Freiburger Hof is a commercial building which was originally constructed as a hotel in 1890. The historic building is situated in a central location in Freiburg city centre and was badly damaged in the Second World War. Massive destruction was sustained in particular during what was known as the night of bombing of 27 November 1944: The historic mansard roof structure and parts of the building were hit and gutted by fire. Reconstruction work after the war went on into the 1950s. During this time, the height of the building was increased with a fourth floor and crowned with a simple pitched roof. Its use also changed: In the post-war period, Freiburger Hof was initially a residential and office building, whereas nowadays the building is only used for commercial purposes.
In order to create more space for offices and commercial premises, the intention was that the makeshift pitched roof should be removed and replaced with a generously sized roof level. It was crucial here to follow the specifications for the preservation of historic buildings. For instance, it was necessary in particular to preserve the historic facade and, for the purpose of adding an extra floor to the building, design a roof structure which blends harmoniously into the cityscape.
Once the architect's design had been presented, the question was then how to implement this design in practice. In joint discussions between the architect, the structural engineer, the client and the carpentry firm that was called for consultation, the decision was then taken relatively quickly to construct the entire roof level in a solid wood design.
The client decided, as part of the adding of a roof level and roof renovation work, also to have all of the windows, some of which were still single-glazed, replaced with thermally insulating windows on all floors. At the same time, the plan was to renew the late Renaissance facade; it is made up of sandstone elements and plastered brickwork. The sandstone elements were restored, fully painted and the ornaments were regilded.
Project Addition of storey and roof renovation
Location Freiburg im Breisgau, Germany
Construction Period 2013
Client W&S Maschinenbau GmbH
Wood construction carried out by Steiger & Riesterer GmbH
Architect Höfler + Stoll GbR
Statics Göppert Bauingenieure
The new roof level had to be designed and planned from a structural engineering point of view in such a way that it does not place any additional loads on the building. At the same time, the client wanted it to be constructed with a particularly sustainable and ecological design. These specifications combined to produce a special design with a solid wooden structure. The shape of the roof resembles an upturned ship's bow, made from binderholz CLT BBS.
The new, cantilever mansard roof structure extends over a basic area measuring around 15 x 50 m. It was constructed on a BBS ceiling, which is also cantilever and was laid on the existing building. This meant that the additional loads on the building structure could be kept within the right framework in the truest sense of the word. In respect of the statics, they are of a magnitude such that they are negligible in terms of calculations.
The 24 cm thick BBS elements are perfect for such a structure. In addition, they offer enhanced thermal protection in summer and have persuasive sound insulation characteristics. But above all it was possible to deliver the project in a near record-breaking construction time thanks to the high level of prefabrication of the elements: just six months elapsed between when the old pitched roof was demolished and when the new roof level was occupied!
The execution of the work was a challenge. The timber construction company Steiger & Riesterer had to implement the construction project with business still being conducted at the business enterprises based in Freiburger Hof and the surrounding buildings. In addition, the building is located on a very busy road with trams running down it. It was therefore only possible to deliver the solid wood elements and materials once a day, in a time slot just before the start of rush hour.
The key to success once again proved to be the technical and economical performance capacity of the system format BBS 125. This was apparent both in machining and when it came to delivery. The BBS elements can easily be combined thanks to their static loading capacity to form a free-spanning roof support structure that is over 15 m wide, thus enabling waste-optimised use of materials. Another advantage is the low weight of the small-sized BBS elements. This is because construction sites where there is a confined amount of space, such as at Freiburger Hof, restrict the size of cranes and thus also the maximum weight of the construction elements.
The timber construction company Steiger & Riesterer is also located just a few kilometres away from Freiburger Hof and operates a dedicated machining facility for cutting the BBS 125 elements precisely to the nearest millimetre. This meant it was possible to fabricate the elements just in time and take account of the usual changes of plan and dimensions when working in an existing building. In view of these cumulative advantages, all arguments quickly favoured the use of the flexible and economical system format BBS 125.
The work was implemented swiftly and smoothly. The timber constructors mounted the roof support structure on site on two separate mounting frameworks. To do this, the first step was to screw in each case one component for the pitched roof to one component for the flatter roof section. The carpenters then laid this half roof element on a second framework and screwed it to the second half of the roof which was prefabricated as a mirror image. In this way, section by section the full roof shape was created from two roof segments in each case.
Within six weeks, we built up a 600-square meter roof space, adding another story constructed in solid timber. That's only doable if the construction processes are optimally coordinated and premium-quality materials are used delivered by reliable producers on a just-in-time basis. These were the preconditions that could be counted on with binderholz. The closely coordinated collaboration with our on-site carpentry workshop went as smooth as silk and simplified our work significantly in site and project management. A top priority for the client was the use of a material that guaranteed the required statics while simultaneously adding little weight to the existing building. The use of binderholz CLT BBS elements showed the client this was possible with a renewable and natural material and confirmed his decision to choose products from binderholz.
Photos: © binderholz, Höfler & Stoll Architekten, Steiger & Riesterer GmbH, Ralf Killian - www.killian-fotografie.de