Planned as early as 1813 during the reconstruction plans of the dockyard, construction of the working Mast and Boat House was in an advanced state in 1821. The superstructure was designed by the navy’s architect, Edward Holl, however it was not completed until after his death. His successor G.L.Taylor, a Civil Architect, completed the designs and oversaw the work. The rest of the building design and construction was overseen by John Rennie but it was not completed until after his death in 1830. The reason for the building’s construction was that when Rennie and his partner Joseph Widbey visited the dockyard in 1808 they found ‘The carpenters’ saw-pits, treenail and wedge stores, joiners’ shop, and mast houses are all most inconveniently situated’ and that ‘the mast-pond is situated in the open shore, surrounded by a wooden fence, and consequently exposed to the ravages of the worm, and also to damage by the high tides and storms’. The site of the mast-houses and mast-ponds were chosen so to be well out of the way in the busy dockyard but to be sufficiently close so as to not be an inconvenience, so boats and masts could be easily supplied to vessels as required.
This building is one of two that were constructed to store small boats and masts. In between the buildings the mast pond could be found with a tunnel leading under the presently remaining building to the shore. A slip beside the tunnel allowed the boats to be pulled up and transferred to the mast-pond. The second building, now demolished, had mast-locks constructed underneath to store the masts. The mast-pond was filled with water from the tunnel leading out to the River Medway - inside this tunnel were two flood gates that controlled the flow of the water and the depth of the pond. The flood gates were still in place in the 1980’s, however it is not known if they still remain.
The slips were to the north east side of the building and boats were brought up through three arches (that still survive although are bricked up) and were pulled into the boat house. Some carried through the building to a slip that launched them into the mast-pond. The mast-pond was positioned beside the Great basin where the boats masts were installed, Rennie designed the mast-houses so that there would be from 60 to 100 feet between them and the Great Basin, giving plenty of space to install the masts. Once the masts were resized and shaped to standard measurements they went into storage - to stop them drying and splitting they were stored underwater in the mast-locks with some of them stored undercover in the mast-house.
The building is constructed of yellow-brick in Flemish bond and is two storeys high with a slate covered iron roof structure and a parapet at the eaves. Inside the ground floor is an open space disrupted by cast-iron columns and would have held a trusseltree pit at one end and four saw-pits and the other end, all of these have disappeared leaving no trace. There is no evidence to suggest the boats would have been lifted with anything other than man power although a painting shows a possible boiler house being situated alongside the building, suggesting some form of machinery driven by steam may have been used. The building was made up of 14 bays, each with a doorway installed with double doors and a semi circular fan light giving access to the slip on one side of the building and the mast pond at the other. Most of these opening survive but have been mostly adapted. A cast iron lintel, used to hold the weight of the western end of the building has the date ‘1825’ cast onto it. The iron work of the building was designed in detail in 1823 as shown on plans held at the National Archives in Kew, they are very similar to the plans and construction of the Lead and Paint Mills found at Chatham Dockyard and also designed by Holls in 1818-1819. The second floor was originally one complete floor, however the middle section of the floor has been removed leaving a gallery. This may have been done to allow the light from the sky lights in the roof to illuminate both floors.
By 1864, when a revised Ordnance Survey map was released, the Mast House to the north east of the mast-pond is shown to have been converted into an engineers shop, this building was demolished in 1980 with the mast pond being filled in to make way for a modern warehouse. Building 26 still stands but is (2009) currently empty although sometimes used as a warehouse.