In densely built-up areas, it can quickly happen that higher parts of buildings "steal" sunlight from others. A creative and innovative solution was found for this problem in Groningen, Netherlands. There, architect Dennis Hauer of Urban Climate Architects designed a unique building with green roof areas, which trace the path of the sun's rays. The planned completion date is at the beginning of September 2019. And to a certain extent, Hebel autoclaved aerated concrete (AAC) proved to be a decisive puzzle piece in its realization.
This spring marked the climax of the construction of the De Woldring Locatie in Groningen, a new housing development for young people with 435 apartments. With 16 floors and 50 meters in height, the building is the most striking appearance on the Reitdiep, a body of water that flows across the northern half of the city. The building is of an impressive size, but it is the architecture that stands out in particular – more precisely the unusual design of the roofs.
The design is unique in its kind and not without reason, as architect Dennis Hauer explains: "The building stands on a special plot, a compact location between the water and a continuous street. The façade on the street side shields the building from it. The two side walls are perpendicular to the street, so that the complex encloses the courtyard, so to speak. "
The building is enclosed, you might say. "The light in the courtyard and the apartments were really big challenges in this project," Hauer recalls, "We have been looking for an optimal balance between the number of apartments and the quality of life, which depends not insignificantly on daylight."
A light study was the answer: Using special software, the daily solar radiation was displayed in relation to the complex during seasons. The results of the study are reflected in the roof design and positioning of De Woldring. For example, the highest tower is aligned with the evening sun. No less consistent is the design of the roof and façade, which ensures that as much daylight as possible enters the building. Hauer explains: "The path of the sun has a very extreme curve, a kind of parabola. Due to that course, the roof shape was drawn in a certain way as a counter-shape, so that all apartments get as much daylight as possible as well as the inner yard. And this also leads to a much more interesting building than usual in such residential complexes. The silhouette is very complex. To make the construction possible, we tried to divide the roof as far as possible into straight sections. "

From a timber frame construction to AAC

The roof is made of six parts and was initially designed as a timber frame construction, says planner Zlatan Balalic from Van Wijnen, the contractor of De Woldring. "But gradually we found out that a timber frame construction can cause physical and structural problems: the construction carries with it the risk of vapor accumulation under the roof and the height of the building would make the realization difficult with the possible wind suction and pressure, so we had to look for an alternative building material. " That was not an easy task, says Balalic. A number of alternatives were discussed. "Unidek roofing sheets, steel sheets, steel and panels, concrete roofing sheets; not a single material offered the solution we were looking for. The alternative was either too heavy or not available in the desired sizes. "

Missing puzzle piece

Until somebody from the construction team suggested AAC. One colleague came up with the idea, "Isn’t that all?" Balalic explains: "Autoclaved aerated concrete, we did not even think of it. So we contacted Xella and had exploratory talks, we were quickly convinced the Hebel roof panels proved to be that missing part of the puzzle - thanks to the low weight of Hebel products, only minimal changes were necessary to stay within the design, while AAC is moisture-regulating, sound and fire-resistant and fits our image in terms of budget and aesthetics gave only benefits."

Roof tiles completely made to measure in the factory

A special aspect of the project is that Xella, in close cooperation with the manufacturer Abt Wassenaar from Haren, was responsible for the complete planning of the roof construction. "We drew all the roof panels and static connections in 3D, then our model was integrated into the contractor's BIM model," says Xella's draftsman Michael Ricardo, explaining that the slope of the roofs was particularly challenging: “Especially since we had to take wind and water loads and the wind suction into consideration."
Despite the structural challenges, the design of the architect has always been "decisive," assures the draftsman. "In order to achieve this, all the panels in the Xella factory have been completely made to measure. Each roofing membrane has a unique number." Arrived on site, the "puzzle" of the roof can be laid.

The next challenge: building in a compact location

In addition to the physical and structural challenges in the construction of De Woldring, there was another daily challenge to solve: building in a compact location. Here, too, the Hebel roof panels offer a solution, says sales manager Jan IJzerman of Xella: "Since they are completely tailor-made, there is no additional waste on the construction site. In addition, we deliver just-in-time, which means we do not need unnecessary storage of building materials. "Xella has a great deal of logistics planning experience, including the assembly of roofing panels and associated safety requirements IJzerman says: "That's what makes this project so special for us – we offer the customer complete relief – not just with the right building solution, but also in the areas of technical consulting, engineering, planning and installation. It fits our changing role in the construction process, rather than just being a supplier, we are increasingly becoming a construction partner who thinks in solutions. "