Durable thatch with stiff-stem grasses
• Thatch is the most commonly used roof covering in the world, although it is barely recognized by construction experts. In India, for example, some 40 million houses are thatched. Almost any vegetable material, from the bark of trees to finely-tapering water reeds, can be used, though grasses, reeds and palms are most common.
• Traditional types of thatch have short durability and performance, but in certain regions (N.W. Europe, Southern Africa, Japan) skilled workmanship produces good quality functional roofing, with life expectancies between 25 and 70 years.
• Thatch uses renewable, local materials requiring minimal or zero artificial energy input in production, and costing less than most other types of roofing. Their application is labour-intensive - an important advantage in terms of employment generation. At the end of their useful life, thatching materials can be composted or compacted for use as fuel.
• The main drawback is its combustibility, but this is significantly reduced through good quality workmanship and common-sense precautions. Thatch is also susceptible to biological decay and weathering.
• The best thatching materials are stiff-stem grasses and reeds of 1 to 2 metres length and up to 10 mm diameter at the cut end. They should be straight (no bends at nodes), tapering and preferably hollow stemmed, as solid culms tend to dry out slowly and thus rot quickly.
Materials: Harvesting and Processing
• Thatch may come from three different sources: first from naturally occurring indigenous vegetation, secondly as a byproduct of food or cash-crop agriculture, and thirdly through the cultivation of a plant grown specifically for thatching.
• Water reed is most durable, but cereal straw (mainly wheat, but also rye, barley and rice) is more widely available. The less artificial fertilizer is used, the less susceptible they are to fungal attack.
• Harvesting is best done by hand, as modern combine harvesters break the straw. The mature (fully grown, dried) stem is cut about 5 cm above the ground.
• To facilitate tight and even thatching, the straw should be combed (with a hand-held rake) to remove dead leaves and other debris, then bundled and stored in a dry place. (The labour involved in combing the straw will be amply repaid, as it lasts more than twice as long as uncombed straw.)
• The bundles should measure 55 cm in circumference at the binding, which is tied about 30 cm from the cut end. Once bundled the straw is ready for thatching.
• Almost any shape of roof with a minimum pitch of 45° can be thatched. Thatch will mould itself to any curve except a convex-shaped roof.
• Pole timbers and split battens may be used, and simple configurations work best, that is, valleys and other changes of roof pitch are not recommended.
• The structure should be capable of supporting up to 40 kg/m2, which is the weight of the heaviest material - reed.
• A tilting board, 35 mm thicker than subsequent battens, fixed along all the eaves and barges at eave level, is essential to force the first course into tension, making the rest of the thatch more tightly compacted.
• Roof-work tools: pen-knife for opening bundles and cutting ties; leggatt (thatcher's mallet) for beating the thatch upwards to tighten the thatch coat; trimming knives for tidying completed work.
• Grass is sorted: short grass for eaves, gable edges and top course; long grass for rest of roof.
• Thatching begins at a right-hand verge (unless the thatcher is left-handed) and can be worked in vertical lanes (more common) or horizontal sections.
• The first course of thatch performs the same function as the foundations of a wall, and as it has the greatest vulnerability to wind damage, it needs to be very secure.
• Thatch is placed in horizontal layers, approx. 20 cm thick, secured by stitching, layer by layer, at approx. half-way between cut end and ear. Layers overlap as tiling, so fixings are covered and protected. Total thatch thickness is 30 cm. After fixing, the grass is wedged tightly into the ties with a leggatt. The compacted surface forms a pitch, identical to that of the rafters, and exposes only 2 - 3 cm of each stem. A slight lip should be left at the top of each course end will be driven back with the next course to form a neat and invisible junction.
• The ridge is the most vulnerable part of the roof and can be made of a variety of very durable materials, eg half-round burnt clay tiles, sheet metal, ferrocement, but they are expensive and detract from the appearance of the roof. More appealing and cheaper is a flexible grass wrapped over the apex, covering the upper course fixings and held with horizontal stitching.
• Material requirements are approx. 10 bundles grass per m2 of roof area; tough local string or steel wire for fixing ties. Experienced workers should fix 10 to 20 m2 per day.
• Thatch roofs are generally not suitable for rainwater collection, unless a wide gutter - 30 cm minimum - is provided. A method called " tile substitution", developed and tested by Nicolas Hall, makes collection at eaves more efficient.
• Burnt clay tiles are substituted for thatch on the first course, producing a hard, straight cave.
• By doing so, the eaves are significantly strengthened (increasing the life of the roof); only a 10 cm gutter is needed (cheap, easily available, easily fixed); and the fire risk is considerably reduced.
• The main drawback of collecting rainwater from thatch roofs is that debris will first be washed off, contaminating the water. Hence, methods should be employed to discard the first flush of debris laden water.
• A competently-laid grass thatch might last up to 40 years or more, though a grass ridge will need renewal every 8 - 10 years.
•Thatch is combustible and common-sense is the best protection against fire: avoidance of high building densities (urban areas); avoidance of open fires near thatched buildings; avoidance of chimneys, or careful design and construction only at the ridge, well insulated, regularly swept; protection of all electrical fitting in the roof space. In addition, the underside of thatch can be protected by fixing an incombustible board ceiling to the rafters.
• Chemical treatments to reduce the risk of fire, organic decay and weathering are possible, but none are cheap, permanent or of good value, and prohibit rainwater collection.
Further information: Bibl. 12.02, 12.03 and 23.11 or contact Nicolas Hall, 48a Hormead Road, London W9, U.K.
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