Thatching is the craft of covering a roof with dry vegetation such as straw, water reed, sedge, rushes and heather, layering the vegetation so as to shed water away from the inner roof. It is probably the oldest roofing material 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 now the choice of well-to-do people who want their home to have a rustic look.
The tradition of thatching has been passed down from generation to generation for thousands of years. Few descriptions of the building techniques exist, especially in tropical regions.
In equatorial countries thatch is the prevalent local material for roofs, and often walls. There are diverse building techniques from the Hawaiian Hale shelter made from the local ti leaves and pili grass of fan palms to the Na Bure Fijian home with layered reed walls and sugar cane leaf roofs and the Kikuyu tribal homes in Kenya. The colonisation of indigenous lands by Europeans greatly diminished the use of thatching.
Records of European thatch date back to before the Middle Ages, when the first villages were established. The creation of villages brought with it the need for readily available, inexpensive, and durable building material, such as thatch. “Thatch houses built in close proximity helped to account for the frequent and disastrous fires that swept through the narrow streets of medieval cities.” Eventually the authorities wrote the Ordinance of 1212, arguably the first building regulation in force in London, prohibiting the building of new thatch roofs and demanding the whitewashing of existing ones with plaster daub.
Early settlers to the New World used thatch as far back as 1565. Native Americans had already been using thatch for generations. When settlers arrived in Jamestown, Virginia in 1607, they found Powhatan Indians living in houses with thatched roofs. The colonists used the same thatch on their own buildings. In the early years of the 19th century thatching was in decline. The commercial production of Welsh slate had begun in 1820 and the mobility which the canals and then the railways made possible meant that other materials became readily available. To compound this, the Napoleonic Wars raised the price of wheat and straw to a prohibitive level in Europe. The number of thatchers declined, as the tradition became regarded as unfashionable.
Technology in the farming industry has had a negative impact on the popularity of thatching. Use of the material declined following the First World War in particular, and with the invention of the combine harvester and the need to develop shorter stemmed varieties of wheat, the long straw once produced was no longer available. The increased loss of water plants and wildlife occurred with the shift from open ponds to cattle troughs and piped water for animals. With it came the decline in availability of rushes, and other wetland vegetation used in thatching.
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 ancient buildings preserved lower layers of medieval thatch over 600 years old.
Water reed, which was used in East Anglia and Eastern England, is a one-coat material; weathered reed is usually stripped and replaced by a new layer. 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. The lifespan of the thatch is also dependent on the skill of the thatcher, but other factors need to be taken into account, such as climate, quality of the materials used, and the pitch of the roof.
Thatch is fastened together in bundles with a diameter of about two feet. These are then laid on the roof with the butt end facing out and secured to the roof beams, after which they are pegged in place with wooden rods. The thatcher adds the layers on top of each other, finishing with a layer to secure the ridgeline of the roof. This method means thatch roofs are easy to repair, can endure heavy winds and rain and only need a stable supporting structure.
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, Indian and pioneer houses were often constructed of palmetto-leaf thatch.
The thickness of the thatch decreases over the years as the surface is gradually eroded. A thatched roof can be thought to be nearing replacement when the fixings are close to the surface. “A roof is as good as the amount of correctly laid thatch covering the fixings.” Water penetration, when it occurs is minimal and is usually due to capillary action. The presence of moss is not necessarily detrimental to the thatch.
The life of a thatch can be extended by appropriate repair. Some guidelines are:
1. Follow the advice of professional thatcher.
2. Do not move around on thatch unnecessarily.
3. Trees should be kept well back from thatch and never allowed to overhang or brush against the roof.
4. Do not let non-thatchers fit netting, flashings, etc., without advice from an experienced thatcher.
5. Television aerial erectors etc., should be required to keep off the thatch as much as possible.
6. Do not allow standing on the ridges or the use of ridges as working platforms.
7. Long Straw thatching should be securely netted to avoid bird penetration as is sometimes the case with combed wheat and some water reeds.
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 reduce the spread of flame. Most thatch fires do not begin on the surface of the thatch (the surface is usually damp in the UK), but in the thatch surrounding the chimney. While it is true that some fire retardants may expedite 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.
Thatch has some natural properties that are advantageous to its performance. Firstly, it is naturally weather-resistant. When properly maintained, thatch does not absorb large amounts of water. There should be no large increase to roof weight due to water retention. In a well-designed roof, the uppermost inch or so of the thatch is the only portion that feels the effects of the elements. A roof pitch of 45 degrees or more allows precipitation to travel down the steep slope of the roof and reach the ground long before it can penetrate the structure.
Thatch is also a natural insulator. When whole vegetation is used, thousands of pockets of air exist between and within the stems of the grasses that make up a thatched roof. These air pockets give the roof the ability to insulate a building in both warm and cold weather. At least a foot thickness of thatch on top of a roof also helps thermal retention. Norfolk reed, commonly used to thatch roofs in the United States and Britain, offers an insulation or R-value of 40. Conventional building insulation, such as fiberglass, usually rates between 30 and 50.
Thatch is not by its nature prone to wind damage. Experience of hurricane force wind tests up to 100 mph in California has shown that, when applied correctly, thatch has good wind resistance. However, older thatched roofs, particularly in wind-prone areas can benefit from well-fitted netting.
Thatch is a versatile material when it comes to covering irregular roof structures. This fact lends itself to the use of second-hand, recycled and natural materials that are not only more sustainable, but need not fit exact standard dimensions to perform well.
Thatching can be sustainable -- if crops are managed ecologically, then the resource can be renewed regularly. Many of the natural thatching materials are improved by regular harvesting. For example, reeds, marram grass, broom, heather, and juniper all regrow in more usable forms. Thatch can be recycled to be an excellent fertilizer.
As local materials always tend to harmonize with the landscape surrounding their place of origin, thatch, as a natural material, will blend well with a rural environment. Thatch has an ecological advantage because it is produced by natural processes that do not use scarce and expensive resources of energy.
Being an organic material, thatch is susceptible to decay and decomposition and precautions must be taken to minimize the possibility of this process taking place. In warm, wet climates thatch is prone to fungal attacks.
Animals can cause damage. Birds looking for food, gathering nest-making materials or nesting in the roof itself becomes a greater possibility when the plant material is not processed appropriately for its intended use. Rodents can cause extensive damage when present in the house. The quality of design and building can greatly affect the performance of the roof. If built and/or maintained inadequately, then problems such as vulnerability to wind damage and prolonged damp conditions are issues.
Thatch can be maintenance intensive. The maintenance cycle varies based on thatch type, roof pitch, the degree of shade or exposure and the kinds of materials used.
Thatch has fallen out of favour 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 one of the few thatched buildings in London (others can be found in the suburb of Kingsbury), but the Globe's modern, water reed thatch is purely for decorative purpose and actually lies over a fully waterproof roof built with modern materials.