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THATCHING
Thatching is the art and craft of covering a roof with vegetation such as straw, water reed, sedge, rushes and heather. It is probably the oldest roofing material and has been used in both tropical and temperate climates throughout the world. Thatch is still commonly employed by traditional builders in developing countries, usually with low-cost, local vegetation. In Western Europe it is no longer a roofing material of the poor, however, and the costs of thatching a roof have increased markedly over the past 40 years.
Thatch material
There are more thatched roofs in the United Kingdom than in any other European country. The traditional material in most of England is wheat straw, which is now produced by specialist growers. Good quality thatching straw can last for more than 45-50 years when applied by a skilled thatcher. Traditionally, a new layer of straw was simply applied over the weathered surface. This has generated accumulations of thatch 2.0 m thick and in 250 ancient buildings preserved lower layers of medieval thatch over 600 years old [1].
Water reed, which was historically used in East Anglia and Eastern England, is a one-coat material; weathered reed is usually stripped and replaced by a new layer. Today almost half of England's thatched roofs are thatched with water reed, 90% of which is imported from Turkey and Eastern Europe. Although water reed has been known to last for more than 70 years on steep roofs in dry climates, modern imported water reed on an average roof in most parts of England will not last any longer than good quality wheat straw.
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Letts, John 2000. Smoke Blackened Thatch: a unique source of late medieval plant remains from Southern England. Reading & London: The University of Reading and English Heritage
Flammability
Thatch roofs do not catch fire any more frequently than roofs covered with 'hard' materials, but thatch fires are difficult to extinguish once they take hold. Old buildings often have poor quality chimneys, and most fires occur in the winter when hot gases break through a poor quality flue or chimney and ignite the thatch surrounding the chimney. Insurance premiums are higher than average because when a fire does occur, the damage is more severe and the thatch is more expensive to replace than with a standard tiled/slate roof. Workmen should never be allowed to use an open flame near thatch, and nothing should be burnt that could fly up the chimney and ignite the surface of the thatch. Spark arrestors also usually cause more damage than good as they are easily blocked and reduce air flow (it should be noted that not all experts agree on this point).
A spray on fire retardant or pressure impregnated fire retardant can greatly reduce the of spread of flame. Most thatch fires do not begin on the surface of the thatch in any case (the surface is usually damp in the UK), but in the thatch surrounding the chimney. While it is true that some fire retardants may expediate the decay of the roof, it is not the case with all well formulated products.
On new buildings a solid fire retardant barrier can be constructed over the rafters making the thatch sacrificial in any fire. If fireboarding is used, it is essential that a ventilation gap is left between the boarding and the thatch so that the roof can 'breathe'and thus reduce the rotting of the thatch.
Modern usage
Thatch has fallen out of favor in much of the industrialized world not because of fire, but because thatching has become very expensive and alternative 'hard' materials are cheaper - but this situation is slowly changing. There are almost 100,000 thatched roofs in the UK, and in some parts of England 1 in 4 new roofs are being thatched.
New thatched roofs were forbidden in London by the Normans in the 12th century, and existing roofs had to have their undersides (within the roof space) plastered to reduce the risk of fire. The Great Fire of London in 1666 had nothing to do with thatch. The modern Globe Theatre is now the only thatched building in London, but this modern, water reed thatch is 'icing on the cake' and actually lies over a fully waterproofed roof built with modern materials.
Examples of thatched building forms
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