Thatch

Thatch is a roof covering made from dry vegetation, most often water reed, cereal straw, sedge, heather, palm leaves or grasses, laid in thick layers on a steeply pitched roof. Water runs down the outer stems and drips off the eaves before it can soak deep into the bundle. Thatching is one of the oldest building crafts and is still practised on every inhabited continent.

How thatch works

Thatch does not form a waterproof skin. Instead, steeply angled stems carry water from one to the next along the surface, so only the outer few centimetres get wet. A pitch of about 45-55° is typical, and a pitch of at least 50° helps rain run off before it penetrates. Areas with heavy snowfall, such as parts of Japan, use even steeper roofs.

Main materials

• Water reed (Norfolk reed): the most durable thatch in northern Europe.
• Long straw: threshed wheat or rye straw, traditional across much of lowland England.
• Combed wheat reed: straw combed to remove grain and leaf, typical of south-west England.
• Sedge: a wetland plant used mainly for ridges.
• Heather, broom, rushes, marram and bracken: local materials in upland and coastal areas, now rare in Britain.
• Palm leaves and grasses: including coconut palm in East Africa (makuti), pili grass in Hawaii and alang-alang in South-east Asia.

Lifespan and maintenance

Good water reed on a steep roof in a dry climate can last 50 years or more, and some roofs have lasted over 70 years; modern imported reed on an average English roof is more likely to give about 40 years. Long straw roofs have lasted 25-50 years. The ridge takes the most weather and usually needs renewing every 10-15 years. In Britain, many old roofs have been recoated again and again ("spar coating") rather than stripped, building up layers more than 2 m (7 ft) thick and in some cases preserving thatch over 600 years old.

Insulation

Thatch is a good insulator. Historic England gives theoretical U-values of about 0.29 W/m²K for 300 mm (12 in) of water reed and 0.23 W/m²K for the same thickness of long straw, close to modern replacement-roof standards without added insulation.

Fire

Fire risk is often overstated but is real: fire can spread very quickly once thatch ignites. Most thatch fires start from chimneys, wood burners or electrical faults. UK guidance recommends well-maintained, lined chimneys swept at least twice a year, loft smoke alarms and fire-retardant boards under new thatch.

Distribution

England has an estimated 50,000 thatched buildings, and the UK as a whole about 60,000 thatched properties (2013 estimate). Thatch is also traditional in Ireland, the Netherlands, Denmark, northern Germany, Japan, sub-Saharan Africa, the Pacific islands and South-east Asia.