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Concrete community for South Bank's Coin Street

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Creating dense community housing above car parking at Coin Street in central London required some innovative technical solutions, explains Mathew Lunn

Site B represents the latest stage in the development of a group of sites totalling approximately 2.28ha on the South Bank of the Thames by Coin Street Community Builders (cscb).

Until World War Two the area had primarily been riverside warehousing, industrial and residential buildings, but after the war much of the area was cleared for redevelopment. By 1970 it had become largely derelict, and in the decade following 1974 a number of local community groups fought a highly publicised campaign to oppose plans for a major office development in the area. As an alternative they presented a mixed-use masterplan, developing the sites for residential uses, open public spaces and light- industrial workshops. An initial public enquiry from 1978 to 1980 was followed by a second enquiry in 1981 and 1982 and actions in the High Court and the Court of Appeal. With public opposition to the commercial development of the area growing, the developers finally capitulated and in 1984 sold their interest in the sites to the glc which in turn sold the freeholds to cscb, the organisation born out of local community groups.

The structure of cscb was developed to ensure that the original ethos of the community group's campaign, including the provision of affordable high-quality housing and local involvement, would be maintained. All of the cscb committee members must be local residents and profits are reinvested in future developments. The formula has proved a successful one: following an initial residential development on Site C, cscb has developed the first phase of the Oxo Tower and a further residential development in Broadwall, Gabriel's Wharf, the Bernie Spain Gardens and the riverside walk. These phased developments have also demonstrated the durability of the community group's masterplan.

In 1997 a limited design competition was held for the development of Site B which Haworth Tompkins won in collaboration with structural engineer Price and Myers and services engineer Atelier Ten. The brief called for an unspecified number of large family houses, flats and maisonettes plus open space and a community training centre, all to be built alongside a new commercial car park. At present the community training centre is still at the feasibility stage, while the car park and housing development are at week 17 of a 75-week construction programme.

Site B was formerly occupied by warehouses, which were demolished when the site was cleared to basement level over ten years ago. In the intervening period the site has been used as a car park. The retaining basement walls remain and it was therefore an obvious decision to place the new car park below ground and develop the housing from ground-floor level upwards. However, the general site levels needed to be reduced to accommodate the car parking while permitting street level access to the houses.

Initial site investigations established that the general ground profile of the site is peaty alluvium on top of five to six metres of gravel, all overlying London clay. Over this, in many locations, was made ground. Ground water was found at about 500mm below the founding level of the new car-park slab and was surprisingly constant in level considering the proximity of the site to the Thames, varying by only 50mm with the tide. The site investigations further established that the foundation structures of the warehouses largely remained in place and were generally founded on the gravel layer.

The new structural basement slab has therefore been suspended from concrete footings. Each of these consists of a 750mm thick mass-concrete base founded on the gravel layer with a 650mm reinforced pad on top. By limiting the extent of reinforced-concrete works below the water level, the extent of de-watering measures has been reduced. Additionally, where new bases coincide with existing bases, the existing base has been broken down to the required level and incorporated into the mass-concrete element of the new footing, thus reducing both the extent of excavation and the volume of mass concrete required.

The structural grid of the car park has been dictated by the requirements of the car-parking bays, aisles and circulation. A number of grid arrangements were tested to establish whether column positions could be co-ordinated with the housing structure above but this proved impossible. Instead transfer structures between the car park and the housing have been used. In one direction a change in ground-floor slab level between the housing and external areas has been exploited to form a transfer beam, while in the opposite direction the party walls between dwellings are reinforced concrete, again acting as transfer structures.

The insistence of building control upon applying the requirements of Section 20 of the London Building Acts has obliged us to provide a minimum of four-hours fire separation between the car park and the housing. To achieve this, the ground-floor slab separating the housing and car park is constructed with lightweight aggregate concrete and the cross-sectional sizes of all the supporting columns have been increased to provide additional cover.

It was considered essential that the residents should be as oblivious of the operation of the car park as possible. The structural slab with a 125mm floating screed insulation provides the necessary acoustic separation. Ventilation to the car park is mechanical supplemented by natural ventilation via the entrance ramp. There are no openings around the perimeter at street level drawing fresh air directly into the car park - air is brought in via louvres discreetly located adjacent to the pedestrian entrances to the car park. Exhaust air is discharged at roof level five storeys above.

In the central courtyard the ground-floor slab has been designed to accommodate a landscape build-up of more than 650mm. The landscaping works form an integral element of the development. The reinforced concrete in this location includes Everdure Caltite additive to waterproof the slab and do away with the need for a waterproof membrane. The initial cost analyses by the quantity surveyor indicated that a polymer-modified bitumen system would be the most cost-effective method of waterproofing the slab. However, the contractor offered such a large cost saving for the use of Caltite that it demanded consideration.

The housing is planned as three linear blocks around the perimeter of the site facing Cornwall Road, Upper Ground and Coin Street, enclosing a private communal garden for the residents. The fourth edge of the site is to be developed as the community training centre to enclose the central courtyard.

The scale of the new housing is intended as a reflection of its urban setting and acknowledges the context of the streets it addresses. The houses in Coin Street and Cornwall Road are three stories high with an 'attic' roof-top room at fourth-floor level. The houses in Upper Ground are three-storey with two-storey maisonettes above. This height is a response to the busy urban thoroughfare and the mass of the London Weekend Television Centre on the opposite side of the road.

A simple series of principles has been established in planning the dwellings. All the houses have street-level entrances and private gardens opening on to the communal courtyard. All the flats and maisonettes have large balconies capable of accommodating a table and chairs. The bedrooms overlooking the courtyard have balconies.

The structural frame of the housing is again reinforced concrete with flat slabs for simplicity and ease of construction, supported on columns in the party walls with dense concrete blockwork between. The thermal and acoustic properties of concrete have been exploited, the mass being used to store passive thermal energy and to provide acoustic separation between dwellings.

Environmental considerations have been at the forefront of the design, and were one of the key factors in its success at the competition stage. Each of the dwellings is fitted with mechanical background ventilation. The advantage of this system is the comfort control for residents: ventilation rates can be controlled and heat recovered from exhaust air to preheat the supply side of the system - which would have been impossible with fixed background trickle ventilators or air bricks. Each house also has solar water heating providing a significant portion of the energy required for heating domestic water.

The elevational treatment of the houses acknowledges their dual aspect, addressing both a dense urban environment and a private landscaped green space. The street elevations are expressed as a deep brick screen with window reveals at 225mm deep being a full brick in depth. The brickwork is supported on stainless-steel angles bolted into channels cast into the edges of the structural slabs. Where the elevation steps back at the upper-storey levels, the treatment is lighter with vertical pre-weathered zinc cladding. The construction also becomes lighter, the roofs being timber joists supporting a cold ventilated construction, again finished in pre-weathered zinc. Zinc-clad enclosures on top of the roofs contain the ventilation and heat-recovery plant.

On the garden elevations the concrete frame is expressed, with the edges of slabs and the faces of columns on party-wall lines exposed. The primary cantilever members of the steel balconies are grouted into sleeves cast into the slabs. The balconies have timber decking: Vitex Cofassus, a naturally durable hardwood from managed sustainable sources. It requires no preservative treatment or applied finishes, thus reducing maintenance costs. The same material is used for horizontal timber sunshades. These are an essential element of the balcony structures and are repeated at eaves level to minimise excessive solar gains on these elevations. The balconies are accessed from the bedrooms via pairs of glazed aluminium doors. Together with sections of vertical timber weather-boarding, these are inserted within the structural openings formed between the expressed columns and slabs. Again the timber is selected to be naturally durable and is unfinished to reduce maintenance. The palette of materials has been carefully selected to weather and mature with the landscaping.

Due for completion in spring next year, the housing will maintain the standards established by previous developments while providing quality housing at affordable low rents for local people.

Mathew Lunn is project architect at Haworth Tompkins Architects

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