Hay barn at Ynysmaerdy Farm (former winding engine house of Llantrisant Colliery)

The building is a hay barn at Ynysmaerdy Farm, which was formerly the winding engine house of Llantrisant Colliery. It is a gabled structure made of rubble sandstone with red brick dressings and a corrugated iron roof. The barn is built into sloping ground, featuring two storeys on the east side and one storey on the west side. It has four bays on the front and back and three bays on the gable ends. The windows have segmental heads with deep, stepped surrounds and pronounced keystones, all made of red brick, reflecting the typical style of Powell Duffryn houses. Red brick is also used for the quoins, a deep eaves cornice, and a string course at the main floor level.

The engine house originally faced the former shaft on the east side, where the bay to the left of centre has two smaller openings on the upper floor, one of which is offset to accommodate winding cables, as well as two openings at the semi-basement level. The west elevation features large, evenly spaced windows. The gable ends have ventilator oculi with four stressed voussoirs and wooden louvres, central doorways flanked by windows, and single windows at the semi-basement level. At the time of inspection, the south gable end was overgrown with vegetation.

Inside, the building has wrought iron roof trusses, concrete floors supported by steel beams, and a core wall designed to support the winding engine. There are also remnants of plasterwork in the former engine room.