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Texture images: Thatched roof

Thatching is the craft of building a roof with dry vegetation such as straw, water reed, sedge (Cladium mariscus), rushes, heather, or palm branches, layering the vegetation so as to shed water away from the inner roof. Since the bulk of the vegetation stays dry and is densely packed—trapping air—thatching also functions as insulation. It is a very old roofing method and has been used in both tropical and temperate climates. Thatch is still employed by builders in developing countries, usually with low-cost local vegetation. By contrast, in some developed countries it is the choice of some affluent people who desire a rustic look for their home, would like a more ecologically friendly roof, or who have purchased an originally thatched abode.

Thatch is popular in the United Kingdom, Germany, the Netherlands, Denmark, parts of France, Sicily, Belgium and Ireland. There are more than 60,000 thatched roofs in the United Kingdom and over 150,000 in the Netherlands. Good quality straw thatch can last for more than 50 years when applied by a skilled thatcher. Traditionally, a new layer of straw was simply applied over the weathered surface, and this "spar coating" tradition has created accumulations of thatch over 7’ (2.1 m) thick on very old buildings. The straw is bundled into "yelms" before it is taken up to the roof and then is attached using staples, known as "spars", made from twisted hazel sticks. Over 250 roofs in Southern England have base coats of thatch that were applied over 500 years ago, providing direct evidence of the types of materials that were used for thatching in the medieval period.[6][page needed] Almost all of these roofs are thatched with wheat, rye, or a "maslin" mixture of both. Medieval wheat grew to almost 6 feet (1.8 m) tall in very poor soils and produced durable straw for the roof and grain for baking bread.

Technological change in the farming industry significantly affected the popularity of thatching. The availability of good quality thatching straw declined in England after the introduction of the combine harvester in the late 1930s and 1940s, and the release of short-stemmed wheat varieties. Increasing use of nitrogen fertiliser in the 1960s–70s also weakened straw and reduced its longevity. Since the 1980s, however, there has been a big increase in straw quality as specialist growers have returned to growing older, tall-stemmed, "heritage" varieties of wheat such as Squareheads Master (1880), N59 (1959), Rampton Rivet (1937), Victor (1910) and April Bearded (early 1800s) in low input/organic conditions.

In the UK it is illegal under the Plant Variety and Seeds Act 1964 (with many amendments) for an individual or organisation to give, trade or sell seed of an older variety of wheat (or any other agricultural crop) to a third party for growing purposes, subject to a significant fine. Because of this legislation, thatchers in the UK can no longer obtain top quality thatching straw grown from traditional, tall-stemmed varieties of wheat.

All evidence indicates that water reed was rarely used for thatching outside of East Anglia. It has traditionally been a "one coat" material applied in a similar way to how it is used in continental Europe. Weathered reed is usually stripped and replaced by a new layer. It takes 4–5 acres of well-managed reed bed to produce enough reed to thatch an average house, and large reed beds have been uncommon in most of England since the Anglo-Saxon period. Over 80% of the water reed used in the UK is now imported from Turkey, Eastern Europe, China and South Africa. Water reed might last for 50 years or more on a steep roof in a dry climate, modern imported water reed on an average roof in England, when thatched by a proficient craftsman, is more likely to last roughly 40 years. The lifespan of a thatched roof also depends on the skill of the thatcher, but other factors must be considered—such as climate, quality of materials, and the roof pitch.

In areas where palms are abundant, palm leaves are used to thatch walls and roofs. Many species of palm trees are called "thatch palm", or have "thatch" as part of their common names. In the southeastern United States, Native and pioneer houses were often constructed of palmetto-leaf thatch. The chickees of the Seminole and Miccosukee are still thatched with palmetto leaves. Makuti thatching in East Africa uses Cocos nucifera.

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