Cynon Valley main Store Building (formerly the Casting House at Gadly's Ironworks

The Cynon Valley main Store Building, formerly known as the Casting House at Gadly's Ironworks, features twin gabled ranges oriented with a long side facing the furnace bank. The exterior is constructed from dark rubble with red and yellow brick dressings, and there is yellow brick patching on the upper gable ends and under the eaves, possibly indicating past re-roofing. The building has pitched slate roofs, boarded eaves, and plain bargeboards. The gable ends include brick-dressed oculi that are blocked, along with twin brick-dressed arched openings, three of which have traceried fanlights on the southeast end. A small, later addition is present on the northwest gable end, which retains one traceried fanlight.

The long flank of the building has seven bays with arched small-pane window frames set into brick-faced arched recesses that were formerly open. Bolted plates are visible on the spandrels.

Inside, the building features twin boarded roofs supported by metal-rod trusses, and there is a central arched arcade made of stonework with brick dressings. Brackets on the spandrels of the windows may have previously supported rails for travelling cranes.