An eye for architectural detail

While the purpose of museums and galleries is to showcase the exhibits inside, it’s no coincidence that many of the world’s most famous museums are also amongst the architectural world’s most celebrated buildings. Look at the Louvre in Paris, the Guggenheim in Bilbao and London’s own Tate Modern, for example.

Founded in 1815 when banker and spice merchant, Johann Friedrich Städel bequeathed his house and art collection as a public institute that would bear his name, The Städel Museum in Frankfurt continues to use the same imposing but unremarkable building as its main exhibition space today. However, there have been numerous extensions and modernisations during the course of the museum’s 200 year history and the latest, the €35 million ‘Garden Halls’, not only provides a distinctive and contemporary underground exhibition space but has also created an external landscaping feature which is both unique and in keeping with the Städel’s status as the home of one of the most significant art collections in Europe.

Dual Purpose
The creation of an extension to the museum below ground provides a self-contained, contemporary exhibition space that extends the available space without creating any architectural conflict with the existing traditional façade. However, the real genius of this building is not in the physical space of the new underground building but in the roof of that building, which provides natural light and also forms the substrate for a new garden area that is a striking green space in the daytime and provides eye-catching feature lighting at night.

At the heart of that achievement is the perfect geometry of 195 round skylights or ‘Eyes on Art’ as the Museum calls them, positioned in a grid formation on the undulating domed lawn that forms the roof of the Garden Halls extension. These ‘Eyes’ bring sunlight into the new building during the day and enable artificial light to illuminate the lawn from within the extension at night. The idea is simple in concept, spectacular in its realisation and technically challenging for architects Schneider + Schumacher who won an international design competition for the 3,000 sq m extension. The roof substrate of the extension had to be both sufficiently well waterproofed to ensure that the underground extension was safe from water ingress and suitably root resistant as the base for the garden lawn/green roof planting – and with 195 Eyes on Art creating so much detailing, this was no easy task!

Liquid System
The solution to meeting this challenge was to use a cold liquid applied waterproof membrane and waterproofing specialist, Kemper System, was brought in to advise on the specification and application of a suitable membrane.

Explains Stuart Hicks from Kemper System UK:

“Kemper System Germany was involved with planning the roofing element of the Städel Museum project from the early design stages to ensure that the design concept was deliverable and could provide a lasting water-tight solution. The architects wanted to use a long-term and homogenous waterproofing product that would be durable enough to withstand footfall and flexible enough to cope with the contours of the dome-shaped roof. The chosen system also had to avoid any mechanical anchors to the skylight structures and seal the complex detailing accurately and this was a significant challenge that could only be addressed by a reinforced liquid membrane.”

In addition to the practical considerations for the waterproof membrane specification, the architects and the Museum were keen to ensure that the chosen system also answered the environmental and sustainability aims of the whole project. As a result, Kemper System’s Kemperol 2K-PUR cold liquid-applied membrane was chosen.

Stuart continues:

“Kemperol 2K-PUR is a solvent-free system based on a castor oil formulation using sustainable crops. The solvent-free resin is used in conjunction with a non-woven reinforcement fleece, which is manufactured using recycled plastic bottles, and the two elements are installed in a single process to create a seamless, monolithic membrane that cannot delaminate and has a service life in excess of 25 years.”

Single Process
The reinforced concrete extension is supported by just 12 interior columns and the outside walls, rising to a height of 8.2 metres at the centre of the domed roof. Each of the circular Eyes on Art skylights have a diameter of between 1.5 and 2.5 metres and the circular bulls eye glass windows are each housed in a square concrete flange that raises the top of each Eye above the roof substrate, allowing the finished glazed circles to lie flush with the top of the green roof build up.

Stuart explains:

“The complex network of a grid of concrete squares topped off by circular windows made for a geometry that was extremely difficult to seal, which is why only a liquid applied system could really have answered the needs of the project. Even so, installation required careful planning and specialist skills and several meetings were held on site involving Kemper System, the architects and the contractor, PB Flachdachbau to ensure a good end result.”

Kemperol 2K-PUR is suitable for application on any substrate and forms a permanent bond. Particular attention was required to ensure suitable preparation of the substrate and sealing of the light domes and PB Flachdachbau erected heated tents to enable preparation of the substrate and installation of the waterproofing membrane despite very cold and inclement weather. The concrete surfaces that lie beneath the skylight flanges were either sanded or blasted, depending on individual requirements at different parts of the site and the installation team applied Kemper System’s solvent-free Kempertec EP5 primer, which can be applied in temperatures as low as 50C.

Once the primer had cured, PB Flachdachbau began the painstaking process of applying the Kemperol 2K-PUR membrane, using small rollers to apply the liquid system to awkward areas. The installation was completed in sections with the application of resin followed by laying of the reinforcement fleece, which was cut to size and shape on site to fit the contours of each detail exactly, with overlaps to ensure that the system’s full integrity is maintained throughout the roof/garden surface. Once the fleece had been laid, the installation team rollered the surface to remove any air bubbles and ensure complete saturation before applying more resin and leaving each section to cure.

Stuart continues:

“Because Kemperol 2K-PUR is solvent-free, there were no issues with nuisance odours, either for the installation team or for visitors to the museum.”

Green Legacy
Once the entire roof/garden surface had been waterproofed, the green roof build up could begin, transforming the studded roof into a spotted garden. The green roof is a grass only surface with interest provided by the gentle slopes of the dome and the Eyes on Art, with the addition of coloured lights in the evening.

Stuart adds:

“Green roofs have become relatively commonplace and Kemperol 2K-PUR has proven long-term root resistance in compliance with FLL standards. For the specification of this green roof, however, the specification challenge was not only to select a product that was suitable for the roof build up but to ensure that the chosen system was applied with expert precision to leverage the durability and flexibility of a membrane that will provide reliable protection from water ingress for generations of art lovers.”

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