Melin Plas-du is a long, single-storey and attic building, constructed of rubblestone under a pitched slate roof of some antiquity. A stone forming part of the jamb of the main doorway to the ground floor, now enclosed within a modern extension, has the date 1197 scratched into it; the date is repeated in a clearer and presumably later carving. Upstairs, the timber roof trusses at the south-east end are wooden-pegged at the apex and at the halved joints of the raised tiebeams; and the purlins rest on brackets mortised into the trusses. The north-east end has a modern roof structure, probably nineteenth century, of softwood with bolted joints. Below is the kiln, with its perforated tiles resting on slate lintels preserved under a concrete floor when the drying room was converted to an office. The waterwheel and all internal machinery have disappeared, but a boarded semi-partition inside the south-east gable may be part of the former hurstbox. It is said that there was a small pelton wheel on site latterly. The south east and south-west sides of the mill building have been enclosed by modern garage and workshop extensions.
W J Crompton, RCAHMW, 23 September 2010.