Government Powder Magazine

A Grade I listed government powder magazine built in 1763–5 to designs by James Gabriel Montresor of the Royal Engineers, located at what became the Ordnance Board's principal centre of gunpowder storage following the Act of Parliament of 1760.

The building is constructed in Flemish bond brown brick with Portland stone dressings. It has a rectangular plan beneath a shallow-pitched slate roof with stone-capped gables. Stone parapet copings are returned to the gable ends to form open pediments, which frame semi-circular lunettes; the lunettes were originally louvred but have since been blocked in brickwork. The long elevations feature dentilled eaves and ventilation slits. At the centre of each long elevation is a small flat-roofed porch with blocked windows to the returns and outer doors set in recast segmental-arched eared architraves. Similar porches flank the gable ends, flanked by single lights with flat gauged brick arches.

The interior comprises two long barrel-vaulted aisles separated by a thick dividing wall pierced by nine arched openings to facilitate internal movement. Softwood timber partitions, partially reconstructed, divided each aisle into 48 storage compartments, each designed to hold approximately 200 barrels. Numbers were painted on the tie beams above each compartment, with many of these surviving. The compartments comprised studs tenoned into floor plates and upper beams; studding survives beneath the beams marked Nos 7, 8–9, 10, 15–16, and 25–26. Copper sheathing on timber inner doors partially survives, along with timber linings and timber pegging and softwood boarding to the inner walls—all elements designed to prevent accidental explosion or damage to powder. The boarded floor was originally ventilated underneath, with brick ducts visible at the base of the east porch. Two timber travelling cranes occupy each aisle, originally manually powered from their gantries, running longitudinally along grooves routed into the horizontal beams and along upper guide rails braced by diagonal struts. These cranes represent uniquely early examples of a type of structure that would later influence the development of industrial buildings. The roof is supported by 17 king post trusses of unusually heavy construction.

This is the only surviving building from the original group of five magazines designed by Montresor, each with a capacity of 5,000 barrels (one source states 10,000 barrels), built 1763–5. The other four were demolished shortly after 1973. The Purfleet depot was established following Parliamentary legislation in 1760 that made it the principal centre of gunpowder storage for both the Army and Navy, surpassing in size the Tower of London and Upnor Castle on the Medway. From 1787, it received powder transported from the Royal Gunpowder Factory at Waltham Abbey. The depot also included barracks for officers and men, a proof house for testing powder, and a clock tower over the entrance archway. The site remains of great significance, together with the 1770s magazine at Priddy's Hard opposite Portsmouth dockyard, as the most outstanding surviving example of the distinctively British type of magazine with twin barrel vaults, reflecting a critical period in Britain's development as a naval power in the decades following the Seven Years War. The timber overhead cranes are of particular historical importance as uniquely early examples that predate their introduction into factory and warehouse spaces in the nineteenth century, with related evidence surviving at Priddy's Hard and Morice Yard, Devonport, dating to the 1740s.