Sunday20 August 2017

Haus H27D apartments by Kraus Schönberg Architects

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The architect’s apartment building in Constance, Germany, shows an attention to material detail in keeping with its medieval neighbours

Clinging to a spit of land on the edge of Lake Constance, at the border of Germany and Switzerland, the medieval city of Constance (Konstanz) is a place steeped in a palpable sense of history. An early Roman settlement, it became a strategic trade hub in the middle ages and the spiritual epicentre of a diocese that stretched from Stuttgart to cover half of Switzerland.

Its historic centre is remarkably intact today, mainly due to the fact it was not bombed by the British during the war — because the city cunningly kept its lights on at night, thus fooling bombers that it was actually part of neutral Switzerland.

As a result, the town’s momentous past can still be felt in the dense network of crooked cobbled lanes that radiate from the 7th century cathedral, a tight urban grain formed by a very particular kind of block pattern.

Thick rendered facades of approximately 10m bay widths line the narrow streets, behind which each plot extends 30m deep. Gardens or yards sit behind these street-facing buildings, with an outbuilding to the rear of each plot, behind which an alleyway (formerly an open sewer) extends the full length of the block. This same pattern, of outbuilding, garden and street-facing building is then repeated the other side of the alleyway to form a symmetrical rhythm — of a kind similar to the Georgian terrace-and-mews pattern in London, only here with a delightfully disorderly medieval bearing.

Kraus Schönberg Architects

Source: Ioana Marinescu

To the rear, the wall is divided into a slender structural grid of posts and lintels.

Kraus Schönberg Architects has synthesised a deep understanding of this historic grain in a new mixed-use building, tightly shoehorned into one of the last remaining plots in the medieval centre of Constance. Haus H27D, which comprises four apartments above a first floor gym and ground-floor clothes store, replaces a low-rise kebab shop that had occupied a shed-like cabin on the site for some years, in the middle of this pedestrianised shopping street. After an invited competition, Kraus Schönberg’s scheme was chosen, being the only one to make maximum use of the full 30m plot depth, with a clever reinvention of the city’s medieval typology.

Perforated facades

Sandwiched between a mid-20th century department store to the north and a lower medieval building to the south, the building’s facade stands as a striking monolithic presence in the street. Cast in-situ in fair-faced concrete, the 500mm-deep wall is punctured by a broad grid of openings in a proportion derived from an analysis of the fenestration patterns of the perforated facades of its neighbours.

The rhythm of these lochfassades characterises the nature of Constance’s street fronts, and any new buildings must be careful to fit in.

“The conservation people are very powerful here,” says Tobias Kraus, as we stand looking up at a sheer face of exposed concrete, distinctly at odds with the creamy render and shuttered windows that line the road. The department store to the left presents a modern face of continuous ashlar, but it is punctured with windows in the precise location of the original building on the site, in a decidedly postmodern attitude to heritage. Kraus Schönberg took an entirely different approach.

“We have essentially made a medieval building,” says Kraus. He describes how the neighbours’ walls are thick slabs of straw and clay meshed together in a timber frame — a monolithic, breathable construction that the architects have reinterpreted with the use of aerated in-situ concrete. Their mix includes Liapor, an expanded clay aggregate that is both thermally insulating and heat accumulating, as well as semi-permeable to water vapour, naturally regulating the internal humidity.

The addition of these ceramic balls means the mix cannot be vibrated too much during the pour, to prevent them rising to the surface. As a result, the finish has an inconsistent, slightly pock-marked surface, giving it the same rugged, massive quality of its timeless neighbours. Despite its contemporary aspect, this building could almost be the stripped masonry core of a much older structure.

Kraus Schönberg Architects


The massing has also been carefully designed to mediate between its neighbours’ changing heights. While rival schemes proposed a conventional stepped elevation, Kraus Schönberg’s layered facade has a rear plane at the height of the department store, which continues the orthogonal line of the plot, while a secondary plane in front continues the height of the lower building along the angled line of the street — as if the facade has been prised open to form a balcony level at the third floor.

At this point, too, the grid-like nature of the elevation is forcefully expressed by its decapitation to form a balustrade — which, approached from down the street, reads like the crenellated top of some ancient fortification.

“We are interested in the difference between the perception of a wall, punctured with holes, and a structural frame,” says Kraus, gnomically. This, it seems, was the driving question in developing a constructional language for the project, which changes as you progress through the depth of the building. The street-facing facade is very much expressed as a defensive wall, perforated by deeply set windows.

To the rear of the building, the wall is thinner and the reveals shallower, while further back still, the idea of a wall is dissolved entirely into a slender structural grid of posts and lintels. But throughout, whether wall or frame, these planes are subtly articulated as stacks of separate T-shaped figures, as if to echo a primal, trabeated form of construction.

Kraus Schönberg Architects

Second and third floor

In plan, the medieval typology has been cleverly reworked to provide the maximum possible floor area for the apartments. The main spaces of kitchen, dining, bathroom and two bedrooms are housed at the front of each floor, leading to a living space further back that faces south-west on to a courtyard. From here, a narrow glazed corridor, framed by an open loggia, extends to the very back of the site, where an extra bedroom and utility room are housed — a vertical reinterpretation of the outhouse.

“Our main reference was Rear Window,” says Timm Schönberg, referring to the classic Hitchcock suspense film. “That sense of one person looking opposite into the other person’s window. We like the idea that here you are your own neighbour in your apartment.” Although hopefully without the murders.

‘We like to work directly with the builders on site, and use their knowledge’

The apartments are compact, but arranged with an open plan layout and sufficient amounts of glazing that they feel surprisingly spacious, while the views through to the stacked rear of the site provide an extra sense of roominess. The interiors employ a simple palette of exposed concrete soffits and walls, with industrial smoked-oak flooring, as well as oak-framed windows, cheaply sourced in the Black Forest at great length — but all with a high-quality finish that belies the €1,428/sq m construction cost. The openable windows and loggia spaces are edged with balustrades of spindly oiled steel rods, each slightly warped and fixed at an irregular angle to give a sense of movement. Such details were never explicitly drawn, but developed with the ironmonger on site.

“We don’t draw very much where we don’t have to,” says Schönberg. “We like to work directly with the builders on site, and use their knowledge as much as our own. The reality is that if you do complicated drawings of details, they will often be lost in translation in the building process.”

The architects explain how they like to be close to their sites, opening an office in Hamburg for their Haus W project (2007), in order to allow this close level of supervision. For the Constance project they took this one step further: Kraus moved into a flat that backs directly on to the plot — so close in fact that he could climb over his balcony on to the scaffolding to talk to the builders, while contractors could pass him samples through the window. “We did a lot of material experimentation on site,” he grins.

View of Constance, Germany, from the building.

View of Constance, Germany, from the building. Photo: Ioana Marinescu

The hand-crafted nature of the project is particularly evident in the stairwell, which winds tightly down a corner at the front of the plan, its irregular geometry denying the use of precast components. Instead, the hand-built shuttering has left a stepped, deeply sculptural, imprint on the curvaceous soffit of the stairs, to which splinters of wood still cling.

“The human hand always makes mistakes,” says Kraus. “We are not machines, but we thought we could use this to our advantage.”

It is haptic touches like this that bring this project to life — which, in other hands, could run the risk of being overly austere, clinical and machine-built. The choice of on-site casting throughout (apart from the pre-cast internal party wall), as well as the lightweight aerated concrete, has left an imperfect finish, but one that gives the building its character, one that celebrates the marks of making.

“It is also very easy to demolish,” says Schönberg, matter-of-factly. “An ancient building burnt down here recently, and it took so long to separate out all of the materials. With this lightweight concrete, it is very cheap to knock it down and recycle the whole thing.”

Kraus Schönberg
Structural engineer Fischer & Leisering
M&E Greiner Engineering



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