Ynysfach Engine House

Ynysfach Engine House is a 4-storey rectangular blast engine house built from dressed blue Pennant sandstone, featuring white-ashlared limestone quoins and dressings, topped with a hipped slate roof that has deep eaves. It reflects the architectural style of the Crawshay ironworks. The windows have segmental heads with deep voussoirs and dressings, and the small-paned sash windows have been replaced.

The north elevation has the main entrance, which is accessed by a modern stair and features a round-headed arch, with one window above it. The principal facade on the east side is 4 bays long, with equally spaced windows on three storeys and a semi-basement that has round-headed arches, along with cast iron brackets at the north end. The south end, which faces the furnaces, has no windows. The west elevation includes six irregularly spaced windows, cast-iron fittings, and a tall blocked archway at the south end.

The building is not accessible. It is reported to have new floors and partitions added, with the bob wall for the beam engine running through the center of the structure. Cast iron brackets for the engine's supporting structure and components are visible in many areas.