Inside Number 87

AHMM solves the puzzle of a complex gap site in London with a crafted block of interlocking concrete flats, writes Tony Whitehead

Known unpretentiously as 87 Weston Street, this new mixed-use building in Southwark takes its place among some of the UK’s most notable concrete architecture. The London borough is home, for example, to Charles Drake’s groundbreaking concrete house, built in 1873, and the iconic National Theatre completed in 1976.

And now, just around the corner from the concrete tower of the world’s tallest hospital, Guy’s, is the latest addition to this eclectic gallery. Developed by Solidspace and designed by Allford Hall Monaghan Morris, number 87 is a residential and commercial project of just 31,000ft2.

Small it may be, but for future generations there are features here that will surely epitomise the concrete of our time. It is a bespoke building. Each of its apartments is subtly different and the ground-floor office space is a one-off. All feature the carefully crafted, as-struck interior concrete currently enjoying such favour with the capital’s designers and developers.

It is not, however, a concrete-frame building. Rather 87 is a concrete shell, and its load-bearing walls create a stepped series of eight interlocking apartments above the ground-floor office. The overall shape is reminiscent of the impossible 3D puzzles found in expensive toy shops.
“Because of the way the apartments interlock, and because we wanted to express that arrangement in the fenestration, a standard grid-based slab and column concrete frame was never going to work,” explains AHMM’s project architect Marion Clayfield. “So instead we have the in-situ reinforced-concrete shell, and although the exterior is clad in grey brickwork, we have expressed the concrete throughout the interior of the building.” Which meant of course that the choice of finish was everything – even more so than when visual concrete is used in a larger or purely commercial project. As Clayfield points out, the domestic scale is smaller, and the occupants are closer to the finish for longer periods of time.

A test process involved having three 2m x 1m concrete sample panels made up in the yard of the contractor, Oliver Connell. A retarder was used on two of these to expose some of the aggregate – achieving a light-etch and a medium-etch finish. A third panel was plank-marked and this proved the clear preference. (The architect and contractor both use the term ‘plank-marked rather than ‘boardmarked’ to emphasise the discernible joint pattern of the finish.) “It’s more tactile, offering a warmth in the smaller domestic setting of the apartments especially,” says Clayfield. “And the grain of the timber as expressed in the concrete helps to hide or tone down any inconsistencies [see “Dancing with concrete” boxout].”

The concrete itself was a standard mix, though with 50% ground granulated blast-furnace slag (GGBS) cement replacement. “We included this both to reduce the cement, and therefore CO2, involved in the construction of the building and also because of the colour. Adding GGBS makes it a little paler. We originally wanted a 70% GGBS mix but when we tried this out, in the unseen basement area of the building, we found that strike times, particularly in winter, were too long and would have proved costly in terms of the building programme.”

Reverting to a 50% mix, explains Clayfield, reduced strike times from up to four days to 48 hours – but also kept the carbon footprint of the material within acceptable limits. This was vital as the building aspired to, and achieved, a BREEAM Excellent rating.

Leader of the gap

Roger Zogolovitch had plenty of time to consider how best to develop the Weston Street site. His company, Solidspace, has owned it since 1985 and until recently was actually based there. “We have thought seriously about development here – in conjunction with AHMM – for at least 10 years,” says Zogolovitch. “That kind of timespan has given us a certainty about our ideas.”

It is no surprise that the final design was based on a concrete structure. “Being in architecture for more than 50 years [Zogolovitch is the “Z” in celebrated practice CZWG], I’ve spent a lot of time thinking about materials and I have an urge to use them elegantly and simply. That’s why I like concrete. There is no need to hide it. You have a structure, and it’s apparent what the material is.”

He has also found concrete well-suited to Solidspace’s preferred type of development – the gap site. “These sites are a valuable resource,“ says Zogolovitch, “but they are often intricate and constrained. Some see that as difficult, but I see them as an opportunity for the architect’s ingenuity to make a difference.”

In this context, he adds, concrete‘s flexibility and ability to flow into almost any required shape can prove invaluable: “These are qualities you need when you carve out these spaces,” says Zogolovitch. “Concrete is often the solution because it is admirably sculptural and in the depth of its section it is pretty much the same whether spanning horizontally or in a wall vertically. It also offers very good fire and sound protection without
having to add to it. It is very simple.” Protection is a word Zogolovitch uses often: “We live in abrasive times,“ he says. “We are assaulted by traffic and noise and the pace of life. A home should offer protection – like a cocoon. And there is something wonderful about well-engineered windows fitting into a solid concrete wall that gives
that sense of peace and solace.” Hence, the name Solidspace – less a brand than a philosophy for living.

The building features photovoltaics and green roofs as part of its environmental credentials,but the thermal mass of the concrete is also key. Clayfield says that since the interior concrete walls and soffits have been left exposed, these remain in contact with the air and can thus readily absorb excess heat in summer, or during the day, and release it slowly overnight. This passive temperature modification results in cooler interior temperatures in summer and reduced heating costs in winter. A thick layer of insulation between the interior concrete walls and the exterior brickwork lowers the U-value of the external walls to a thrifty 0.15W/m2K.

Entering the apartments, it is apparent that the interiors display the distinctive style that has come to characterise many of Solidspace’s projects. Each apartment is split level, with different levels for working, sleeping and eating. The plank-marked concrete aesthetic is complemented by timber window frames and fittings in oak or walnut – the grain of the timber reflecting that expressed in the concrete. But while almost all the walls are plank-marked (there is hardly any plasterboard), the ceilings are smoother, having been formed from standard paper-faced ply boards.

“The exposed concrete meant we had to think about services very early in the design process,” adds Clayfield. “For example while lighting in the kitchen is on a track, elsewhere we have cast-in conduits in the 200mm slabs. Obviously that means the lighting and electrics have to be designed and coordinated with the slab design before the concrete is poured.”

Projecting out from the building on two sides are six precast concrete balconies and these are unusually deep, thrusting out 2.8m and adding a playful touch to the building’s puzzle-style exterior. The balconies are cast in one piece with a light etch. They cantilever out from the slabs via a thermal break connection, but each is also held in place by four substantial steel brackets. “Some of the balconies reach almost to the canopy of mature trees outside, and it is possible to turn around, look back and get a great view of the building.”

Residents who do this might note other precast details. The deep window and door reveals have been finished in pale precast concrete, as have the building’s entrance canopies, which feature number signage embossed in the concrete. There is no precast inside the building however: even the stairs within the two cores have been formed from in-situ concrete. “These are quite difficult as the (potentially uneven) top of the concrete is the part that you see,” says Clayfield. “They also have cast-in space for steel nosings to the treads. The contractor  trowel-finished them and did a very good job.”

With its smooth soffits and plank-marked walls, the ground-floor office space has the same finishes as the flats. The main difference is that this is an open, single-level space with an extraordinarily high ceiling. A lack of interior walls to support the ceiling slab means that columns are required here, but these have been kept to a minimum (just four) by the deployment of two large concrete downstand beams. These add a dramatic sculptural quality while also spreading the loads above.

Roger Zogolovitch, founder of Solidspace, says: “The ambition here was to make a calm, peaceful and creative space, and to use volume is an important part of that environment. So we have a 4.5m ceiling height and almost 5,000ft2.” Zogolovitch is a keen advocate of concrete’s ability to protect building occupants from the stress of contemporary life (see “Leader of the gap”), but in the office space it has also been used to create an imaginative lighting regime. For example, a recess cast in along the downstand beams enables lighting to be set within them. In addition, and since not all the office space is topped by the building above, a trench-like slit running along one side of the ceiling allows natural light to filter down.

Similarly, at the western end, a large glazed circular hole in the ceiling permits views of the trees and skies above. These features were also cast in – the hole being formed using plywood scored on the back to enable it to be bent sufficiently. “We have tried to use light in a cathedral-like way,” says Zogolovitch. As the daylight slants through the high ceilings to illuminate the sculptural concrete walls, it is easy to see what he means.

Dancing with concrete

Oliver Connell & Son managing director JamesConnell has spent decades in the business, but even he admits that “concrete sometimes seems to have a mind of its own. There are so many factors that can affect how it comes out – you’re never really sure until you strike it.”

But the plank-marked finish which is the unifying theme of 87 Weston Street is neat and regular, the concrete is consistent and the grain of the planks distinct. So how did they do it?

“We have done quite a few plank-marked jobs recently so we know the yards that can supply good-quality timber,” says Connell, After that, it is largely a matter of detail. For example, the 22mm x 150mm planks were all
planed on the concrete side to prevent loose strands of timber getting stuck in the concrete. The planks were also staggered in thirds to avoid vertical lines, and this meant that because of the intricate nature of the job, each piece of formwork was bespoke, taking up to one-and-a-half days to make.

“We couldn’t reuse the formwork, but we did reuse the planks,” says Connell. Having carefully dismantled the used formwork, the planks were brushed and jet-washed before being reused up to three times.

The highest walls, in the double-height sections of the apartments, were up to 7m and done in two pours. In order to neaten the horizontal line between the two, concrete was first poured to halfway up the top plank of the
first formwork. This last plank was left in place and effectively incorporated into the formwork for the upper pour.
The efforts of Connell’s team did not go unappreciated by his client. “How they operated was a pleasure to behold,“ says developer Roger Zogolovitch. “The way they coordinated and orchestrated everything – it was like watching a well-rehearsed dance troupe.

Photos: Tom Rothery, Rory Gardiner, Mike Abrahams

Architect: AHMM
Structural engineer: Form Structural Design
Main contractor: Bryen & Langley
Concrete frame contractor: Oliver Connell & Son