ABOVE AND BEYOND

Redevelopment
Building on top of an existing structure can be liberating for an architect otherwise constrained by the restrictions of the site – as these two examples demonstrate

Words Jan-Carlos Kucharek

In straitened times, there’s particular pressure on architects to squeeze what value they can from a site at minimum cost, proving that recessions really can become hotbeds of creativity. And certainly, when faced with run-down buildings that are more of a drain on a developer’s resources than an asset, imaginative architects can prove their worth. But when a structure is built out to its demise, the only way to get more value from the site is up. When refurbishing, the economics of the new works are crucial to the developer’s bottom line, calling for serious consideration of the existing structure. But done well, a design can transform a building and turn it from a liability into an asset.

Architect Duggan Morris found itself in just such a situation recently on a job with developer McDonald Egan, which was saddled with a commercial and storage property in Catford, south east London. Of B1 and B8 Use Class, it nestled in a residential area that constrained the site in terms of overlooking. The firm first submitted a scheme to demolish the existing building and convert the site to full residential. This was thrown out by the planners, intent on maintaining the local employment value of the site. ‘When we went back to the client,’ says practice partner Joe Morris, ‘we knew we were going to have to adopt a far more leftfield approach. We knew we had to keep the existing buildings and the employment-generating component of the site in place, and that drove us to the rooftop solution augmented by a two-storey, two-unit residential development to the street.’ The site would realise a far greater value with what turned out to be a budget of £1.3m – about £1,600/m2.

Even with only one extra storey to play with, and with limited roof area, Duggan Morris managed to fit five 55m2 two-bedroom units on the site, as long as they could deal with serious overlooking issues with the neighbours. It was clear a new structure would be required, but there was no guarantee that the existing strip foundations would be able to take the additional load, and anyway the architects wished to avoid any remedial works to the existing building – and recourse to Building Control. If the apartments were going to be built on the roof, a ‘structure within a structure’ would be required. ‘We ruled out the option of a lot of underpinning to reinforce the existing structure. Instead we decided to slide 12 new steel columns into the building, offset 200mm from the strip, sitting on their own pad foundations,’ explains Morris. With a new row running centrally through the existing building, and their torsional effects limited by the timber floor, and with 203mm by 203mm steel beams running between them, there was a solid base for the new structure – one that was completely independent of the existing.

Metsec walls have been used to form the new structure on a 350mm thick floor zone that hovers about 500mm over the original timber roof, just allowing it to clear the parapet. The walls have been insulated, weatherproofed and timber-boarded either side with an external rainscreen of larch cladding. Morris says the firm did look at a pre-fabricated, engineered solution, but that ‘there were no economies of scale to be had, and the numbers just didn’t stack up’. The gap between the old and new structure also performed a key function, acting as a void for the new services to run over to the stack where they pass down through the existing envelope. ‘The approach meant acoustic and fire compartmentation issues could be dealt with in one stroke,’ says Morris. Cantilevering the flats over the building line to point away from the opposite properties directed views away from the immediate neighbours to the allotments beyond. Light issues – namely, how to get light into the second bedroom in the middle of a deep flat plan – were resolved with an ingenious long and narrow balcony, giving just enough space to install a glazed door at its end. Apparently the evening light is ‘spectacular’.

Spectacular is a word that might also be used for Young Architect of the Year 2008 finalist Kraus & Schönberg’s design for German developer Garbe, ironically in the ‘Little Germany’ area of Bradford. Hanover House, a grade II-listed building, sits right in the middle of a conservation area, but the developer knew that to realise a profit from the refurbishment of the old warehouse to residential apartments, it would have to expand to increase the net areas. The architect’s radical solution was apparently an interpretation in microcosm of the rooftops of Bradford. Its proposal was folded plates of metal abutting and pulling back from the parapet, creating a dramatic artificial landscape of plateaus and troughs.

Kraus & Schönberg, which won the 2009 German Timber Award for its ‘Haus W’ in Hamburg, built in KLH engineered timber, was keen to use the product on Hanover House, knowing the intrinsic strength to weight properties of the material would mitigate loadings on the existing structure below. This, a 5m by 2.5m grid of cast iron columns, was not suitable for the kind of free plan the firm sought, so from the outset the practice wanted a floor structure robust enough to take the new imposed loads. ‘Ideally, we would have installed a 140mm KLH floor all the way across as, being cross-laminated, it would have spread the load beautifully,’ says practice partner Timm Schönberg. ‘Instead, however, the d+b contractor installed 300mm timber beams with an 80mm timber ply build up. It was a bad move, as it meant that when loads were not transferring directly from above onto the beam, a post was required.’ But while this was a troublesome consideration, it did not affect the basic principle of the design. A shell of 140mm KLH will form both roof and walls. The main structural load of the new extension is taken by three cranked glulam beams springing from the party wall of the neighbouring warehouse, which then crank down to tie to the floor plate. ‘It was a nice solution, because as long as you accepted these would be the balcony locations, the beam at this point becomes a glulam exterior wall, and no additional bracing would be needed.’

The shell was modelled in 3-D Microstation. Because the firm ‘wanted to stay in control of the geometry’, it double-checked KLH’s model before the information was uploaded into the factory CNC machine. ‘The set out and cutting was so accurate that the whole structure was CNC cut in Austria, brought to the UK and constructed on site within a weekend,’ says Schönberg. The architects thought unglazed areas of the new roof would have to be local slate, but the whole form took on a homogeneity when the planners allowed the firm to specify pre-patinated zinc by VMZINC, which is a similar colour to slate, but has the advantage of being able to wrap ridges and eaves points. As a result, hidden gutters were neatly set back from the ridge, draining onto the timber floorplate and back to the services stack at the rear of the building. With the whole building refurbished, a car park built and the roof extended for a mere £1.5m – that’s E1280/m2 – Schönberg admits there was a crudeness to the construction process. The razor sharp finish to the roof evidenced in the design was compromised by the thicker eaves detail, but it doesn’t seem to bother him – he concedes that sometimes an architect ‘should be happy to accept the ugliness of things’. But accepting that things must stay as they are has proved the most potent aspect of both these projects: the old and the new clash in conscious and considered ways, lifting a project to new aesthetic levels.