Thatch

Thatch was the roof covering for most buildings until the end of the Middle Ages, and remained the norm in rural areas until the mid-nineteenth century. For most of the twentieth century thatch was only used in conservation work; however, with the new resurgence of interest in the material, partially associated with the reconstruction of the Globe Theatre in London (Fig. 4.32) thatch, once again, has become a current construction material.

Materials

The three standard materials for thatching within the UK are water reed (Phragmites australis), long straw (usually wheat) and combed wheat reed (also known as Devon reed). Water reed is associated with the Norfolk broads, the fens, south Hampshire and the Tay estuary, but much is imported from Turkey, Poland, Romania and China. Long straw is the standard thatch in the Midlands and Home Counties, while combed wheat reed is more common in Devon and Cornwall. Water reed is the most durable, lasting typically 50 to 60 years, but long straw and combed wheat reed last approximately 20 and 30 years respectively, depending upon location and roof pitch. All thatched roofs will need reridging at 10 to 15 year intervals; in the case of water reed this is often done with saw sedge (Cladium mariscus) which is more flexible than the reed itself.

Both long straw and combed wheat reed are often grown and harvested specifically as thatching materials to ensure long undamaged stems. Long straw is threshed winter wheat, whereas combed wheat straw is wheat with any leaves and the grain head removed. An alternative to combed wheat reed is triticale (Triticale hexaploide), which is a cross between wheat and rye. Triticale produces a more reliable harvest than other forms of wheat straw and it is indistinguishable from combed wheat reed when used as a thatching material. Water reed for thatching is usually

Fig. 4.32 Thatched roof - Globe Theatre, London. Photograph: Arthur Lyons

between 915 mm and 1830 mm in length. Typical lengths for long straw and combed wheat reed are 760 mm and 915 mm respectively.

Appearance

Long straw roofs show the lengths of the individual straws down the roof surface and are also characterised by the use of split hazel rodding around the eaves and gables to secure the thatch. To prevent attack by birds they are frequently covered in netting. Combed wheat reed and water reed both have a closely packed finish with the straw ends forming the roof surface. A pitch of about 50° is usual for thatch with a minimum of 45°, the steeper pitches being more durable. The ridge, which may be a decorative feature, is produced by either wrapping wheat straw over the apex or butting-up reeds from both sides of the roof. Traditionally hazel twigs are used for fixings although these can be replaced with stainless steel wires. The durability of thatch is significantly affected by the climate. All materials tend to have shorter service lives in warmer locations with high humidity, which encourage the development of fungi. Chemical treatment, consisting of an organic heavy metal compound, may be used, preferably on new thatch, to delay the biological decomposition. Thatch is usually laid to a thickness of between 220 and 400 mm.

Properties

Fire

The fire hazards associated with thatched roofs are evident; however, fire retardants can be used although these may denature the material. In the case of the Globe Theatre in London, a sparge water-spray system has been installed. In other new installations, the granting of planning consent has been facilitated by the location of permanent water drenching systems near the ridge and by the use of fire-resisting board and foil under the thatch to prevent internal fire spread. Electrical wiring and open fire chimneys are the most common causes of thatch fires, although maintenance work on thatched roofs is also a risk if not carefully managed.

Insulation

Thatch offers good insulation, keeping buildings cool in summer and warm in winter, a typical 300 mm of water reed achieving a U-value of 0.35 W/m2 K.

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