Cosway Street is a 49-home residential development in Marylebone, central London, and forms part of the first phase of Westminster City Council’s Church Street masterplan. The private housing scheme, which will cross-fund 28 affordable homes on neighbouring sites, is distinguished by a curtain-like precast facade that varies in rhythm and tone as it ripples across the length of the building.
“The idea was to craft the building as if it had been carved from brick,” says Sundas Rohilla, project architect at David Miller Architects (DMA), which delivered the scheme from an initial design by Bell Phillips. “However, the fluting means there are a lot of ‘corners’ and ‘seams’, so we opted for an offsite solution using precast brick panels to ensure control over quality and a consistency of finish and details.”
The building occupies a potentially awkward site. Although it lies within a conservation area facing a grade II*-listed church, Cosway Street had become a fragmented part of the streetscape, bounded by roads with the Bakerloo underground line running directly below. To protect residents from the rumbling noise and vibration of the trains and traffic, the building was designed with a reinforced concrete frame. Typical spans are 6.5m x 7m, with 800mm x 200mm blade columns located in the party walls to minimise impact on the internal layout. The flat slabs are 250mm deep.
Due to the Tube lines, the structure becomes more complicated below ground. The ground floor slab acts as a transfer structure to accommodate the car park grid line and the no-build zone around the tunnels. With minimal space for piling, the building is supported on a raft foundation, which has been decoupled from the super-structure using steel-laminated rubber bearings to reduce vibration. Even the base of the lift shaft is isolated from the foundations.
The robust structure also helped to facilitate the scalloped facade, which is suspended from the frame in storey-high units. Windows and Juliet balconies were all built in, while service penetrations were designed out: “We used the balconies to conceal services, with downpipes and rainwater pipes recessed into either side of them, also hiding the joint between the facade panels and the traditional brickwork lining the recesses.”
DMA worked closely with precast facade specialist Decomo to establish the geometries of the fluted shapes, which were inspired by the arched windows and circular columns of the adjacent church. The Belgian manufacturer developed its own model from the shared BIM, taking panel fabrication off the critical path while it worked out how to create the different curvatures. In all, 117 different shapes of brick were needed in each of three brick colours.
Colours, gradation and finish were all tested in large, 3m-wide, prototype panels. “We needed a range of colours that allow very subtle changes in hue as you move around the building,” says Rohilla. The mass of the building is reduced not only by these different brick tones but by the subtle gradation of the mortar, with five different colours that get lighter as the five-storey building rises. The panels were built using half-bricks and were pointed offsite in the factory.
The depth of the scalloped panels was also exploited to improve the fabric’s environmental performance. Again, the quality control of the manufacturing process was vital, ensuring that the non-combustible insulation filled every inch of the curving cavity. This was part of a sustainability strategy that shows a 35% improvement on Part L. The building also includes photovoltaic cells on a green roof, as well as an option to install air-source heat pumps in future.
In total, 3,500m2 of facades were delivered, comprising 285 precast panels. The panels were installed on site at a rate of eight per day, taking just four months in total.