A hybrid concrete frame and hand-placed facade help Pollard Thomas Edwards deliver 91 mixed-tenure homes on a near-impossible site next to Walthamstow station
Juniper House is what Justin Laskin, partner at architect Pollard Thomas Edwards, calls a “head and tail” building. Facing the busy Walthamstow Central station in north London, its public face is an angular 16-storey tower in dark brick and white precast concrete, with sharp corners that cantilever over the streetfront. Behind this, the building takes on a different form, stepping down to a lighter-toned nine-storey block, before unfurling its tail – a row of two-storey homes, threaded between a railway line and a row of gardens.
The council-led development includes 91 low-carbon homes. Half of these – mainly apartments in the tower – are for private sale. The rest are affordable, some for social rent, others shared ownership. The low-rise block contains the larger for-rent family homes, all of which have their own front door. There is also a nursery, tucked away on the eastern edge of the site, and a London outpost of the University of Portsmouth in the tower podium. “We quite liked the idea of the building having bookends – the beginning and end of the educational journey within a single town-centre site, with homes above,” says Laskin.
If this sounds like a lot of life to squeeze into one urban development, the 0.4ha site itself was not an obvious choice. A triangular sliver of brownfield land, it was bounded by a railway on one side, a sloping major road on another, while the third was flanked by terraced houses. There was only one small back road for vehicular access. “It was unbelievably constrained,” says Laskin. “A lot of people weren’t even sure it was developable.”
These constraints bore an overriding influence on the design, with the architects working closely with the construction team from the competition stage. “We were thinking about the logistics even before coming up with architectural drawings, because we knew access was going to be so limited,” says Laskin. “There was pretty much no room for material storage and we could only get one crane on the whole site. That impacts not only your choice of structure, but also the facade.”
The building has a reinforced concrete frame to the podium level, and in-situ flat slabs with precast columns on the floors above. “I think you would have struggled to get any other material to work on a site like this,” says Laskin. The facade, meanwhile, had to be able to be installed without a crane, which meant handlaid brick and very small precast elements.
The window frames are divided into as many as ten pieces – corner units and connecting horizontals and verticals – while the horizontal bands around the two-storey homes are made from panels just 1m long. “We tried about a dozen different ways of doing it, with different sizes of profiles. In the end, this just felt like the most time-efficient method – and I think it’s worked.”
At lower level, where joints would be more noticeable, the team specified glass-reinforced concrete (GRC), which could be cast in more lightweight, thinner, and therefore larger, panels. “The limit was always what two people on site could lift,” says Laskin. Matching the GRC and precast was a challenge, requiring the architects to investigate a range of different products: “We worked hard to make sure they sat comfortably next to one another. They’re poured in different places with different mixes, but still had to look like the same material.”
With its stepped form and angular footprint, the tower was never going to conform to a regular structural grid, but PTE worked with structural engineer Price & Myers to rationalise the column spacings as far as possible, ensuring that the majority of slabs could be limited to a depth of 250mm. “That collaboration really helped us,” says Alex Bradshaw, project engineer at Price & Myers. “If we said, ‘we need to introduce a column on this line to achieve 250mm’, they were up for that. We always challenge column grids where we can: even half a metre can make a real difference in terms of the overall embodied carbon.”
The cantilever over the north-western corner was also made more structurally efficient. Although the overhang helped the architects to maximise the building footprint, the initial design transferred the load via a 1m-thick slab across the whole first floor. Price & Myers spent a long time rationalising this element: “By the time we’d finished, we were able to concentrate the load on specific corners of the floorplate, which meant that two-thirds of the slab could revert to 250mm,” says Bradshaw.
Cement replacement also reduced the building’s embodied carbon, with a 50% GGBS specified for the in-situ elements. And as the design of the 880mm x 220mm precast columns changes at the ninth floor, the reinforcement could contain a lower proportion of steel due to the reduced loads.
In 2021, Price & Myers calculated the as-built embodied carbon at 386kgCO2e/m2 (lifecycle stages A1-A5). However, there is an important caveat: this calculation assumed the use of a CEM I mix throughout and a global average reinforcement carbon factor, which was common practice at the time. In reality, Bradshaw says, the GGBS used in the concrete would lower the embodied carbon by about 10%, while the steel would have been procured from a UK CARES supplier, and therefore made from recycled material, lowering the associated carbon by 55%.
This would reduce the overall embodied carbon by over 20% to 300kgCO2e/m2. “Even over a four-year period, we are still struggling with how best to compare calculations from the early years of embodied carbon reporting, due to the ever-changing material factors and procedures,” he explains.
In terms of operational carbon, the scheme’s total emissions have been calculated at 75.05 tonnes CO2/year, which is 65% better than the Part L (2021) baseline. The design team adopted a fabric-first approach, which involved integrating a 250mm layer of insulation behind the brick outer wall. The depth of the wall build-up introduced a significant cantilever from the tower’s structural frame to the facade, which the structural engineers reduced by extending the floor slabs 25mm into the cavity, lessening the amount of steel support needed.
To protect against overheating, the south facades are heavily profiled with deep balconies and recessed high-performance windows, while the majority of flats are dual aspect, enabling cross-ventilation. Roof terraces give all residents access to outdoor space. The low-rise “tail”, meanwhile, is veiled from the afternoon sun (and neighbouring gardens) by a screen of obliquely angled brick-piered balconies. A thermal comfort analysis for the residential units and communal corridors confirmed that all assessed spaces were compliant with CIBSE TM59 overheating requirements.
Heating is provided by communal air source heat pumps and PV panels, and is supplied to homes via underfloor heating. Because of the low ecological value of the site, measures such as bat boxes, blue and green roofs have resulted in a biodiversity net gain.