Semper Aula has been restored and can be used again

In 2020 ¨C almost 150 years after the hall was inaugurated ¨C it was time for a thorough conservation and restoration of the auditorium. As part of the preparatory work, the condition of the hall was analysed and aspects of architectural history, materials technology and the preservation of historical monuments were examined. The actual work then lasted three years until the end of 2024. The focus was on the extensive conservation and restoration of the historic surfaces and the technical renewal of the auditorium. This required a large number of experts.

Team of experts united

The work was supervised by various experts and institutions, including:

• the external page Cantonal Office for the Preservation of Historic Monuments in Zurich
• the Chair of Construction Heritage and Preservation at ETH and
• the Real Estate Management department.

Together with the architects from the Zurich-based firm external page Ruggero Tropeano Architekten, they formed the project's supervisory group, thus combining the necessary expertise.

¡®The discussions in the supervisory group were extremely important and enriching. The exchange with our colleagues from the ETH Real Estate department was also excellent. The result is therefore not the achievement of individual decision-makers or planners, but of an interdisciplinary collaboration,¡¯ emphasises Silke Langenberg, Professor of Preservation at ETH.

Aula of great cultural significance

The main building, with its most valuable room, the Semper Aula, is listed in the inventory of protected monuments of cantonal importance and is also a listed building of national importance. This makes it one of the buildings in Switzerland worthy of protection.

¡®The cultural and social value of the room is immense,¡¯ explains Christiane Illing, the architect in charge of the project, ¡°not least because it is the last room designed by Gottfried Semper to be largely in its original state.¡± She continues:

This is also the reason why the room should only be used for representative occasions if at all possible.

Various measures taken

The preservation measures were challenging and affected the entire hall. The aim was to preserve the condition at the time of construction and to make as few changes as possible to the original substance, while at the same time finding solutions that would support today's use.

What measures have been carried out?

• The unstable parquet floor was replaced.
• The fragile oil paint on the various components was strengthened and
• cracks in the panelling and stucco work were repaired.

To do this, the professionals had carried out extensive material analyses in advance in order to do justice to Semper's historical materials and colour scheme.

New parquet flooring with sprung floor

During the inspection prior to the restoration, various defects in the floor came visible:

• Due to the large span of the beams, the statics of the hall floor were only partially serviceable. Beams sagged and additional steel girders were therefore installed.
• The substructure of the parquet flooring, which had only been replaced in 1996, was unstable. There were a number of collapsed areas in the floor socket area.

Accordingly, the structure, including the parquet flooring, was completely redesigned and rebuilt by skilled craftsmen using high-quality workmanship.

¡®We decided against repairing the floor and in favour of rebuilding it because the repair would have involved a great deal of effort and a less than satisfactory result,¡¯ explains Peter Kohl, project manager in the Real Estate department.

¡°The new floor construction, including the new parquet, now provides better statics and acoustics and a uniform appearance for the entire room,¡± Kohl continues, ¡¯this would not have been possible with a repair. Cost considerations also contributed to the decision.¡¯

Challenging stucco layer

All wall sections, bases and capitals of the columns and pilasters were photographed in advance in 2020. The condition was mapped, and the various damages and the measures to be taken were then noted.

In practice, this meant:

• loose pieces of stucco were reattached
• cracks and defective areas in the stucco were filled with a specially developed stucco compound coloured to match the grey of the auditorium
• layers of paint on the stucco base were secured and
• the surfaces were cleaned.

Modern technology and improved acoustics

The auditorium's acoustics have also been improved and the lighting technology updated to meet today's usage requirements. To this end, fabrics were stretched across the four large arches and sound-absorbing surfaces were created. In addition to the new floor, the acoustics have been improved by the addition of the padded chairs that were originally made for the auditorium.

However, the work on the main building is far from complete, explains Prof. Dr. Ulrich Weidmann, Vice President for Infrastructure and Sustainability at ETH Zurich. ¡¯The various measures will keep us busy for decades to come while operations continue.¡¯ We will continue the work this year and start the next major phase in 2026 with the renovation of the windows and fa?ades.¡¯