North Smithery (So 23)

This is a disused smithery, originally designed in 1808 by Sir Samuel Bentham, Inspector General, and extended in 1847. A railway cutting bisected the building in 1879, and a subsequent extension was demolished in the mid-20th century. The building is constructed of roughly coursed Dunstone rubble with limestone ashlar dressings. It features a truncated brick chimney in the southeast corner and a corrugated sheet hipped valley roof. Inside, there's a network of cast-iron columns.

The smithery has a rectangular plan, with a central line of columns leading to the valley roof, and spaces on the west and east sides for forges. It is two storeys high, with eleven windows on the east and west elevations, and six windows on the north and south elevations. The exterior walls are plain with a plat band and rusticated quoins, and the windows have square-headed plat surrounds, some boarded up, while others contain 12-pane ground-floor and 9-pane first-floor cast-iron casements. A railway tunnel runs at an angle through the building, passing through segmental arches with rusticated architraves on the west and north elevations. The rear elevation shows stone corbels at ground-floor level, cast-iron brackets, and other evidence of former industrial processes.

Internally, a central aisle is supported by tall cast-iron columns that lead to the valley roof. Lower, tapered iron columns support a gantry crane in the western section, along with an early 19th-century hand-operated traveller. Late 19th-century purlins cover original queen-post trusses. A corrugated iron tunnel, built around 1879, now encloses the train track. Evidence remains of 18 small forges in the east half and 5 in the west half; the positions of their flues can still be seen in the roof, which originally had ridge louvre vents. Steam-powered machinery was installed from 1845, followed by a larger stack.

Historically, this smithery is of considerable importance as one of Sir Samuel Bentham's innovations to the Dockyards, and is considered to be one of the earliest and least altered buildings of its kind in the country.