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Specifier's choice: Crown Street Buildings, Leeds

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Using a contract that involved some elements of Contractors' Design saved Allford Hall Monaghan Morris doing unnecessary work on Crown Street Buildings, Leeds, but still allowed it to specify some unusual materials. Jonathan Hall and Sarah Hunneyball talked to Sutherland Lyall

Allford Hall Monaghan Morris is currently building Crown Street Buildings, a mixeduse development in the middle of Leeds. It is on a site between the up-and-coming Calls, with its fashionable bars and warehouses, and the city's main shopping zone. Opposite the curving north side of the roughly triangular development is Leeds' Grade I-listed corn exchange, and just below it is the city's old Cloth Hall. To the south is a railway viaduct leading to the main railway station and opposite the short side of the site, on Calls Street, is a mix of Victorian buildings converted to bars and trendy shops.

The ground floor and basement of this new hollow triangular scheme are intended for commercial use of the same kind, with shops on the ground floor and a pub/nightclub and restaurant mooted for the basement. On the three floors above are apartments, with penthouses on a set-back upper floor. Apartments and shops on the east side are built into the shells of four Victorian terrace houses. And it will be colourful, with infill panels across the north elevation in shades seguing from blue through green to yellow.

The site had been vacant for some time when the client, Welbeck Land, bought the site with a scheme attached. There were some planning issues and Welbeck decided to start again with the design. It ran a limited competition, based on outline schemes. AHMM's was the most convincing proposal, and the practice was appointed, inheriting the other consultants from the earlier scheme.

The site was restricted by its proximity to the old corn exchange, the presence of a service road between the site and the railway viaduct, and by the need to elevate the housing (15 per cent of which had to be social housing) above the night life specific to the area; noise generated by late-night drinking would be exacerbated by the scheme's own proposed night-life facilities. The planners insisted there should be no parking on this city-centre site and the presumption was that most occupants would not have much in the way of family. One of the planning conditions was that there had to be retail use.

The single-storey apartment floorplans are straightforward, with the minimum of circulation space. There is a dividing wall down the middle, with an entrance alongside the kitchen leading to a living room, and a door leading laterally to the bathroom, which serves a bedroom front and back.

Hall says: 'We had developed a basic plan for prefabricated dwellings for Peabody at Raines Dairy in Hackney, in which we were trying to minimise circulation areas. The plan of the flats at Leeds is based, to an extent, on that.'

Access is from walkways that run around the internal court. They are held away from the building to maximise privacy, although on the east the walkway runs inside the rear facades of the gutted terrace houses hard up against the new facades of the new flats.

Contractor design This was a JCT 98 with contractor design.

Hall paraphrases the way this works: 'There is an initial package up to stage D with some rates developed. You get a builder on board and you produce the design and then negotiate the detail. And then you get novated to the contractor.

'The thing is you don't end up doing a lot of abortive drawings that you would do to protect the lump-sum tender price, or when the tenders come back too high or when the contractor wants to change things. So there is efficiency in the process and you develop a relationship with the contractor at an early stage, and develop during the negotiation process when you are firming up the spec with the builder and the client. It means you end up with a solution that is acceptable to everyone.'

What AHMM doesn't do is also act for the client, with a Chinese wall between the two teams. Hall says of this increasingly popular mode of proceeding: 'Generally it has its attractions for the client side because they have the benefit of an architect on board and it saves them getting someone else to report, but it does give a potential conflict - so our insurers have advised us.'

He adds comfortably: 'It's quite a complex building and it's going reasonably smoothly. We are building on bell pits. That's the way they used to mine in the 18th century, which meant they ended up with these big bell-shaped holes underground. So the foundations turned out to be a concern to the engineers.'

Digging the basement dealt with much of the void problem, but extra mass concrete was poured around the piling just to be safe.

High-quality elevations The elevations to the three floors of apartments take the form of a brick grid with a random pattern of coloured infill panels and glazing. Hall says: 'The good thing about contentious sites is that you end up using good-quality bricks, and other materials as well. The Victorian pressed bricks from Furness Brick are based on bricks used in the local warehouses. For the Victorians they were bog-standard rather than expensive specials. We have great bricklayers on site and they say they enjoy what they are doing because the bricks are so nice.' Unlike Victorian construction, these bricks are deployed on the cavity-wall principle. There is no clever stuff with Victorian lime mortar either because, Hall says, 'we have to have movement joints for structural reasons'.

The infill panels are made up of blocks of coloured, glazed Sicilian lava stone by Pyrolave and supplied by Surrey firm James & Taylor. Hall says: 'We suggested this as part of our planning proposition. The planners loved it - and everyone else seems to like it.

And it's interesting stuff to use. The lava stone itself comes from the south of France or Sicily. It is given a fired-on colour glaze finish so you end up with a quite deep surface with a kind of variegated glaze and quite richly textured.'

Hall's team came across it almost by accident, but saw the connection between it and the faience terracotta used on arcades just up the street in the centre of Leeds. Like that model, the individual stones are laid in ashlar pattern rather than the stack bond architects often use to denote non-loadbearing masonry. The big shifts from this paradigm were in using colour, and gradating it from one side to the other of the long north elevation.

Hall says: 'We have been working with the suppliers, James & Taylor, and they have been extremely helpful in cutting, fixing, and specifying the stone. They were instrumental in advising us how to use and handle it, and have been on site while the subcontractor completes the installation. The contractor decided to use some of his own tradesmen and we have been doing sample bits and pieces, so it's been a bit of a learning process, and now we have ironed out any niggles about joints and tolerances. It is costefficient in the manufacturing process and you can have different colours without too much extra cost.'

Airedale Glass fabricated the glazing system from Technal Fxi46 aluminium sections and Saint Gobain glass. Hall's team took advice from Doncaster acoustic specialist SF Garritt about ambient night-time noise and based the performance specification on these findings - and asked Pilkington whether glass by itself could meet the basic requirements.

The insulated render used on the internal court elevations is Marmorit. Hall says: 'We did a performance specification so you have no control over which manufacturer is chosen. The main contractor came back and suggested Marmorit. We had originally specified a traditional German system straight on to the blockwork using a mesh reinforcement and Rockwool insulation. But early on the substrate was changed to a Metsec framework. Even so, for housing we have to have NHBC approval. NHBC said we had to have a drained cavity. So we now have a cavity between the insulation and the Metsec framework.'

Job architect Sarah Hunneyball explains that, although most of the access walkways in the internal court have some kind of shelter overhead, they are exposed to the elements.

Made from ply on timber joists between steel beams, they are cantilevered off the main structural frame. Rather than paint the ply, the architect specified a Decothane lightweight no-slip finish based on quartz, fixed in a resin body.

The vertical glass planks over the residents' entrance are Reglit. Hunneyball says:

'We looked at another manufacturer but Reglit was cheaper.' So, too, was local kitchen firm Howarth Timber, when Magnet was the maker first thought of. And the contractor reckoned Shires offered a better deal than Armitage Shanks, which AHMM had originally specified. In the same way, Trocal replaced Sarnafil. Sometimes, as in the case of architectural ironmongery and preformed sheeting, it made more sense to go to local suppliers, but the Deva taps, Kaldewei baths and Kone lifts remained as specified.

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