortar. At the struts, they are half-lapped and nailed into the seatings. Once the purlins and wallplates are in place, the rafters can be fixed.
The use of long nails (minimum 125mm) is important to ensure a good fixing. Nailed timber packers may be used if the purlin or wallplate is too low due to lack of straightness. If they are too high the rafter may be notched by up to 12mm. 50mm x 25mm tiling battens are now fixed to the rafters. Timber packers can be used to make up any differences in level caused by lack of straightness between rafters.
How the project contributes to protecting the environment
- increases the efficiency with which the produce of man-made forests can be utilised.
- demonstrates that a natural resource - timber thinnings - currently mostly being wasted, or at best used for fuel, can effectively and economically be used in the construction of permanent buildings.
- reduces, to some degree, the demands on natural forest.
- encourages the use of a resource whose availability is set to increase dramatically. The Food & Agriculture Organisation of the United Nations (FAO) estimates that the area of plantations, worldwide, is set to increase three-fold in the next thirty years.
- encourages the use of a resource with perhaps the lowest production energy of any construction material.
- The life of the timber poles is extended by preservative treatment. Both the preservative used and the resulting treated timber are non-harmful to human beings and have minimal negative environmental impact - rather they conserve an otherwise biodegradable resource.
Project benefits
In addition to such obvious benefits as the provision of high quality, low cost shelter and the increased awareness of environmental issues, the project has introduced local communities to the necessary training and skills which enable self-help construction projects of this nature to be undertaken. Furthermore, the use of treated timber poles in the housing project has eliminated the need for costly sawn timber which must currently be imported to Sri Lanka.
Funding
The project was funded by the Department for International Development of the British Government (formerly Overseas Development Administration) under an Engineering Division research project.
Partner agencies:
TRADA / TRADA Technology Limited
Department of Civil Engineering, University of Moratuwa
Lanka Evangelical Alliance Development Service (LEADS)
State Timber Corporation
For further information, please contact:
Lionel Jayanetti
Overseas Operations TRADA TECHNOLOGY
Chiltern House
Stocking Lane
Hughenden Valley
High Wycombe
Buckinghamshire
HPI4 4ND
United Kingdom
Tel: +44 (0) 1494 563091
Fax: +44 (0) 1494 565487
E-mail: dljayanetti@ttlchiltern.co.uk |
ITDG would like to acknowledge Lionel Jayanetti, the head of TRADA International, for providing the original paper on TRADA Technology’s approach to affordable shelter in Sri Lanka.
This document is an output from a project funded by the UK Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of the DFID.
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