On the former milk factory site of Aichach, a district town in the centre of the Munich – Augsburg – Ingolstadt metropolitan area, stood a three-storey building from 2009, until autumn 2020. It was and still is used as a commercial and office building, but has now grown by one storey. The “energy farmers” who plan and operate solar and wind power plants are located on the second floor. Following the boom that had been going on for years, their space requirements had gradually increased, which is why they finally decided with their landlord to increase the existing mineral building. In order to enable this during ongoing business operations, the best solution emerged as an increase in timber construction. Schlamberger Moosbichler Architects from Augsburg provided the design for this. The staggered storey jumps inwards all around compared to the existing building and enables generous roof terraces, especially on the long sides. The increase appears like the coronation of the existing building and thus enhances it.

Frame construction made of BauBuche with glued laminated timber for the roof ceiling

As the supporting structure for the 37 m long, 11 m wide and 4 m high staggered storey, the structural engineers chose a wooden frame made of BauBuche of strength class GL 75. It is formed by 3.10 m high BauBuche columns (w / h: 20 cm x 24 cm ), which are placed in external wall levels in an axial grid of 5.35 m, as well as 6 cm raised double girders made of BauBuche (w / h: 2 x 20 cm x 44 cm) with a length of 10.40 m transversely from column to column tighten. At the front of the stacked storey, the edge beams are then only half as wide at 20 cm. To connect the wall and roof elements, a kind of ring beam made of BauBuche (w / h: 10 cm x 44 cm) was laid on the column heads.

16 cm thick glued laminated timber panels function as the ceiling or roof. Tied to fit perfectly, the 60 cm wide elements were laid across the BauBuche beams and screwed to them. The whole thing was installed by the carpenters of the general contractor Gumpp & Maier on the insulated and sealed, 40 cm thick reinforced concrete ceiling on the second floor. Before that, the BauBuche components were given a varnish coating in the factory to protect them from moisture.

Slim cross-sections increase the cost-effectiveness and aesthetics of the extension

The advantages of using BauBuche for this structure, compared to glued laminated spruce timber, are its higher strength and density, which enable significantly slimmer cross-sections with the same load. This not only increased the cost-effectiveness of the addition, but also provided the desired look for the office unit in the attic. The slim construction also corresponds to the large glass facades on the long sides made of triple insulating glazing, each of which accommodates a mullion and transom construction. These are arranged in front of the BauBuche supports so that the wooden skeleton remains visible on the inside. The “remaining areas” of the building envelope include 4 m high, prefabricated and insulated timber frame construction elements. The 33 cm thick and a maximum of 11 m long elements are made up of an 18 cm deep, fully insulated frame made of solid structural wood, on which a 22 mm thick OSB board was attached on the room side, which both stiffen the elements and is airtight layer. This is followed by two layers of plasterboard. On the outside, the wall elements were given a 3.5 cm thick wood fibre insulation layer including a facade membrane to protect them from the effects of the weather. Finally, the horizontally structured, rear-ventilated facade made of larch wood rhombus strips on a substructure. This wall structure achieves a U-value of 0.19 W / (m²K).

The vertical stiffening of the construction takes place via the wooden frame construction outer walls as well as two inner walls placed transversely to the longitudinal direction of the building, the frame and threshold of which are made of particularly stable and dimensionally stable laminated veneer lumber. These inner walls stand on steel girders, via which the loads can be transferred directly into the reinforced concrete columns and walls of the existing building without placing any load on the top concrete ceiling. The horizontal bracing is provided by the glued laminated timber roof pane screwed onto the BauBuche beams.

Heat measurement and fire alarm systems enable the visible wooden construction

The wooden components of the extension of the building to be classified in building class 5 were calculated “for burn-up” and meet a fire resistance class of F90. In other words, they were “topped up” with an extra layer of wood all around, the thickness of which is chosen so that it takes 90 minutes to char until the load-bearing cross-section is reached. This so-called “hot measurement”, in combination with an automatic fire alarm system, made it possible for the wooden components to remain visible. The facade cladding and the outer wall insulation were also allowed to be made with combustible building materials, since the entire length of the attic is more than a meter away from the edge of the existing building and consequently a fire spread from the attic to the floors below via the facade can be ruled out.

The cantilevers of the ceiling construction with the canopies and the attic clad with titanium zinc sheets, which was clearly drawn down to the terrace level on the east side, were reinforced with laminated veneer wood panels.

Emission-free heat supply via modern infrared modules

Since the highly insulated building only requires a minimum of heating, the company opted for an emission-free solution made of modern infrared modules. These do not require expensive floor, wall or ceiling installations and, in contrast to sluggish surface heating, heat up in a short time. Since it is electricity-based, a separate power supply was installed on the flat roof with a photovoltaic system on an area of ​​154 m². In addition, a ventilation system ensures the continuous exchange of air with a heat recovery rate of 80%. The new top floor has usage units for offices, meeting rooms and a small canteen. Thanks to the integrated planning and a high degree of prefabrication, it was possible to add the timber structure to the three-storey mineral building in just eight months.