Material Quality

Life expectancy of a thatched roof is dependent on the quality of the materials used. The key points which contribute to longevity of the thatch, water reed or cereal straw on a roof, which can be measured in raw materials are:

  • Height of Cut – Stems should be cut as low as possible to ensure they retain the section of high lignin content which gives strength to the stem and resistance to weathering.
  • Moisture Content – Maintaining the thatch at a moisture content <17.5% will retard degradation by micro-organisms.
  • Clean material – removal of immature or broken stems. A mixture of stem diameters will slow down drying after rain creating an environment for decay.
  • High specific bundle weight – will indicate material with higher lignin content and longer life.
  • Low salt content – salt is hygroscopic and will draw and hold moisture in the stem.
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Cutting height

Harvesting technique is important as durability of the stems is lost when it is cut too high. Under some harvesting conditions, the strong butt ends closest to the ground are being lost because the cutter bar of the harvester is set too high.

Material that is cut too high will have lost the section which is high in lignin content. It is this which gives the stem resistance to early decay. The stem will be thinner, will also absorb water more readily and be slow to dry out. High cut material will have a reduced life expectancy and should be rejected.

Moisture content

It is important to keep moisture away from the material after cutting and making sure thatching techniques keep the moisture out once the material is secure on a roof. The aim is to maintain a moisture level <17.5% and to select material with tough butt end stems capable of shedding water quickly when the roof is subject to multiple wetting and drying cycles. The micro-organisms which cause decay cannot grow and multiply when the moisture is below 17.5%. The organisms are not killed by low moisture, but their development is retarded.

Clean Material

Stem diameter and wall thickness affect the way in which the reed dries. Also, damaged stems will allow uptake of moisture raising the risk of decay.

High specific bundle weight

The gap between the reeds will affect the rate of moisture loss after wetting through rain on the roof, and in turn will influence thatch longevity. Stems with a great variety of diameters will cause a dense surface which will slow the drying of the thatch. But stems of similar size which make heavy bundles are likely to be high in lignin and have a longer life expectancy

Salt and mineral interactions

Reed grown in salt-containing soil will be softer and have high salt content which attracts moisture, reduces drying rates and encourages microbial and fungal decay.

Water reed which grows in saltwater may have a small amount of salt within its structure but mainly surface salt which can wash away on reed beds as well as roofs.

If there is deterioration in a water reed roof and there is no obvious cause, the possibility of salt in the reed must be considered. The combination of high-cut reed which has lost the durable section, and a high proportion of ‘salt in the stems’ results in a softer more absorbing reed; orientation of the building and position; inclination angle of reed stems can also affect thatch longevity.

Roof Thatchers - National Society of Master Thatchers