The missing pieces of the jigsaw in a unique roof design

This spring, the Woldring development in Groningen was topped out. The big new complex consists of 435 apartments for younger residents. Standing 50 meters high, and with 16 residential storeys, the Woldring Complex has the most striking profile on the river Reitdiep. Despite its massive dimensions, it is the architecture that catches the eye – especially the roof made of Hebel panels, with its many ultra-steep pitches.

The design is unique of its kind, and with good reason, as architect Dennis Hauer, of Urban Climate Architects, explains. “The building occupies a narrow strip of land between the water and the public highway. One façade of the building backs on to the road, while its two side walls run at right-angles to the road. The wings of the complex, so to speak, em- brace the inner court.”

An architectural hug

Another neat way to put it is that the building holds the site in a hug. “Sunlight for the courtyard and incident light in the apartments were major challenges of this project,” reflects Hauer. “We’ve tried to strike the ideal balance between the number of apartments and quality of life, in which daylight is a factor.”

Daylight survey yields unusual result

A survey of daylight design parameters offered a solution. Special software plotted the daily course of the sun around the complex, in all seasons. The survey’s findings were then input into the position- ing of the Woldring Complex and the design of its roof. Thus, the tallest block faces the evening sun. Even more radical are the roof design and the façade that maximises daylight penetration. Hauer adds, “the sun follows quite a tight curve – a kind of parabola. And the roof was drawn as a sort of counterform to this path, so that all the apartments, as well as the inner court, make the most of the avail- able daylight. The result is a much more interesting apartment block than most. The massive proportions are severely pruned. To make sure the design was feasible, we tried to subdivide the roof into straight sections wherever possible.”

From timber frame to AAC

The roof consists of six parts and was conceived as a timber frame structure, recalls planner Zlatan Balalic from Van Wijnen, the general contractor at the Woldring site. “But as we progressed, we realized the physics of a timber frame can cause structur- al problems. With a timber structure, condensation may accumulate under the roof, while the wind pressure and suction on a roof at that height (50 meters - ed.) would be incompatible with a timber carcass. Anyway, it was a risk we could not take, so we had to search for another building material.” It was no easy task, reports Balalic. A series of alternatives was considered: “Unidek roof panels, sheet steel, steel plus panelling or concrete roof slabs: none of these alternatives was quite what we were looking for. Either the material was too heavy or it did not come in the dimensions we needed.”

Missing piece of the jigsaw

Then someone in the construction team came up with the idea of AAC. Balalic remembers: “How about autoclaved aerated concrete? was one colleague’s suggestion. Gosh! We hadn’t thought of that. Then we got in touch with Xella for outline discussions. Soon we were convinced of the merits of AAC.” Hebel roof panels turned out to be the missing piece of the jigsaw. Balalic continues, “the lightweight He- bel products necessitated only minor tweaks to the structure to keep us within design specifications. At the same time, AAC is breathable, soundproof and fire resistant. It fits the bill in terms of budget and aesthetics. Everything about it was a plus.”

Fully prefabricated roof panels

A special feature of the project was that Xella handled the entire design of the roof structure, in close co-operation with structural engineers abtWassenaar, of Haren. “We drew all the roof slabs and static connections in 3D, then input our model to the developer’s BIM”, explains Xella’s draughtsman Michael Ricardo. He identifies the particular challenge as the pitch of the roofs, some of which were laid at an angle. “Then we had to allow for wind and water loads and suction.” Despite the structural challenges, the architect’s design “was always the blueprint,” notes Ricardo. “To conform to it, all panels were fully prefabricated at the Xella factory. Each has a unique number.” As soon as these ‘pieces’ arrived on site, they were fitted into the ‘jigsaw puzzle’ of the roof.

Next challenge: building on a compact site

After the physics and the structural challenges, another puzzle presented itself daily at the Woldring development. This was how to build on a compact site. Once again, Hebel roof panels were the answer, records Xella Sales Manager Jan IJzermann. “They’re bespoke, generating no extra waste on site. We deliver just in time, eliminating unnecessary storage of building materials.” Xella is highly experienced in logistics planning, and also relieved the contractor of the assembly of the panels, which has to comply with safety regulations. “That’s why this project is special for us as well. We offer an all-in service to our clients: not just the right building solution, but technical advice, engineering, planning and installation. This reflects our changing role in the building process. Rather than just another supplier, we are becoming a development partner, one that thinks in terms of solutions.”

De luxe housing for Groningen students

On the outskirts of Groningen, the Woldring largescale residential complex towers above the banks of the Reitdiep waterway. Planned for young people, it boasts 435 apartments plus commercial space on the ground floor. It was designed by Urban Climate Ar- chitects and built by Van Wijnen and ITBB, on behalf of Woldring United BV. Xella supplies and fits the Hebel roof panels used in the unique roof construction. Floor space in the apartments ranges from 30 to 50 square meters. A few luxury penthouses are also available. The Woldring Complex is located on a former industrial estate along the Reitdiep, where the City Council has earmarked 13 plots of land for residential development. The strip of land forges a more robust link between Groningen city center and the university campus. The Woldring residential development should encourage students to move closer to campus. Handover was planned for early September 2019.

Xella Nederland BV Mildijk 141, 4214 DR Vuren, The Netherlands www.xella.nl