The mechanism of thatch deterioration is poorly understood Preliminary observations were made on 16 roofs, all thatched with combed wheat reed. Thatch wears away from the exposed surface of the roof. Three distinct zones (the outer, middle and inner) develop and appear to represent three stages in the decay of thatch. The inner zone is a region of mainly microbial activity, the middle a region of microbial and invertebrate activity and the outer a region of abiotic activity (weathering). The inner is the earliest and the outer the last, stage. Although decay typically takes the form of a basidiomycete-mediated white rot process, a soft rot process mediated by imperfect fungi may occur occasionally. The rate of decay in areas of soft rot is faster than in areas of white rot.