This study examined the occurrence of macrofungi and the decay of roofs thatched with water reed, Phragmites australis. Sampling from 20 north- and 20 south-facing roof sides showed that several ascomycetes usually associated with reed in situ are common on thatch. The only basidiomycetes recorded were Mycena species. There was no significant difference in the representation of macrofungi on north- and south-facing roof sides, but Mycena spp. only produced basidiomes on surfaces facing north, suggesting that the dry environment of the south side prevents fruiting. Eleven species were recorded in total, and the average of 2·4 species per roof did not increase with the age of thatch or degree of decay. The same species were generally present on young and old thatch, and no successional stages of fungal communities could be distinguished with increasing roof age.

Deterioration of thatch occurs at the exposed surface of the roof and progresses inwards. Within the layer of reeds outer, middle, and inner zones develop representing different stages in the decay. The zones move inwards as thatch deteriorates. A comparison of the rate of decay among roofs with pitch 30°, 45° and 60° showed that the innermost zone appeared ca 20 cm from the exposed reed butts in both of the steeper roofs, whereas it was no longer present in the roof with low pitch. This suggests that the depth of the zones depends upon the roof slope, and the outer and middle zones move inwards at a higher rate in roofs with a low pitch, resulting in an increased rate of deterioration.

Phragmites thatch appears to harbour its own characteristic macrofungal community, with certain Mycena species likely to represent the principal decomposers. A common feature of fungi occurring on thatch is that they must endure unfavourable conditions.