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 annually 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 than 17% 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.