Metsä Wood’s new birch plywood mill was inaugurated in Pärnu, Estonia in October 2018. Due to the tight construction schedule, the use of prefabricated elements was the best option. The installation of the elements was two times faster than building on site. The advantages of using Kerto® LVL (laminated veneer lumber) beams and wooden roof elements combined with other materials were clearly visible in the speed and cost of the construction.

An important investment for Metsä Wood’s competitiveness birch plywood products, the construction of the Pärnu mill began in April 2017 and was completed one year later. The building consists of a production hall with social facilities and an office. The total area of the building is 29,300 m², which is roughly the same size as four football fields.

The lead design office in the project was the Finnish company Sweco. Project manager Aare Uusalu says this was the first time for him to work with wood in such an extensive way. “We designed the building using wood because of the green thinking of Metsä Wood. Kerto LVL was also a cost-effective material, as the lightness of the elements made assembling fast”, he says, capturing the essence of wood element construction – fast, light and green.

Kerto LVL beams for load-bearing structures

The load-bearing frame of the building is made of concrete and glulam. The roof elements act as part of the bracing.

The roof rests on this load-bearing structure that combines glued laminated timber beams and steel beams, and finally, Kerto LVL S-beams as the load-bearing structure of the roof elements. The roof elements are comprised of wooden panels with mineral wool insulation and PVC waterproofing. A single element has a width of 2.5 metres, a length of 18 metres and a thickness of 400 millimetres. The underside of the element is covered with plasterboard. The elements were connected to the structures with wooden screws. This benefitted the construction, as connections could be made through the panel.

The elements have a ventilated air gap between the insulation layer and roofing panel. The elements are covered with the final roofing material at the element factory. Interior finishing panels were also pre-installed. The factory set included thermally insulated roofing elements on wooden structures covered with roofing membrane, smoke hatches and parapet elements. Immediately after the installation of the roofing panels, the joints were filled in order to turn the building weatherproof.

All in all, installation turned out to be fast and easy: “The time schedule was tight and challenging to manage. In principle, this prefabrication method improves the pace of construction work, so other installations can start much quicker”, says Lauri Susi who worked as a project manager for the subcontractor Timbeco Woodhouse, company responsible for manufacturing the roof elements.

Kerto LVL – a fire-safe building material

When building the Pärnu mill, fire safety was analysed through modelling. “With modelling, we were able to prove that the fire resistance of timber was efficient without additional measures, so there was no need to cover the wooden surfaces with additional fireproof paint like we do with steel”, says Aare Uusalu. The mill has a fire resistance rating of R30.

Kerto LVL is a fire-safe material, as it behaves predictably, and its exact charring rate is known. In the event of a fire, it performs better than many non-combustible materials: when the product is exposed to fire, its surface becomes charred, which protects and insulates the product, thus slowing down the burning process.

The main benefit was that installation is essentially twice as fast compared to common solutions – steel or concrete beams and steel profiles.

Marek Sööt, the Project Manager at main contractor Nordecon

A fast, practical and cost-effective wooden roof

For the designers, the extensive use of wood in an industrial building was a new approach.

“When we build industrial buildings in Estonia, we normally use concrete and steel”, says Aare Uusalu. “This was my first experience of using wood so extensively in a large building and my first time using Kerto LVL. It is a strong material, and the elements were light and easy to instal.”

Marek Sööt, the Project Manager at main contractor Nordecon, also says that using prefabricated elements sped up the building process. According to him, using timber elements with concrete structures for the roofing of large buildings significantly improves the pace and reduces the budget: “The main benefit was that installation is essentially twice as fast compared to common solutions – steel or concrete beams and steel profiles.”

According to Sööt, it was a natural choice to use wood materials for a mill that processes wood, and aesthetic and green values also played a part in this. In the mill’s offices, timber is used both in the facade and the exposed parts of the interiors: “Of course, there is also the aesthetic point of view. It is certainly nicer than standard solutions”, he concludes.

The new birch plywood mill has a production capacity of 50,000 cubic metres. The full capacity will be reached by the end of 2019.