Thatch roofing - The construction procedure
Thatching makes use of materials that are naturally available - grass or reed.
The stalks of thatching grass are normally hollow and about 3 mm thick. Dekriet stalks, however, are solid and about 3-4 mm thick. The quality of the material improves with cultivation and regular cutting. Some thatchers consider that the quality of material that is cut by hand is superior to that of material cut by mechanical means. Hand cutting will produce about 50 to 100 bundles a day. A mechanical cutter and binder will process about 6000 bundles a day.
After cutting and loosely bundling, each bundle is shaken briskly to dislodge all loose material. The bundles are then cleaned by passing a sickle through them. This removes the remaining leaf growth from the lower two thirds of the stalks.
The grass is then remade into bundles. These bundles are each tied with a thong of twisted grass or with twine and packed in heaps about 2m high and 3 m in diameter at the base.
When the thatch is to be used for the area immediately above the thatching battens, where the underside will often be exposed within a room, the material should be combed to ensure that the stalks are perfectly clean. A comb is made by driving a number of round wire nails into a approximately 300 mm length of horizontal pole.
The thatcher in general thatch construction normally uses five tools:
- Sickle - This is used for hand cutting as well as for cleaning the cut bundles.
- The thatching spade - This is usually a home-made implement consisting of a board with a handle on one flat side, rather like a plasterers float. Several metal blades are secured to the other flat side. This tool is used to dress and shape the thatch in position.
- A straight needle - When it is possible to have an assistant working on the underside of the thatch, a straight needle, about 300 mm long, is used to 'stitch' the thatch to the roof battens.
- A curved needle – It is used to 'stitch' the thatch to the roof battens when it is not possible to have an assistant working under the roof surface.
- A climbing hook - S-shaped climbing hooks are used to give the thatcher a foot rest when working on the roof slope.
A typical small thatching team consists of four men; one to pass material from ground to roof level, two thatchers working on the external roof surface and one working under the roof to assist those working on the outside. Such a team can be expected to lay about 10 m2 of thatch in a day. Before each bundle is passed to the thatcher on the roof it is butted against a butting board, or on level ground, to ensure that the butt end is even and that any sharp ends are blunted. The bundles are normally thrown up to the thatcher. The grass is used in bundles as cut and laid on the roof with the butt end lowest. As each bundle is laid on the roof the thatcher cuts through the twisted grass or twine that secures it. He lays the first bundle on the corner, at an angle of at least 45°, thus exposing the butt end at the eaves and at the verge. Each bundle in the first course at eaves level is secured to the second batten with tarred sisal cord thatching twine at 75 mm intervals. In this process of stitching the straight needle is used, where one man can work under the roof. If it is not possible to work under the roof the curved needle is used. Subsequent courses are secured, either with a poplar stick or with a length of 4 mm diameter galvanized steel wire. The thatch is laid, two bundles thick, to a total minimum thickness of 150mm. Each successive layer conceals the poplar stick or wire that secures the previous layer. As thatching proceeds a layer of selected stems is spread evenly on the roof battens to a thickness of about 12mm. This gives a neat appearance inside the roof. On top of this layer a laminated foil of aluminum and building paper reinforced with fiberglass is laid as a protection against fire. Thatching then proceeds, course by course, to the ridge level until complete.
Thatched roofs are generally constructed with dripping eaves; rainwater gutters and downpipes are not normally provided. Eaves overhangs should be at least 600mm and some provision should be made at ground level, around the building, to prevent erosion due to water dripping from the eaves. This can either be in the form of a concrete apron or paved surround.
A thatched roof will normally last for about 25-30 years if properly laid. Aesthetic advantage of using a thatched ridge has been mentioned previously. A disadvantage of using such a ridge is that it will require renewal every 4-6 years. As maintenance of a thatched roof invariably results in dust and pieces of straw being dislodged from the roof, the provision of a reinforced cement ridge, suitably waterproofed and coloured, may be preferred.