Number 7 Slip Cover and Machine Shop Number 3

This is a dry dock cover, constructed between 1852 and 1855. It was designed by Colonel GT Greene and built by Grissell and Peto as part of Chatham Dockyard. The structure is a cast- and wrought-iron frame with corrugated-iron sides and roof.

The building has a rectangular aisled plan. Externally, it is a single-story, 10-bay range with a wide gable displaying the iron frame. The central bay has an open section leading to a canted upper brace between two columns, with diagonal bracing above. The aisles are sheeted in above the ground floor in three stages, with horizontal ties to the corner posts. The roof features two tiers of continuous clerestory lights and a ridge lantern.

Inside, the building has H-section cast-iron columns connected at a lower level by a cast-iron beam with parabolic bottom flanges. It includes cast-in supports for gantry cranes to the main roof and aisles, wrought-iron roof trusses, and longitudinal lattice and trussed aisle beams with diagonal bracing. Many members carry the inscription “H & MD GRISSELL/LONDON 1853”.

Historically, dry dock covers were introduced to Navy yards from around 1814 to protect wooden ships undergoing construction from the weather, but became largely obsolete by the 1860s with the advent of metal shipbuilding. Number 7 was later used for submarine building. The slip cover represents a significant advancement over earlier designs, moving towards a rigid portal-braced frame, a development fully realized at the Sheerness Boatstore. It was designed not only for weather protection, but also to accommodate travelling cranes over all three aisles. It is an important example in the evolution of engineering frames, comparable to a former slip cover from Woolwich Dockyard and the nearby Boilershop.

The building forms a notable group with the 1837 timber slip cover and the 1845-7 iron covers to the south, and the surviving iron sheds within the former Dockyard to the north.