Bags of ingenuity and a refusal to be overshadowed lie at the heart of this successful North London office renovation
Standing in the shadow of architect AHMM’s Angel Building in London’s Islington (literally – it’s across the road, due south of it), it was never going to be easy for practice Stiff+Trevillion to compete with that Stirling Prize-shortlisted refurbishment; although if it does, it’s been through stealth, setting itself up in formal opposition to its richer cousin, rather than by playing it at its own game. Hence, to counterpoint its neighbour’s high performance full height curtain walling, Stiff+Trevillion decided to adopt a more contextual approach for its £9.5m refurbishment, hearkening back to the traditional terraces of this now affluent area. The result is a striking trabeated facade of Petersen Tegl brick, sparkling through its use of highly contrasting grey hues.
The reinvented 10-4 Pentonville Rd was built in the 1980s by practice EPR as the sister building to the then BT Angel Centre. Developer Derwent inherited them from London Merchant Securities on merging in 2006. It is actually two adjacent buildings, built concurrently, separated by a public access road that remains sacrosanct even in its new form. Unlike the pink granite spandrels of the Angel Centre, the architect had here had opted for a more restrained low-rise, cut-price Miesian classicism, its bronze anodised spandrel panels marking the lines of the reinforced concrete structure – the rest of the facade flush to them with heavily-bronzed, low solar gain glass. Curiously, the elevation was articulated by three full-height vertical protrusions along the façade, having neither rhyme nor reason in its Miesian aesthetic. Practice partner Michael Stiff believes they were the architect’s concession to planners’ concerns for the local urban streetscape.
Stiff explains that to make the development economically viable, Derwent needed the net lettable areas to be optimised, which led to a design proposal linking the two previously separated buildings with new floorplates sailing over the dividing road. To this,the firm has applied its unifying brick skin, creating depth where formerly there was none. Despite refurbishment, however, two separate receptions remain, like two heads on the one body – the public road confounding any attempt to fully link them. These became the subject of some architectural ingenuity, to make a ‘positive’ of what could easily be viewed commercially as a ‘double negative’.
‘With both permissions finally achieved, it was proposed to knock through the two buildings to create a single large floorplate. It’s a permission that was won more by canny degrees than bravura’
Clever plan
An ingenious approach to the linking of the buildings treated them as two separate applications. Stiff explains that this developer-driven approach meant that the planners dealt with it as two minor, rather than one major, application – and it could be handled at officer rather than committee level and determined in eight rather then 13 weeks. This had interesting repercussions – one building, No.4, went through with only one condition, while No.10, complicated by daylight/sunlight issues thrown up by a small build out at its rear, was refused by a different officer. Being a refurb, there was no requirement to reduce the existing ratio of glazing to solid, but in the course of steering the scheme to approval the planners began to grow interested in the facade’s materials – influenced by those on the already permitted scheme. With both permissions achieved, it was then possible to knock through the two buildings to create a single large floorplate. As such, it’s a permission that was won more by canny degrees than bravura.
Fundamental to the project’s financial viability was the design team’s strategy for the new build component that obviated any need to underpin the existing foundations. As an aside, project architect Ed Mullett suggests that the Eureka moment came about when engineer AKT II reminded them of concrete’s ability to gain strength incrementally over the years – but the fact is the team could also take advantage of changes to the engineering loading criteria over the same period, opening up the possibility of making the existing structure work harder. AKT II director Andrew Ruck speaks of the typical engineering overprovision at the time it was built, born of British Council of Offices guidelines governing commercial developments in the 1970s and 80s. Then, engineers were working to design loads of 5kN/m2, whereas British Standards and Eurocodes for commercial buildings now sit between 2.5+1kN/m2 and 3.5+1 kN/m2. These new working standards have effectively created greater redundancy in the Pentonville Road building, something AKT II capitalised on. As designed, the 140mm thick lightweight concrete composite floors sit on new 300-400mm steel beams spanning the road, impinging on the side walls of both buildings and forcing the existing reinforced concrete structure to ‘sweat’. Only at a few points did the existing concrete columns need to be beefed up with concrete jackets.
All these extra loadings are inevitably transferred to ground, so it was AKT II’s job to ensure the foundations were strong enough to take them. Ruck explains that none of the team was keen on underpinning – the building was close to underground Tube tunnels, limiting what could be done; and from the client side, any works would have had time and cost implications on a job already pressurised by being built out during a recession. Again here, and confident in the integrity of ‘good, firm London clay’ the engineers questioned another engineering code guideline – that the thick concrete pile caps had no bearing at all on the foundations’ ability to take load. AKT II argued that this was not the case, re-appraising the building’s engineering model to take account of them, and presenting their findings to Building Control, which was minded to accept them. This single move, says Ruck, probably saved the client about £250,000 on the bottom line and 6-8 weeks on a 12-month programme.
Stable loads
With the foundations safely taking the additional imposed loads, the architect was free to build its Danish National Format (longer and slimmer than standard) bricks wall off them. The engineers’ only concern was that no further load was transferred from the used for this wall back to the main concrete structure. To ensure this, Mullett explains that they detailed the hand-set wall tied back to a Metsec frame, which then tied back to the existing RC frame. This kept the lateral stability of the wall at regular intervals across the facade without bearing off the concrete structure. The separation of one from the other was good for the programme too. Stiff explains that even though the brick construction was at one point running four months behind schedule, this had no effect on the project’s critical path because the off-the-peg bronze anodised aluminium Schueco cladding system – with lighter customary mullion extrusion – was installed independently of the brick wall in front of it.
Traditional lime mortar was specified, and its ability to deal with building movement through ‘microcracks’ meant the architect got the facade it wanted – a seamless one without any expansion joints at all. Stiff adds that a lot was riding on the consistency of the mortar colour in the raked joints, aided by the fact that main contractor Sisk bought a huge batch of it, storing it in dry silos to ensure even colouring across the facade. Consistency of the brick mix was also provided by the fact that Petersen Tegl batched the bricks for delivery pre-mixed using an automated selection process, so bricklayers on site could pick them off the palette and lay them randomly.
The boosted net internal area – from 43,000 to 53,000ft2, a gain of 25% – was due not just to the infill floors between the buildings, but to shunting the air handling plant from the basement to the roof, which was strengthened when the upper floor mansard was built out to create a top floor stepped, rather than angled, facade. Small variable refrigeration flow units replaced older chillers, making room for solar thermal heating. Mullett explains that achieving BREEAM Excellent on the building would have involved more than large PV arrays, so the client was content with its ‘Very Good’ status. This does not seem to have bothered the commercial market– the building was pre-let before it opened at a higher per ft2 value than that at the sister Angel building.
It has been a striking reinvention of a tired and dated building, but it is interesting to note that the architects could not bring themselves to fully eradicate all trace of its former guise without at least some acknowledgement of the modernist aesthetic at its core. Outside the ground level entrance are two 6m high solid mirrored steel cruciform columns, rising the full double height of the new reception areas within, transferring the loads of the brick facade at that point to ground. Read with the anodised facade of the infill floor zone, these twin entrance porticoes come together to form a kind of Miesian Tempietto – sealed like an insect in amber within the building’s bold new brick facade. It all goes to show that you can’t keep a good man down; especially when the man in question was the size of two.