Felin Isaf is a large and spectacular roofless and ruined ivy-clad corn mill, showing evidence of two periods of building. The earlier part is the southern wing, built from rubblestone and rendered on the south-facing gable where the waterwheel was once sited. The wheel may have been as much as 20ft (6.1m) in diameter by 4ft (1.2m) wide, power being taken from a gear ring of about 12ft (3.6m) diameter mounted on the spokes. Its inner bearing was supported on a stone projecting from the mill wall, which bears the marks left by the revolving wheel hub, and the outer bearing was supported on a massive rubble wall. Water came from a 250-metre long leat along the east side of the valley, and along the top of a massive wall to the wheel. Inside the building are four monolithic millstones 5ft (1.5m) in diameter, probably from Anglesey; one of the runner stones had been broken into four pieces and bound together with iron clamps and bands.
The later phase of building produced a very large two-storey cross-range to the north of the earlier mill, with the outer faces of its walls built from massive roughly-squared coursed blocks with some small pinnings. The water wheel was sited at the west end of the south wall, and may have been as large as 18ft (5.5m) in diameter and 5ft (1.5m) wide. The water supply route for this wheel is not clear, but must have been carried round the earlier mill building on an overhead launder. The floor and all internal fittings have been removed.
Close by there are the roofless ruins of ancillary buildings whose purpose is uncertain. A small cottage to the north, still in use, may have been the mill cottage.
W J Crompton, RCAHMW, 9 September 2010.