An impressive historic building from the early 20th century stands on the banks of the Rhine in Düsseldorf. The facade of this landmark-protected building, made of stucco and natural stone, showed significant damage—particularly to the Kylltal sandstone—and required extensive restoration.
Challenge
In a prominent location, just a few minutes’ walk from Düsseldorf’s Königsallee, the multi-story residential and commercial building is situated in the well-known Carlstadt neighborhood. The historic building dates back to 1910–11; its historically protected facade consists of white stucco surfaces and solid reddish-brown Kylltal sandstone. The street-facing lower ground floor stands out visually thanks to a dark basalt lava base.
Over the decades, environmental factors and the building’s immediate proximity to the Rhine have caused increasing damage to the street-facing exterior. In particular, extensive flaking was observed on the surface of the sandstone. Philipp van Noppen, the Düsseldorf-based architect commissioned to oversee the restoration work, explains this issue: “In some areas, a water-repellent treatment applied in the early 1990s—which was not carried out properly—had led to the damage.” Water had seeped through cracks in the joint material into the water-repellent layer, causing the stone surface to erode and flake off.
The loosening and falling stone material also posed a danger to passersby. Nevertheless, it was not possible to begin the necessary restoration work immediately in the summer of 2018. “The largest house martin colony in North Rhine-Westphalia nests along this row of houses here on the banks of the Rhine, and their breeding season lasts until the end of September,” says van Noppen, explaining the unusual circumstance. As a result, he had to postpone the work to the structurally unfavorable period from October to March. “After the site survey in July, however, we made good use of the long lead time over the summer to have the natural stones prefabricated and to coordinate the schedules of all the trades,” says van Noppen.
Site Survey
The two-day, detailed site survey was conducted using a cherry picker. Together with master tile, slab, and mosaic setter Johannes van Noppen and master stonemason and stone sculptor Peter Rüb, the architect examined the damaged facade. “The natural stone facade consists of 35 to 40 centimeters of solid masonry. In several places, we had to remove up to six centimeters of damaged stone material before we found sound core material,” reports Johannes van Noppen, adding: “The outer corners were even more severely affected by the weather. In some places here, we had to chisel out up to eight centimeters.”
In addition to the main problem of spalling and numerous cracks in the plaster, the construction specialists found heavily weathered and crumbling corbel stones—particularly in the upper third of the facade—that had to be completely replaced. In addition, moisture had penetrated numerous natural stones through the damaged stone surface. As a result, the old iron anchors corroded severely, causing further damage to the stones. “After all, one cubic meter of iron turns into three cubic meters of rust,” says Johannes van Noppen, explaining the enormous destructive force generated by the corrosion process.
Restoration work also had to be carried out on three of the five existing balustrades. In some places, old, crumbling restoration mortar was found, which, according to the architect, was the result of a “repair carried out without professional expertise.” Otherwise, the upper sections of the historic natural stone railings were relatively well-preserved due to the sheet metal covers.
However, significant damage was evident at the base points. Here, driving rain had led to extensive material loss. “In some places, the loss of material had progressed so far that we could no longer determine the cross-section,” reports Johannes van Noppen. To reconstruct the historic elements with attention to detail, the architect created templates as well as plan and working drawings.
Kylltal Sandstone
The historic facade stone is Kylltal sandstone from near Bitburg in the Eifel region. “One of the most important tasks for the restoration was finding sandstone that matched in color and joint pattern,” emphasizes Philipp van Noppen. He found what he was looking for at the Bamberg-based company Natursteinwerk Hermann Graser GmbH.
The family-run company owns 21 quarries in Germany, where granite and sandstone are mined and further processed in its own facility. But the medium-sized company also works with “external materials,” such as the large raw blocks of Kylltal sandstone. “In Bamberg, we spent a lot of time together figuring out the right shade. As a result, even an expert would have a hard time noticing the replaced STEINS on the restored facade,” Johannes van Noppen reveals, not without pride.
After the parties involved had jointly selected the appropriate stone material, the stonemasons at the Bamberg Natural Stone Works produced all the necessary cut stones—such as cornices, window reveals, and corbels—including the desired polished or bush-hammered surfaces. Using templates created by the architect, the factory was also able to reconstruct the balustrade elements, which had been severely weathered. In the first step, an industrial robot milled the desired stone pieces, including their profiling. In the second step, to achieve an optimal result, the stonemasons hand-finished the individual stone elements.
Installation of the Stone Elements
After the swallows had left their nests at the end of October, Johannes van Noppen and Peter Rüb were finally able to begin installing the stone elements. In total, the two experts replaced 3.6 metric tons of Kylltal sandstone on the facade. The two-person team installed new corner pieces, particularly in the corner and profiled areas. The pre-fabricated, flamed saw-cut pieces from the Bamberg Natural Stone Works only needed to be cut to the correct dimensions on site before the two installed the corner joints with precision using trass cement mortar (Weber mix 614).
They replaced the old, corroded iron anchors and clamps with stainless steel reinforcements of six, eight, or ten millimeters in thickness, as appropriate. Since all measures had to be coordinated with the historic preservation authority in advance and the replacement of the old sandstone blocks was to be minimized as much as possible, the stonemasons repaired the less damaged sandstone blocks using restoration mortar (from Remmers). In doing so, they used a mixture of the two shades: reddish-brown and variegated sandstone.
“This allowed us to influence the interplay of colors on the ‘new’ facade through our craftsmanship, so that it would blend perfectly with the existing stone facade,” explains Johannes van Noppen. The stonemasons filled smaller cracks with epoxy resin adhesive (Akemi Akepox) to ensure a strong bond, while they reinforced larger cracks with stainless steel dowel anchors (VA 3 x 250 mm) and injection mortar (Hilti Injection Mortar, HIT-MM Plus 330/2), and subsequently sealed the remaining holes on the surface with a suitable restoration mortar. Another measure involved cutting out damaged joint material and repointing the exposed joints with trass-lime mortar for historic masonry (Weber).
