Mines and Countermines Store (Building 409)

This is a mines and countermines store with an attached examining room, dating from 1899/1900. Constructed of brick in English bond with a slate roof supported by steel trusses, it stands close to the Shell Stores and Transfer Shed, sharing a similar distinctive architectural style developed by Admiralty architects following the transfer of responsibility from the War Office in 1890.

The building is long and narrow, with a gabled form across 15 bays, measuring approximately 45.7 metres in length and 11.4 metres in width. The long sides feature prominent square buttresses, capped by a tile-coped concrete pad and a brick eaves band. A chamfered plinth sits set back from the piers. Fixed to the buttresses, at mid-window height, are rectangular steel plates and projecting steel arms relating to a crane rail. Each bay has a nine-pane window set within a cambered brick head and a Portland stone sill, positioned close to the eaves. The south gable end has a pair of framed plank doors flanked by matching nine-pane lights, all set within cambered brick arches, with end pilasters running as a gable verge course. A smaller gable to the right has a pair of plank doors under a straight lintel, and this section features a two-bay return with buttresses and windows mirroring the long sides. The north gable end is similarly detailed, but lacks the added unit. The eaves have a plain board and moulded cast-iron gutter, partially missing on the west side.

Internally, a central tramway runs the length of the building, with continuous crane rails positioned at mid-window height.

Historically, the building was constructed to accommodate the storage of harbor defence mines, reflecting the Royal Navy’s role in offensive minelaying and the destruction of enemy devices using countermines. By 1913, it was being used as a guncotton store. The site itself dates back to the late 18th century, expanding significantly in the mid-19th century alongside the development of land and sea artillery and the navy’s transition to more modern warships. It represents the best-preserved range of structures associated with Britain's history as a global sea power. Further historical information can be found in the description for 'A' Magazine.