Throwing a warehouse party

Set within the Clerkenwell Conservation Area, the Waterman is an ambitious reimagining of four 19th-century warehouses on Farringdon Road separated by masonry party walls. Over the years, it had undergone piecemeal conversion, leaving it with a confusing array of tenancies, no cohesive identity and a highly inefficient single-glazed building envelope that bled energy.

Architect Fathom was taken on by client BGO to turn the building into a work space that not only suited the needs of modern tenants but delivered a markedly better thermal performance. A replacement fifth floor and new sixth-floor extension have increased gross internal area by 900m² to over 6,500m². The refurbishment celebrates the building’s industrial heritage while creating cosy, club-like reception and meeting areas at ground. Upper floors, in addition, have exterior balconies.

Installing high-performing but contextual windows and cladding was a key aspect of Fathom’s challenge but not the only one. Associate Tom Bulmer speaks of the difficulties of this sizeable building retrofit. The practice was appointed in 2019 when the building was tenanted and, he says, it was difficult to ascertain the precise condition of the structure, not least because its timber floors, masonry party walls and cores stagger down more than 3m from its west to east ends.

‘It was a narrow deep plan – quite inefficient and hard to let – so we wanted to make them as open as possible,’ he explains. ‘We connected them across the length of the plan, reducing the cores from four to two. With the four original double-bayed buildings, each 7m bay stepped relative to the next but they have now been rationalised to four stepped levels connected by two circulation cores with double-sided lifts, making each level accessible.’

The result is that level changes are now significant – up to 1m in places. Reconciling these changes with the existing heritage openings – which could not be altered – leads to some shifting relationships of sills to floors throughout. At entrance level, Fathom had to drop the timber/steel floor in places, which meant lowering the lower ground concrete floor slab to maintain head heights there.

Of interest is that Fathom’s successful planning application, including new external terraces within the built line of an incongruous curved roof addition on the rear elevation, contained a proposal to replace all 360 single-glazed timber sash windows with double-glazed ones.

Fathom director Justin Nicholls explains that their condition was bad and the carbon saving argument was just too compelling. ‘The modern replacements specified reduce energy consumption by 4.55 kWh/m2/year – paying back the embodied carbon from replacing the units within six years,’ he recalls.

Given the size of windows and scale of the job, Bulmer says it came down to a couple of firms to supply the performance-specified units, with Gowercroft Joinery winning the job with subcontractor JPJ installations. All upper floors have been fitted with its Chatsworth timber-framed double-glazed sliding sash windows with a traditional weights and pulley system. The high level of grey paint finish could leave you thinking they are aluminium sections but the feel of the sashes as they slide up leaves you in no doubt. ‘They said we were the first firm to specify black cord!’ adds Bulmer.

Given the large surface area the main elevation glazing has solar coatings, with a g-value of 0.4. Limiting solar gain was important – first to fourth floors are passively ventilated with mixed-mode used on fifth and sixth floors. Bulmer explains that, with sill levels as low as they are, it meant installing double restrictors that only allow sashes to open up 100mm, but adds: ‘If purge ventilation is required, there’s a limiter that keeps sashes within the 470mm opening allowed under the 2005 Work at Height Regulations.’ Things were less complex at ground floor where Hardwick timber-framed double-glazed flush casement windows were specified at the front. To the rear, original shopfronts were kept but reconfigured where needed to suit new access and MEP requirements.

The sheer size of the sashes at lower levels (up to 1.28m x 2.7m) is such that, when it came to installing doors on the rear elevation to access the new exterior terraces, planners had to relent on their desire for timber frames.

‘The frame size was so big it would have looked incongruous with the sash windows,’ says Nicholls, ‘so we went a Kawneer AA720 aluminium door system’. On the sixth floor, meanwhile, a Kawneer AA100 curtain walling system cladding system on a steel frame is fitted with tilt-and-turn windows to allow for cleaning.

Kawneer AA720 cladding is used on the fifth-floor terrace externally, with its windows expressed internally like the original punched openings elsewhere and with an 800mm sill level, which the client preferred for space planning purposes. While it emulates the original rear elevation, Fathom seems ambivalent about the result. But it had fun specifying the gold anodised AA100 curtain walling system with 120mm laser-cut motifs emulating the original stucco ones found on the Farringdon Road elevation.

At roof level, a 4m x 2.5m rooflight above the south escape core is in fact a huge bespoke door, which slides open via an actuator if the fire alarm is activated. Supplied by Glazing Vision, it’s an elegant alternative to a standard fire escape door and offers the client future flexibility when it comes to possible split tenancies being able to access the terrace from this core. Its low height kept planners happy too.

Interior designer Fettle was responsible for the fit-out of the members-club-like common areas at ground floor level, with glazed partition walls supplied by Fusion Partitions, which also supplied effective Contempo Glide sliding acoustic pocket doors.

But Fathom was responsible for some of the internal material reuse and circularity, repurposing 3 tonnes of original glass into Ecorok terrazzo style worktops for kitchen areas, as well as the recycling of 5,000 bricks, nearly 200m² of timber flooring and 2.3 tonnes of structural steel within the build. It also specified recycled raised floors, a decision it says was a significant factor in its carbon mitigation under BREEAM.

Bulmer says carrying out the definitive survey after the design-and-build contract was signed led to a lot of  ‘discoveries’, meaning contractor or client had to absorb the costs. Nicholls thinks ‘ideally it should have been a construction management contract – though it would have meant pushing risk back onto the client’.

But the practice is now modifying work processes. At its latest retrofit job in Mayfair, Nicholls says it is colour coding drawings to show whether aspects are surveyed, taken from other info or just plain guessed  ‘in the hope that we can pre‑empt those discoverables!