John McAslan + Partners’ landmark residential tower in north London takes ultra-high-performance fibre-reinforced concrete cladding to new heights
Apex Gardens may not look like a concrete-clad building, but lightweight precast panels were instrumental in both its design and construction. The 23-storey residential complex in north London, designed by John McAslan + Partners, has a brick-slip facade supported on 4,500m2 of ultra-high-performance fibre-reinforced concrete (UHPFRC) panels. This solution reduced the amount of material needed by about half, cut three months off the construction programme and, crucially for such a landmark building, allowed the architects to bring depth and detail to the brickwork.
Rising from a small, triangular footprint on the busy junction of Seven Sisters Road and Tottenham High Road, Apex Gardens is one of the tallest buildings in north London. The tower, which is flanked by two adjoining blocks of six and four storeys, is a fairly conventional structure of in-situ reinforced-concrete flat slabs supported on precast-concrete columns and internal walls. It offers a slender profile to the main arterial road, with its mass broken into three rectangular volumes. These have been designed “to look as thin and blade-like as possible”, according to Heather Macey, associate director at McAslan.
The building form has a considerable impact on the accommodation within. Apex Gardens contains 163 apartments, 60% for the private rental sector (PRS) and 40% affordable housing. The slimline plan allows many of these to be dual aspect and, unusually for a residential tower, the corridors also have windows at both ends. Cutbacks in the main rectangular volumes have made room for a number of spacious terraces – all shared amenities – with views across London.
Such communal areas are a typical feature of PRS schemes, which tend to target young professionals with short-term contracts, furnished flats and managed facilities. In this vein, Apex Gardens also has a double-height reception area, fitness room and co-working space. “Residents don't necessarily have to be in their apartment. You have another place to work if you're not going into the office, which as the pandemic has shown is really important. No one’s isolated,” says Macey.
While the elegant form makes a virtue of the small footprint, the hemmed-in nature of the site presented a number of logistical headaches. “The site is incredibly constrained,” says Macey. “It's pretty much next to Seven Sisters station, as well as two massive road arteries. Loads of people needed to pass by, trucks could only get to the back of the site, there wasn’t much space to unload and only certain places where you could put a crane. Building something 23 storeys tall in that context is quite tricky.”
The design team wanted the building to have a high-quality, natural-looking brick facade, in keeping with the surrounding streetscape and the aspirational nature of the building. But they knew that hand-laying brick in this context would be extremely difficult. Likewise, conventional precast-concrete brick-faced cladding panels would simply have been too heavy and unwieldy for the weight-limited crane, with units typically 200mm thick and between 8 and 10 tonnes.
So instead, they turned to Thorp Precast, which was developing brick-slip systems with UHPFRC cast monolithically onto the back of 50mm-thick brick components. Apart from localised ribbed areas, thickened to accommodate lifting and fixing points, Thorp was able to reduce the overall panel thickness to just 100mm, and the weight by about 50%. This meant the panel sizes could be maximised, ranging from 6.5m to 8m long and 3m to 4m wide, with pre-installed glazing in many of the units. This in turn resulted in less time on site, improved safety and cost savings. The more efficient use of material also reduced embodied carbon by 20-30%, lessened the load on the structure and foundations, and required fewer site deliveries.
The UHPFRC panels gave the architects greater scope to be inventive with the brickwork, allowing them to play with depth in a way that is not usually possible with brick slips. “We wanted the window reveals to be as deep as we could possibly get them,” says Macey. “Because the cladding has a lighter build-up, we were able to fold the brick slip around the corner in a way that's insulated and doesn’t take up too much space. It has that depth of reveal, without having to change the material.”
Rather than a thin skin masking a concrete building, the effect is of an authentic brick structure.
A buff multicoloured facing brick was specified for the tower and a darker monotone buff brick for the lower block; subtle decorative elements include vertical bonds above the windows and some panels featuring a raised, banded motif. The architects could control the tone of bricks and mortar in a way that would be impossible with traditional handlaying. To avoid differentiation between panels – a potential problem if there is a break in construction, or if they have been stored slightly differently – a treatment was post-applied to blend them together into a unified whole. “We asked ourselves, how much differentiation do we want? Is it a good or a bad thing? We wanted it to feel natural.”