The winner of the tendering process was a consortium consisting of the firms G. Hinter-egger und Söhne Baugesellschaft, Oberhofer Stahlbau and IPC Project Consulting in collaboration with the architectural practice Halle 1 and the engineers Herbrich Consult. Their design, implemented in a construction period of nine months, proposed the erection of an additional volume over the auditorium plus three sliding roof elements to cover the entire width of the stage area. This structure is not only rainproof, but storm- and winterproof, so that the stage can now be used all year round.
The creation of the new roof was dependent on the complete removal of the existing covering over the area between the House for Mozart, the Holzmeister stage tower, the Karl Böhm Hall and the Mönchsberg. The overall configuration of the new monopitch roof is formally and structurally related to the roof-scape of the festival district and extends up to the sloping rear rock wall of the Mönchsberg. The visual link with the neighbouring buildings is created by the roof form and the neutral grey sheet-metal covering used, which corresponds to the colours of the urban surroundings. Finally, no elements in this structure impair the view of the Mönchsberg or the Edmundsburg for passers-by or audiences.
The new roof covering to the Felsenreitschule consists of three large areas that slide over each other, two fixed edge beams and five telescopic beams. The last slide into and out of box beams that at the same time form the main girders of the new fixed roof over the auditorium. In an extended state, the telescopic beams span the entire width of the stage space and rest on shoes inserted in the face of the rock wall. The individual telescopic arms are operated by simply maintained spur-gear motors fixed to the sides of each box beam. Power transmission from the motors is by means of a rack-and-pinion system on the underside of the telescopic beams. All motors are accessible from the new stage gallery and are encased to prevent noise emission.
Only when the telescopic beams are fully cantilevered out and come to rest at the ends on two columns, can the three mobile roof areas slide to the rear wall of the stage and close off the internal space. As a means of operation, two spur-gear motors per roof element are mounted on the two fixed edge beams. Here, too, power transmission is by means of a rack-and-pinion system. The entire closing process – the extension of the telescopic beams and all three sections of the roof – takes six minutes. The mechanism is fully automatic and has a low noise level, so that it can be operated in all weather conditions and even during performances.
The positive acoustic properties of the former membrane roof were to be retained as far as possible, a covering that was permeable to lower frequencies. Working in close collaboration, the engineer for building physics, Rainer Rothbacher, the acousticians from Müller-BBM, and the architects decided to line the underside of the mobile roof surfaces with 4-centimetre-deep trapezoidal-section metal sheeting to match the outer covering. The optimization of the spatial acoustics was based on an evaluation of acoustic measurements and data obtained from computer models. Some areas of the matt-black underside of the roof were executed in an unperforated form (reflecting) and others in a perforated form (absorbent).
The soffit over the auditorium comprises cladding with acoustically adapted bays for sound reflection and diffusion, whereas the lining to the side walls is based on the use of sound reflectors. The finishings to the soffits and walls of the hall consist of a double layer of concrete building slabs. To improve and modify the sound quality, equipment was installed to extend the reverberation time (electronic spatial acoustics). Based on measurements made during the first events staged here, in June 2011, Müller-BBM concluded that the internal space possesses excellent reverberation properties across the entire spectrum. Furthermore, the Vivace system for electro-acoustic effects complements the spatial acoustics ideally.
The demolition of the existing roof also allowed the creation of two new storeys above the auditorium. These are supported by two lattice girders set parallel to the stage. The new third floor of the Felsenreitschule houses stores and spaces for technical installations, whereas the fourth floor contains recreational areas, a tea kitchen, offices, changing rooms, studios and two sanitary areas, in addition to storage space. At the junction between the fixed roof and the sliding roof elements, two new lighting bridges, a working bridge and a stage gallery have been installed. These permit the presentation of pieces that could hitherto not have been performed here.
The conversion of the Felsenreitschule has successfully combined the attractions of an open-air stage with the advantages of one that can be used all year round in all weathers for musical and dramatic performances.
