|GATE - 2/85 - Health, Water and Sanitation (GTZ GATE, 1985, 56 p.)|
Technique for Improved Thatch Roof
by S. N. Mehrotra, N. Verma, A. Datta, Y. K. Batra
Thatch is one of the ancient forms of roofing. In India it is still common, expecially for poor people, and may continue to be so for some time to come. Its popularity is due to the fact that practically all the materials needed are locally available or can be grown nearby. Besides, thatching can usually be done with the skill available locally and the roof is fairly economical.
Thatch roofs constructed in the traditional manner have certain inherent shortcomings. The materials used, being mostly organic, are susceptible to natural weathering. Moreover, they are vulnerable to insect attack and fire hazards. A properly laid and well maintained thatch roof may last many years, but it need partial replacement annually or at the latest every other year.
There have been many attempts to improve the quality of thatch roofing and also to increase its live. A variety of chemical treatments for thatch have been developed, but they have not been found suitable due to the high cost of the chemicals and the lack of technical skill available in rural areas. Moreover, the development of such chemical treatment methods was alien to the basic character of the material and the simplicity of its use.
CBRI (Central Building Research Institute) has developed a new method of laying thatch roofing by using manually pressed thatch panels and, to make the roof waterproof and fire retardant, applying non-erodable mud plaster over them.
Preparation of thatch panels
Bamboo splits 20 mm to 25 mm wide are cut from good quality dry
bamboos about 50 mm in diameter and 3000 mm long, so that the splits are strong
enough. Bamboo mats are prepared by placing bamboo splits across each other
about 230 mm apart and tying them together either with Gl wire of 20 SWG or
alternatively with jute rassi (sutli), whichever is more readily available. The
thatch ist cut to the required size for uniformity, separating small undesirable
pieces from it. The length of the bamboo mat is kept to the size of the room and
the projections, but its width is governed by the length of the thatching
material as well as the dimensions of the slope and the overlap in the panel.
The thatch, of the required thickness (about 100 mm), is laid between two bamboo
mats which are tied together with Gl binding wire of 20 SWG. While pulling the
wire to tie it, the thatcher also applies body pressure and jerks on the joints.
This presses the joints together and reduces the thickness of the thatch. In
this way a panel of the required size is obtained. We call it a "prefabricated
manually pressed thatch panel."
Laying of thatch panels
The supporting bamboos or balks are placed in such a way that their position coincides with the centreline of the overlap of the thatch panel. The panels are lifted and laid starting from the bottom, with a 100 - 125 mm overlap so that they are properly supported on the purlin, and to avoid any leakage through joints. The panels are then tied to the supporting bamboo purlins or balk rafters, as the case may be, with 18 SWG wire.
Treatment to make the panels waterproof and fire-retardant
After the panels have been laid and tied to the supporting structure, they are plastered with non-erodable mud plaster to seal the roof and make it fire-retardant. To make the non-erondable mud plaster a bitumen cutback ist prepared with 1.5 kg of 80/100 grade bitumen, 300 ml of kerosene oil and 15 gm of wax for every 0.03 cubic metre (1 cu.ft.) of soil. This cutback is added to mud mortar containing bhusa (wheat straw) or paddy straw, whichever is available, prepared earlier for traditional mud plaster. The paddy straw may be from 20 mm to 40 mm long. The mud porter is applied to the top surface of the roof as mud plaster with a minimum thickness of 12 mm over the bamboo strips. The average thickness of the mud plaster may be increased to 20 mm for proper finishing. When the plaster is completely dry it may have some shrinkage cracks. These are filled with mud plaster. Two coats of gobri wash (containing cow-dung mixed with bitumen cutback) are applied over the entire roof surface on top and are allowed to dry for 24 hours. The well-dried roof surface is given a further coat of bitumen cutback applied with either a brush or a sprayer, as may be convnient. When this coat is completely dry, say after a day, another coar is applied. The thatch roof is thus made waterproof and fire-retardant.
The roof panel prepared by the above-mentioned technique must be strong enough to sustain stresses and live loads as well as loads occurring during construction and repair of the roof. Moreover, since the roof constructed by this technique has mud plaster on top for waterproofing and fireproofing, it has a higher roof load than a conventional roof. Therefore, the supporting roof frame has to be sufficiently strong for this type of thatch roof. Laboratory tests were conducted on various panels and supporting roof frames to test their resistance to water penetration and fire.
A full-sized panel fixed over the supporting roof frame was tested with a uniformly distributed load, to study the deflection of the panel and supporting structure and the durability of the plaster. A small-sized thatch panel simply supported over a similar span was also tested with a concentraded load equivalent to a man's weight to ensure that the panel did not fall during periodic maintenance and repair. The test results showed that the roof can safely bear a live load of 1 kg/cm². The deflection was found to be within permissible limits and no damage to the plaster was found. To test for water-tightness the panels were subjected to a one-hour test with alternate wetting and drying, a water spry being used to simulate rain. The test showed no erosion of the plastered surface.
A few samples of prefabricated thatch panels 1,000 mm x 1,000 mm were subjected to fire penetration and external fire exposure tests (B. S. 476, Pt. 3). It was found that fire penetration time was more than half an hour.
To study the long-temm performance of manually pressed large size thatch panel roofing, a prototype roof was laid over the verandah of the model hut constructed in Shantinagar. The size of the roof laid over the verandah is 5,700 mm x 3,150 mm. Four panels 5,700 mm x 900 mm were used, with an overlap of 150 mm. The supporting roof frame consists of four wooden balk rafters spaced 1,600 mm apart, fixed onto two wooden balk beams, one at the ridge and the other on the balli posts at the ends of the verandah. Bamboo purlins about 50 mm in diameter were fixed over the balk rafters 750 mm apart, i.e. just below where the overlap of the panel coincides with the lower panel. The thatch panels were then tied to the bamboo purlins with 18 SWG Gl wire. The roof was finished with non-erodable mud plaster at least 12 mm thick, with paddy straw as a binder. The roof was completed in January, 1980 and withstood the following monsoon without any appreciable damage.
On the basis of materials actually used and labour required, the cost of a thatch roof works out at Rs. 26.00 per sq. m., including non-erodable mud plaster at the prevailing market rate at Roorkee.
· Use good-quality bamboos for the splits of the panel frame.
· Use a long needle for piercing the Gl wire for better output.
· The Gl wire should be properly twisted and then flattened to avoid any injury to workmen.
· Cut off excess wire, if any.
Where longer bamboo strips are needed, provide a joint with a minimum overlap of 300 mm.
The technique provides a tidier, better-looking and more durable roof. It shelters the dwellers from strong winds and rain and offers protection from fire, etc. It will be economical in the long run because of the reduced maintenance expenditure and its long life, expendet to be 10 years or more.
The technique offers great scope for utilizing self-help and facilitates cooperative ventures at village level.
Tech and Tools
In July, 1985, thousands of people will come to Nairobi, Kenya, for the two world meetings- one governmental, the other non-governmental to mark the Decade for Women. We mentioned this topic on our last issue of "gate", and would like to return to it now with particular reference to the non-governmental meeting "Tech and Tools", which itself claims to be "an appropriate technology event for women at Forum '85".
"Tech and Tools" will be the site of ongoing strategy sessions, demonstrations, exhibits and practical training related to the ways women have used appropriate technologies in their projects. It is not, however, a simple "show and tell" of technologies. The main objective of "Tech and Tools" is to consider technologies from a multi-disciplinary, multi-organizational perspective... that is, not just a consideration of hardware, but to explore, as well, such issues as: What are successful examples of integrating appropriate technologies into women's income-producing projects? What are effective ways to communicate information about appropriate technology to local groups in your country? How can women pressure training and educational institutions to offer courses relevant to their project work with appropriate technology? How can women take control of - rather than be controlled by - technology? Do appropriate technologies relegate women to the fringe of economic productivity?
The organizers include, among other bodies: International Women's Tribune Centre, 777 UN Plaza, New York, NY 10017, USA. Appropriate Technology Advisory Committee (ATAC), PO Box 61221, Nairobi, Kenya. World YWCA, 37 Quai Wilson, 1201 Geneva, Switzerland. World YWCA, South Pacific region, PO Box 623, Nadi, Fiji.
The intention of the organizers is to explore ways to disseminate results and activities to those who were not able to come to Nairobi and are focusing on plans for a follow-up to the world meetings and the women's AT network that may be started there. For example: a "Catalog of Women's Technologies from around the World" will be produced. This information resource will describe the technologies that were featured and discussed in "Tech and Tools", and provide information about the types of project in which they are being used, as well as the experiences of women's groups in using them.