Wooden extension on a century-old department store

The building, which belonged to the department store Herrmann & Vogel during the First Republic, owes its unusual shape to the original development. As a warehouse with offices, it was located in the courtyard of the densely built-up part of the center of Opava. The department store was founded in 1910 by Ervín Herrmann and his friend Otto Vogel, both of whom were part of the Jewish community in Opava. The company survived World War I and the economic crisis of the early 1930s, but it was significantly impacted by the rise of totalitarian regimes. The owners lost the building in Lazebnická first with the rise of Nazism, and soon after for the second time with the rise of communism. The house returned to the family's hands only after the Velvet Revolution. Today, its descendants are breathing new life into it. "The house has a long history that carries all the beautiful, but also tragic traces. It was built by my great-grandfather and he did not live to see its second return. His daughter, my grandmother, received it back, and now my dad and I are taking care of it. It is the only tangible legacy of our once great Opava family," specifies Klára Dvořáková, the great-granddaughter of the original owners.

The task for the architect was to design an extension with terraces, allowing the new floor to be slightly "submerged" and become a natural part of the original historical building, which is located in the heritage zone of the center of Opava. This also led to the strict requirement from the heritage authorities for the roof, which must respect the existing building outline, achieved through large overhangs of the roof above the terraces. The choice of a wooden extension was made for three reasons – firstly, due to the unclear appearance of the lower structure and its different module compared to the extension, secondly due to speed and the desire for maximum prefabrication, and thirdly because the investor is very close to wood.

The house in Lazebnická was built in 1929 according to the design by architect Erich Geldner as a basement iron-concrete building, which was very advanced and almost unique for its time. Concrete was the material of the future, and the house was already then prepared for an extension. The last horizontal structure, the original roof, consisted of a reinforced concrete beam ceiling with a beam spacing of 2 m. The new extension thus rests on the ceiling of the last floor and is not supported by load-bearing walls. Therefore, it was necessary to reinforce the reinforced concrete ceiling with a 20-centimeter concrete overlay for the beams, which, however, was not continuous along its entire length, only in the central part of the extension, as it was not needed in the terrace areas. As is often the case with reconstructions, a surprise arose here when it was discovered that one concrete ceiling beam was missing, although it was drawn in the historical plans. Fortunately, it was in a location where it was not a significant problem from a static point of view.

Structural Solution

The extension has three main naves designed as open office spaces with spans of up to 8 m, with technical rooms, kitchens, an entrance hallway, and social facilities in the middle. The central part was modularly designed, and the other structures were prefabricated flatly. The structure of the individual naves consists of prefabricated frames made of BSH, which were glazed immediately after assembly. The gable walls are made of visible CLT NOVATOP SOLID panels. The NOVATOP OPEN roof elements were entirely prefabricated, including overhangs, filled with insulation and covered with a wood fiberboard, which is hydrophobic. During the assembly of the wooden structure, windows and doors were also stacked on the construction site. The installation of windows occurred parallel to the roof assembly, directly into the BSH frames. The glazing is fixed, frameless, which is a significant financial saving, and a great bonus is the beautiful clean design.

The floor insulation is more or less formal, but it also influences the airborne sound insulation of the assembly. Insulation was done outside with extruded polystyrene, and larch terrace segments, which are easily removable for maintenance, were laid on the grid. Inside, mineral insulation is used followed by a special board for underfloor heating, fermacell Therm25, designed for a dry floor system with integrated pipe conduits for heating or cooling – both functions will be utilized here. Around the entire perimeter, there is a 300 mm installation channel where cables for networks and installations are completely distributed, above which is a removable hatch. Even the column in the BSH frame has a vertical installation channel of 10x10 cm inside, which was created by gluing beams from three pieces (20+10+20 cm). This makes it easy to modify the layout of spaces or incorporate new networks that will be used in the future.

The NOVATOP OPEN roof elements are made from a three-layer bio-board that is airtight and visible towards the interior. Reinforcing BSH beams, which carry the span, are glued to it, trimmed towards the overhang. Due to the trimming, the overhangs appear very subtle. The double-skin ventilated roof is filled with wood fiber insulation for better phase shift, accumulation, and building acoustics. The covering consists of hard hydrophobic wood fiberboards, primarily for immediate walkability during installation.

The roof was covered with high-quality diffusion foil, allowing for zero slope and anchoring. On top of it, sloped wedges made from wooden rectangles were placed, each subsequent wedge being 3 mm lower than the previous one. This is easier for production than making triangles, and it is also possible to effectively utilize offcuts for their production. The wedges were covered with a three-layer board and on top of it, an EPDM foil was stretched.

The roof installation was quick and precise, and the construction was quickly closed. Only the lower covering of the roof overhang was completed on-site, so that the visible ceilings made of bio-board could be as continuous as possible along the entire length of the terraces. Insect and bird nets are not missing. The lintels consist of a 280 mm beam, not for static reasons, but for the storage of blind boxes for external window shading. Photovoltaics are planned on the roof.

Modular Prefabrication

There were several reasons for prefabrication – primarily there was a time limit on occupying the busy street, set by the city for only one week. Other factors included the required quality and accuracy. Conversely, the height of the building at 13.5 m and access only from one side was a challenge for prefabrication, which, combined with a depth of 28 m, imposed weight limits on the modules. The 3D modules were very complex in shape because the architectural design initially did not account for prefabrication. One of the most complex modules had a rounded wall, adjacent walls at an angle of 45 degrees, one wall that overhung, and at the same time incorporated doors. The rounding was done for the first time in this form in production. The arch consists of 15 cm strips of CLT, which are screwed to ribs of bio-board placed from the back, which ultimately serves as shelves in the kitchen. Small transition edges were ground down, and screws were concealed. It is a very effective solution, visible from both sides.

Interior

The interior is dominated by ubiquitous wood. The visible quality of NOVATOP is exceptional and stable, resistant to the formation of surface cracks and has a uniform appearance in all exposed panels. Part of one module was a SOLID wall laminated with oak veneer, which also contained hidden doors, so the veneer decor is continuous across the entire area of the wall. The doors were then carpenter-finished and hinges were mortised. Everything was sanded and oiled as part of the finishing work. The same veneer decor accompanies other interior elements in one of the three offices.

In most rooms, there are locally installed wooden acoustic panels NOVATOP ACOUSTIC on the ceiling to optimize spatial acoustics, and they are also a significant design element that the architect had counted on from the beginning in the entire concept. They cover a 12 cm hollow space for the distribution of air conditioning, which was originally considered only for the modules. Ultimately, this solution was adopted in all rooms, resulting in about 10 outlets that ensure completely uniform and pleasant airflow.

Final Balance

Construction is always a large project. And reconstruction is usually even bigger. An extension on a historical building is therefore a challenge in many respects, but it also offers an opportunity to evaluate the building, add a new dimension to it (literally), and create space for new commercial premises and housing even in densely constructed areas of cities, where it would otherwise be difficult to realize.

In the case of this project, it was possible to move the entire house closer to sustainability. A heat pump, designated for heating and cooling the extension, will also heat the floor with apartments, as well as rainwater and energy from planned solar panels in other parts of the house find their use.

Thanks to prefabrication and the associated speed of construction, the first tenants moved into the offices just 9 months after the work on the original roof began. This aspect is significant from the perspective of investment return.

GRAND OPENING at Lazebnická 3

From September 10 to 12, 2024, there will be a grand opening for the professional public. On Tuesday, September 10, a professional program will be prepared with a debate focused on "Possible Technical Solutions for Extensions, Their Advantages and Limits." Representatives of wooden solutions – NOVATOP and NEMA – as well as Unicont, specializing in brick or aerated concrete extensions, will participate in the debates. The guide will be Kateřina Eklová from the Rethink Architecture team.

The professional program will also be supplemented by Aveton, which deals with spatial and building acoustics at the highest level, and Fermacell, which will present the topic of fire safety of wooden buildings. The program will also feature Petr Stanjura, the city architect of Opava, who will attempt to answer the question of whether wooden buildings belong in the historical centers of cities.

The event is primarily intended for architects, designers, city and municipality leaders, and the professional public. Guest registration is necessary due to limited capacity.

Registration form – https://docs.google.com/forms/d/e/1FAIpQLSfMRlUK5o9ins08S3Y7oybZZwC-SI1jNCPYl7cpWXrYzgDyng/viewform

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Implementation: 2024
Architectural Design: Ing. arch. David Wittassek, QARTA Architecture
Project and Construction: NOVATOP (AGROP NOVA a.s.)
Static Analysis: A2 Timber
Wooden Structure Installation: Dřevostavby MC
Construction Implementation: KODE Stavby
Interior Design: Ing. arch. Lenka Demetrovičová
Investor: private individual