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