Climate Effects on Thatch

A thatched roof constitutes a very hostile environment for micro-organisms to grow; in the past hot dry summers and frosty snowy winters controlled the speed of decay, by keeping thatch relatively dry and controlling fungal growth. Periods of prolonged heavy rain in the recent past have encouraged the growth of moss and algae, not just on thatch but on tiled roofs, trees and garden furniture, with warmer winters further contributing to expanding colonisation on thatch. The present climatic conditions will shorten the life expectancy of many thatched roofs. Research carried out by Kirby and Rayner (1989) suggested that under normal conditions 2cm of a thatch surface is worn away annual by weathering It can be expected that a thatch wearing normally will be wet after rain at the surface to a depth of 2cm, but even after continuous heavy rain the moisture content below the surface will be less than17% deeper inside the thatch. It is believed that many of the current problems associated with early degradation are either inherent in the raw material or are associated with changing climatic conditions.


      
Physical wear on a straw thatch. Algae formed a biofilm on the thatch in the winter and a dry summer has caused the film to contract. This has caused tearing and breakage of the butt ends of the straw











Reports of early failure in water reed thatch, within 4-10 years, were first recorded in 1970, and by 1983 had given rise to considerable cause for concern. At that time the majority of affected reed was home produced; researchers from the Universities of Bath and East Anglia investigated both the potential for infection of freshly harvested reed during storage and also studied the degradation process within thatch. Then as now complaints are associated with soft, weak reed, colonisation of the surface by clumps of organisms that dry out in sunny conditions and cause the surface to physically degrade in windy conditions. This surface colonisation and physical Prolonged periods of rain encourage the colonisation of the straw thatch surface with algae and mosses. These hold moisture and prevent the thatch from drying out interaction with the thatch allows the ingress of water into underlying thatch layers providing conditions suitable for further degradation of the reed by fungi and a subsequent reduced life expectancy for the roof. The main body of research on decay of lignin rich materials has been carried out on wood and wood products. However, there are only a limited number of organisms that form symbiotic functions in the decay process many of these organisms appear to naturally colonise both wood and other decaying vegetation such as thatch. In this particular form of attack, decay is not homogenous across the whole surface of the coat work, but can be seen as “bleached” areas in either zones or patches; this type of decay is not necessarily associated with high wear areas of a roof such as the junctions of dormers, valleys or gullies. Stems taken from within these patches have often lost both tensile and compression strength causing them to collapse and fragment.