Thatching is a craft rather than a precise science and the craftsmen who carry this out will take into consideration many factors which can affect the look and longevity of the roof. The materials are organic and can vary tremendously. The commonly used materials are Cereal Straw and Water Reed. In the case of cereal straw there are many varieties each with their unique characteristics. These can be used on specific features to help achieve a roof which will wear evenly and attain a reasonable life. Cereal straw can be divided into 2 categories resulting from the way it has been processed post harvest. The straw will have been cut with a reaper binder, stooked in the field to ripen the straw and grain and stacked to await further processing.
A Guide to Specification
Combed Straw
The grain is removed mechanically but in a specialised way. A comber attachment is fitted to the threshing drum to process the straw. The comber removes the grain, the weed and leaf from the straw leaving the stems unbroken. This is then mechanically tied into bundle, clipped and is ready to use as thatch.
Long Straw
This has the grain removed mechanically when passed through a threshing drum. The straw is often broken and is a random mixture of straw, grass and weed. This needs further processing before it can be used for thatching. The straw needs to be straightened and the bulk of the foreign bodies removed.
Water Reed (phragmites australis)
Common reed is grown in wetland areas in the UK. Evidence of the use of water reeds for thatching has been found dating back to the Bronze Age at Flag Fen near Peterborough. It is cut by professional cutters who also manage the marshland to ensure there are habitats for wildfowl, reed supply and tourism. The UK produces about 10% of the reeds used for thatching, with the shortfall being made up by reeds produced overseas.
Thatching
Roofs can be thatched directly onto the timber structure beneath or a layer can be applied to a prepared thatched roof – re coating. Any decayed thatch is removed to a firm dry base. This may need packing out and tightened where necessary. The thickness of a new thatch should be in the region of 300 – 450mm depending on the methods employed.
Fixings
There are various fixing methods employed to suit various situations. These range from tarred twine, steel hooks hammered into the rafters, wire fixings screwed into the rafters and battens (this method requires the battens to be adequately secured to the rafters), or a combination of the above.
For recoating the use of any of the above may be used in the preparation, after which split hazel or willow spars which are pushed into the thatch beneath to secure the new thatch. The fixings should be 100 – 150mm from the surface of the thatch.
Ridging
This is an extra layer about 75-80mm thick, which covers the last fixing of the main coat so there is no direct passage for rain water to follow into the thatch. The ridge is held down with hazel or willow spars. There are also several types of ridging which your thatcher will give advice on the best to use.
Netting
Netting can be applied to the whole roof or just the ridge and gables. There should be no gaps larger than the mesh size of 19mm. Galvanised wire should be 20-22 gauge and polythene net should be UVa resistant.
Chimneys
The thatch around chimneys should be removed to facilitate the inspection of the masonry. During this process photographs and identification of the various layers should be carried out, recorded and kept with the client file.
Flashings
The flashing can be either cement or lead fitted to prevent ingress of water.
Conservation
Many properties with thatch are listed and it is the responsibility of both the Owner and thatcher to ensure that the necessary permissions and documentation is in place before any work commences.