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

A gabled structure of rubble sandstone with red brick dressings and a corrugated iron roof. It is built into sloping ground, with 2 storeys to the E and one to the W, and is 4 bays to front and back and 3 to the gable ends. The windows have segmental heads with deep, stepped surrounds and pronounced keystones, all in red brick, in typical Powell Duffryn house style. Red brick has also been used for the quoins, a deep eaves cornice, and a string course at the main floor level. The engine house faced the former shaft on the E side, where the bay L of centre has 2 smaller openings to the upper floor, one of which is offset, to carry winding cables, and 2 to the semi-basement level. The W elevation has large, equally placed windows. The gable ends have ventilator oculi with 4 stressed voussoirs and wooden louvres, central doorways flanked by windows, and single windows to the semi-basement. (The S gable end was overgrown with vegetation at the time of inspection.)

The interior has wrought iron roof trusses, concrete floors with steel beams and a core wall to support the winding engine. There are remains of plasterwork in the former engine room.