An erstwhile colleague once commented: “Why would you want to specify a roofing material which makes perfect nesting material for most of the local wildlife and could go up like a two bob rocket if a spark from a garden rubbish fire landed on it? Who in their right mind would specify thatch?”
Fortunately, several hundred years experience of thatch and thatching has proven him wrong. As a traditional roofing material, thatch has provided effective weather protection for many generations. And yes, there is a fire risk particularly around chimneys but, from someone who has undertaken a number of full scale fire tests on thatch, watching grass grow has potentially more entertainment value! Contrary to popular belief, thatch can take many hours to fully ignite and certainly does not compare favourably with the performance of pyrotechnics. And surely providing a source of natural nesting materials only enhances its environmental credentials!
The use of thatch has had a recent resurgence, particularly within the south west of England. It is estimated that of the current stock of 30,000 thatched roofs, some 2,500 are new build. Where specified as new build the thatched roof must, of course, conform to current Building Regulations.
Thatch is naturally air and moisture permeable – it provides a weather resistant, breathable and highly durable roof covering. However, polyethylene sheeting used to provide weatherproofing during thatching, if subsequently left in place, can impede this air and moisture flow and lead to early deterioration of the thatch. In addition, with the requirement for thermal insulation and fire protection to satisfy Building Regulations, there is a potential for reduced air movement within the thatch, which could also affect its durability. Extensive guidance on specifying thermal insulation, fire protection and electrical installations is available from BRE, local authorities, insurance agencies and the National Society of Master Thatchers .
Types of thatch
Traditionally, thatch materials were locally sourced. However, with increasing demand on UK stocks an increasing proportion is imported. There are three principal types of thatch:
Although traditionally harvested within the fens and marshes of Norfolk (Norfolk reed), Cambridgeshire and a few isolated areas within Dorset, a growing proportion of water reed is now imported from Europe. On site, the reeds are formed into bundles and laid onto the roof in horizontal layers where they are fixed directly to the structural roof timbers by metal rods (sways) and hooks. Once fixed, the rigid nature of the reeds allows them to be dressed into position so that the cut (butt) ends of the reeds present an even finish to the roof surface.
Combed wheat reed (Devon reed)
Wheat straw, similar to long straw, is produced by passing the straw through a reed comber which removes the grain without crushing the straw stem and ties the straws into aligned bundles. These bundles are laid onto the roof timbers or existing thatch layers and fixed and dressed into position in a similar manner to water reed. The downward facing cut or butt ends can be trimmed or clipped.
This is threshed wheat straw, sometimes referred to as crushed straw. Threshing and subsequent storage before thatching must be undertaken with care to minimize damage to the straws. The preparation of long straw thatch distinguishes it from the other two materials. The threshed long straw is shaken into layers, without regard for alignment, and wetted. This process is repeated for several layers that are then left to soak. This wetting process improves the pliability of the straw and allows it to be compressed on the roof surface. The long straw is drawn into yealms and laid onto the roof where it is fixed to structural timbers using sways and hooks. Long straw is not dressed into position, resulting in its distinctive soft or ‘shaggy’ appearance.
Other naturally occurring materials can be used, e.g. sedge, heathers, grasses, seaweed and eel-grass. However, the use of these materials is usually limited to conservation projects and specialist applications. Synthetic thatch products are also available.
Netting and other issues
To prevent birds and vermin from removing material, galvanized steel wire netting is fitted over the outer thatch surface. The ‘shaggy’ finish to long straw is at particular risk and for this reason the whole roof area is normally netted. Water reed does not usually require netting, but any straw detailing, e.g. at ridges, should be netted. The widespread application of netting should be carefully assessed as leaves and debris can become trapped behind the netting, reducing air movement and drying at the surface of the thatch resulting in the build-up of moss and lichen. Netting can also impede the efforts of the fire brigade when dealing with a thatch fire. Heavy gauge plastics netting has recently been introduced. Light gauge plastics netting will not be effective against squirrel and vermin attack.
When refurbishing thatch it may be necessary to strip existing layers back to sound thatch materials or roofing timbers. Specifiers should be aware that where the underlying thatch is of historical significance, e.g. base layers of (medieval) smoke blackened thatch, there can be restrictions to the extent of stripping.
Thatch roofing, new or old, has a unique aesthetic and ‘feel’ about it, and many of us would relish the prospect of ‘living under thatch’. There are moisture and fire related risks but, as with all roofing systems, effective detailing and specification can reduce or eliminate them.