Preserving the original structure and character of a notorious estate originally conceived as a ‘vertical garden city’ helped Mikhail Riches’ Stirling Prize-shortlisted redevelopment cut embodied carbon by 61%. Stephen Cousins reports
The Park Hill estate has been a landmark on the Sheffield skyline for over 60 years, and at the time of construction was a true vision of the future. This ‘vertical garden city’ for 3,000 residents had ‘streets in the sky’; raised access routes wide enough to accommodate milk floats, and direct access to shops and pubs on the ground floor.
Designed by architects Ivor Smith and Jack Lynns, the interconnected tenement blocks were positively received by residents, but over time problems with poverty, crime, drug use and anti-social behaviour brought Park Hill into disrepute, and by the 1980s it had a reputation as one of Britain’s most notorious “sink estates.”
There were calls for it to be knocked down, but in 1998 the buildings were saved from the bulldozers when English Heritage granted it a Grade II listing, making it the largest listed building in Europe. Property developer Urban Splash took over the estate, kicking off the regeneration with architect Hawkins\Brown’s ambitious reimagining of three ‘wings’ in Phase One, completed in January 2013. This replaced the original facades of brick panels and timber windows with brightly coloured anodised aluminium panels and large sections of glazing.
Fast forward to 2015 and the brief for Phase Two, a U-shaped block at the centre of the development, called for a lighter touch approach that would preserve as much of the original building as possible, whilst prioritising energy efficiency. The competition-winning design, by Mikhail Riches, delivers 195 flats and 2,500 m² of ground floor commercial space.
Alim Saleh, project architect at Mikhail Riches tells ADF: “Hawkins\Brown completely stripped the structure back to the concrete frame, gutted everything and started again because that level of internet intervention was needed at that time to really change people’s perceptions of the estate. They did a really good job, but we had to be more sensitive in our approach, in terms of the listed structure and listed status, and from an environmental point of view, to try to keep some embodied carbon.”
The retentionist approach, preserving the original brickwork infills and the majority of the concrete frame, resulted in big embodied carbon savings, while upgrades to the building fabric slashed operational carbon compared to the original building. But finding appropriate and resilient solutions raised complex challenges for the architects, who had to carve out spacious modern flats from a distinctly ungenerous layout, and adapt a structure plagued by tolerance issues and cold bridges.
Unusual lengths
The stark brutalist architecture of Park Hill has always divided opinion and Urban Splash went to great lengths to ensure its new vision for the estate would be embraced by the city.
The competition for Phase 2 of the project, held in 2015, saw five shortlisted practices each assigned a flat in the derelict building and given two weeks to demonstrate their ideas. The brief called on the architects to “live and breathe” the site and “wholeheartedly commit” to it.
Mikhail Riches’ team was inspired by the remnants of lives lived there, including traces of former inhabitation and the ways residents had made their flats feel more like home. For example, some had painted balcony walls different colours, and laid patterned linoleum on their doorsteps.
Back in the 1950s, when designs for Park Hill were first published, the facade was heavily criticised in some quarters for failing to express individual dwellings within the block, regardless of whether apartments were large or small and who might live inside.
“Personalising their balconies was perhaps a way for people to look up at the building and say, that’s my flat, helping give a sense of individuality in the whole,” says Saleh.
Mikhail Riches’ competition-winning proposal sought to redress this failing, installing patterned ‘door mats’ at each entrance onto open-air access decks (known as the streets in the sky’), and coloured balcony reveals to give each flat its own identity. Otherwise, the building would be a faithful restoration, keeping as much of the existing fabric as possible and giving it a thermal and acoustic upgrade.
The original Park Hill layout was complex, a bewildering configuration of 37 different flat types and no repeating cluster. Even the drawings were hard to interpret, with notes written in obscure jargon, forcing Mikhail Riches to retain the same team on the job to avoid misunderstandings.
Existing flat sizes and layouts were reconfigured to meet modern standards and ways of living. A ‘typical’ existing floor plan featured a series of single bay-wide flats – dubbed ‘sad flats’ by the architects – were too cramped. These were removed to ensure all apartments have at least a double-bay wide kitchen/ living space.
“When we tessellated that through the scheme, it enabled double bay wide living spaces on the deck access street side, or on the balcony side, which enabled many more flat typologies,” says Saleh.
Although some three-bed units remain at ‘hinge’ points in the U-shaped plan, most were converted to two-beds with an added ensuite; walls between stairs and landings were also removed to open things up.
Carbon counting
Sustainability was embedded into Hawkins\Brown’s Phase One scheme, which retained and repaired the original 50 year old concrete frame. Phase Two took things a stage further, a key constraint being the preservation of not just of the concrete, but the bricks too.
A condition survey of the structural frame was crucial for the performance specification and repair strategy, which included removing areas of defective concrete and corroded reinforcement. Where exposed rebar was significantly corroded, bars were locally cut out and replaced with new welded rebar primed for repair mortar.
The building had largely stood the test of time, and the only major demolition required was around the entrance areas, where the removal of flat units opened up space for new double-height lobbies. The bricks were largely undamaged and the original mortar only needed repointing where existing windows had been removed. Nevertheless, some 60 years of weathering, decay and mould-growth made it uncertain how well the bricks would clean up to match the original.
Historic England wanted the fire skin (the outer skin of the brick), left undamaged, which ruled out an abrasive, or chemical clean. Instead, a low-pressure ‘Jos’ clean, typically used in stone conservation work, removed any staining without chemicals. The existing concrete frame was also repaired and cleaned as part of the work.
Combustible materials are banned for use in external facades of buildings higher than 18 metres, so apart from the outer skin, the wall build up, including the structural block work and plaster, was removed and replaced with a steel framing system, cavity insulation and new plaster. This made it possible to bring new windows back into the thermal line, with pressed aluminium reveals adding depth to the facade.
Preserving as much as possible for the existing structure, in line with the listed status, raised challenges when trying to resolve issues with thermal performance and cold bridging, and put a focus on insulating every surface of every room. This lowered the already restrictive floor-to-floor height to just 2.215 metre, versus at least 3.1 metres in a modern building.
Extensive thermal imaging work carried out on Phase One and modelled by Greengage, found that the balcony cheeks (side walls) were acting as a major cold bridge, so between 60 mm and 100 mm of Rockwool was applied to the reveals to help insulate concrete beams going back into the structure. Balcony floors and door thresholds were also upgraded.
A lax approach to the original building’s construction raised issues for designers and for contractors on site. The concrete slabs vary in tolerance from floor to floor, sometimes by as much as 60 mm, which informed the installation of cradle and batten floors in flats to create a level surface. Brickwork is not straight or plumb, making it difficult to fit new square windows into unsquare holes.
Removing more of the existing fabric could have made the scheme easier to build and simplified a lot of details, says Saleh. However, it would have had an impact on upfront embodied carbon, which delivered a 61% saving on the equivalent good practice new build. An assessment of post-retrofit whole life embodied carbon revealed a 55% saving over new build.
Cleaner operation
Turning to operational performance, EPCs for the apartments are generally around C, confirms Saleh, but as a retrofit of a leaky concrete tower block, arguably the 87% improvement in operational carbon over the original scheme is the more important figure to consider.
This was enabled mainly through fabric performance upgrades and degassing the building’s heating. The original scheme was connected to a district heating network run on dirty waste heat source, this was stripped out to make way for an all-electric system of panel radiators. Careful analysis identified all the key thermal improvements needed to reduce heat loads and make the electric system viable.
Renewable energy generation does not feature in Phase 2, however plans are in place to retrofit solar photovoltaics to the building, as well as to phases four and five of the regeneration, which Mikhail Riches has been retained to work on.
Sunlight will certainly enhance the subtle palette of turquoises and greens painted on the building’s balcony walls and inspired by the nearby Peak District. The 12 new colours match the front doors to require extensive testing and discussion with heritage experts to finalise. “People have different opinions on colour, making it one the biggest decisions. It took perhaps five years to agree on the approach,” says Saleh.
Historic England underlined the need to be able to ‘read’ the existing brick bands in the facade, which comprised a terracotta brick at the base, an ochre-type brick in the middle, and a whitish cream brick at the top, denoting the different layers of the ‘streets in the sky.’
An early idea to apply a red and yellow palette was rejected in favour of the cooler tones, which work more in harmony with the brick, yet also stand out and give individual flats an identity.
Social streets
The streets in the sky were originally conceived as an outdoor social space where neighbours could meet and children could play. Hawkins\Brown’s design for Phase One stuck to this concept, but entrances to flats were built out into the access decks forming thresholds to each group of four flats.
Phase Two sought to maintain the full original width, which demanded a different approach to define the thresholds. Entrances to two of each group of flats are recessed, and every property has a patterned ‘door mat,’ referencing the lino mats laid by original residents. There are 12 different decorative patterns, each one embedded into the new surface of resin bonded stone on recycled rubber.
The patterns add visual activity to the street and give each unit an area of personal space, whilst also discouraging people from walking directly past front doors by leading them towards the balustrade.
Problems with crime on the original Park Hill emerged, in part, because residents were closed off from the access decks with no view out. Mikhail Riches has addressed this by introducing a full height side light to front doors to allow passive surveillance as well as enhance natural daylight. Alternate flats feature an internal window, between the hallway and kitchen, so residents can see the street while cooking.
“It’s a way of promoting neighbourliness and social interaction, which seems to be working really well,” says Saleh.
Once hailed as the most ambitious inner-city development of its time and a visionary piece of modernist architecture, Park Hill failed to match expectations and soon fell on hard times. By paying respect to the aspects that made the building work, and finding effective solutions to those that didn’t, Mikhail Riches’ sensitive refurbishment should stand the test of time in a way its post-war forebear could not. The project’s success was recently recognised by the RIBA, who placed it on the shortlist for the 2024 Stirling Prize as one of only two refurbishments in the running.