Former filter works, Beside 86 Carthall Road, Coleraine, Co Londonderry, BT51 3PQ

A former water filter house built in 1951 beside Carthall Road, Coleraine, designed by architects Ferguson and McIlveen and constructed by local building contractor John Rainey & Co. The building was opened by the Coleraine, Portrush & Portstewart Waterworks Joint Board following the Board's formation in 1945 to develop a regional water scheme. The filter house represents a good example of post-Second World War utilitarian architecture, though it is not of sufficient quality for listing consideration.

The building is a brick construction aligned north-south on the south side of Carthall Road, comprising a one- and two-storey middle section with single-storey side annexes. The middle section rises to two storeys at its northern end, reducing to single-storey height at the middle and southern ends. It has a flat concrete roof with slightly oversailing edges and a raised parapet across the south end, with cast-iron hoppers and downpipes. Red-brick walls are built on a slightly advanced chamfered base course over a concrete platform. Metal-framed windows throughout are generally fitted with 2x2-paned opening sections hinged to the middle, with all openings having rendered-concrete architraves that project flush with concrete cills.

The principal elevation faces north to the road and is symmetrical, featuring a double-leaf tongue-and-groove entrance door to the ground floor middle, flanked by 2x2 windows on each side. Above the door is a painted panel with bronze lettering reading "Carthall Filter House 1951". Above this panel is a 4x3-pane window with 2x3-pane flanking windows. The east elevation of the frontage has a doorway and 2x2 window to the ground floor and two 2x2 and one 2x4 windows to the first floor. The remainder of this elevation is largely abutted by the east annex, though four pairs of high-level 1x3 windows are visible above. The south elevation contains a pair of large sliding metal-sheeted doors to the middle section, with 3x2 flanking windows. The front section of the west elevation displays three 2x2 windows to the ground floor and a 2x2 window plus an infilled loading door to the first floor, the latter surmounted by a truncated cantilevered steel beam, presumably once a hoisting bracket. The remainder of this elevation is abutted by the west annex, with four pairs of high-level 1x3 windows above.

The east annex has a flat roof detailed as the middle section, with walls that are painted and rough-dashed over a steel frame with infill panels of probable mass concrete. Its north elevation displays three 3x2 windows, the east elevation has two tongue-and-groove sliding doors (one with a picket gate) set between three pairs of 3x2 windows, and the south elevation has three 3x2 windows, one of which belongs to the middle section. The west annex has roof and walls constructed as the east annex, with a large roller-shutter door on its north elevation, five pairs of 3x2 windows on the west elevation, and three 3x2 windows on the south elevation, again with one belonging to the middle section.

The name and date plaque on the frontage is in relief brass individual lettering and represents the only commemoration of the building's opening. Although original filter fittings have been removed and the pipe channels infilled, the structure remains a little-altered example of 1950s utilitarian architecture in a good setting.

The filter house served as a treatment facility for water drawn from four reservoirs: Carthall, Dunalis, Ballinrees and Altikeeragh. Water was initially supplied from the Board's existing Dunalis Reservoir, 3 kilometres south-west of the filter house, where it was chemically treated with ammonium sulphate and chlorine and filtered through sand beds. The larger Ballinrees Reservoir, 1 kilometre south-west of Dunalis, subsequently fed into the filter house once completed, as did the later additions of Altikeeragh near Castlerock and the Carthall Service Reservoir approximately 400 metres north-east of the filter house. This service reservoir is an underground tank surrounded by a security fence and trees which stores water pumped from the River Bann.

Plans for the filter house were first presented to the Board by its consultant engineers in December 1949. The contract was advertised in March 1950 and construction was underway by August of that year. By April 1951 some filters were already in operation, and the site was formally handed over in July 1951. The total cost of the works was £28,209, with the filters supplied by the Turn-Over Filter Co accounting for 50 per cent of this sum, the building for 37 per cent, and a meter gauge by R Platt & Sons measuring water processed for the remaining 13 per cent.

The adjacent caretaker's house and garage were also designed by Ferguson and McIlveen and built by John Rainey & Co, completed in January 1954. The caretaker's house is a two-storey structure with a hipped concrete tile roof, rendered chimney, plastic rainwater goods, and painted rough-dashed walls with replacement uPVC door and window frames. A small domestic garage stands at the south-east.

Approximately twenty years before the present survey, the Department of the Environment Water Service deemed the filter works superfluous following the construction of a new water treatment works at Ballinrees. The premises were subsequently sold to private ownership. The filter tanks were removed from the building and the pipe channels infilled, and a large two-storey extension was added to the caretaker's house. The site is tarmacked all around. The building now functions as a store.