Biodiverse green roof on the clay castle in Porumbacu

Article by Ioana Mircea, Landscape Urbanist, published on Acoperismagazin.ro

Last summer added a new chapter to the history of green roofs in Romania. At the beginning of this year, still in the design stage, the green roof of the restaurant at the Clay Castle in Porumbacu de Sus gradually became a reality, complementing the story woven by the Vasile family in the “Valea Zânelor” (Fairies’ Valley), as they named this magical corner of the Porumbacu River valley. The current image of this place, even while still under construction, indeed looks like it came straight out of a beautiful story, with many visitors comparing the new restaurant to its “green hat,” and the Castle to the famous fantasy worlds of franchises like The Hobbit or The Lord of the Rings.

Reaching this point required a lot of work and careful planning, especially for procuring the necessary materials in advance. As mentioned in the previous article, one of the main criteria underlying the entire project was sustainability: using natural materials, employing local labor and products as much as possible, as well as involving specialists from the necessary fields and even volunteers who understood the message of this project.

Ecosystem and modern materials

Of course, to achieve an efficient drainage system and proper protection of the wooden structure of the roof, modern materials and construction techniques were required. For this purpose, the Bauder range for extensive green roofs was chosen, proven to be one of the best performing in this type of work both in Romania and across Europe. Although the materials used in this range are not entirely natural, it was a small but necessary compromise, not only for constructing the roof but especially for maintaining it in optimal conditions over a long period. The project demonstrates its advantages through the creation of an exceptional vegetation layer, both as a substrate and as an ecosystem, whose preparation took over a year. Seeds collected a year earlier from alpine meadows, similar to the local ecosystem, were used, along with natural materials from the vicinity of the Castle, such as straw, spruce bark residues, peat, and volcanic slag to enrich the soil.

Before the actual implementation of the vegetation, the roof surface also required preparation. This is the first stage of any green roof: creating the substructure of the system, which protects the roof and ensures water retention for the plants while draining excess water.

After laying the decking and installing gutters to collect rainwater and irrigation water, the entire surface was covered with Bauder Plant E waterproofing, a specially designed heat-weldable membrane that also acts as a root barrier, preventing both plants and water from penetrating the roof structure. The 5.2 mm membrane, reinforced with polyester fleece, was welded strip by strip with a torch to ensure perfect sealing. This process took one week and was a critical step to guarantee the long-term structural integrity of the roof. The protection provided by the membrane, together with the rest of the extensive system, can extend the roof’s lifespan by up to 25%.

The second step consisted of installing the Bauder PE 02 polyethylene separation layer. As the name suggests, this layer prevents direct contact between the waterproof membrane and the protective layer applied in the next step, enhancing waterproofing efficiency.

Next, the Bauder Schutzvlies SV 300 protective layer was applied, made of polyester fibers, providing mechanical protection for the lower layers of the roof. With a water absorption rate of approximately 2 l/m², it improves drainage by retaining and evenly distributing rainwater not absorbed by the upper drainage layer.

The following step was installing the Bauder DSE 40 drainage and water retention modules, which interconnect to form a uniform layer that collects, retains, and channels water to the lower layers. Its retention capacity is medium, about 13.5 l/m², with retention cell depth of 40 mm. It was specifically chosen due to the roof’s low slope, offering good water retention. At this point, small “dams” were also created along the main slopes using this element, creating slightly wetter zones for planting species characteristic of the nearby riverbed.

The final step before starting the vegetation installation was laying a 125 g/m² Bauder FV 125 filter geotextile over the entire drainage layer. This layer allows water to pass to the lower drainage layers while preventing clogging with soil or organic material. It was fixed in place by the weight of the plant substrate itself.

Biodiversity on sustainable foundations

All these substructure operations, carried out by Odu specialists, took about two weeks, during which the substrate mixture for sowing and planting was prepared in parallel. The secret of this roof lies precisely in preparing and planting this mixture, with the visual element ultimately capturing the attention of the many daily visitors.

As mentioned, this phase required significant preparation. Since the goal was to integrate the restaurant into the landscape as naturally as possible—a large, imposing element—a biodiverse meadow was created on the roof, similar to local ecosystems. For this, soil collected on site was mixed with sand for better drainage. To further enhance porosity and water retention, volcanic slag, chopped straw, spruce bark, and spruce wood residues from roof construction were added. This approach minimized material loss, reduced construction waste, and therefore the overall impact on the site. These materials were chosen because they are natural, locally available, and fully aligned with the project’s sustainability direction.

Another important role played by the plant residues and slag in this mix is their decomposition into organic compounds, slowly releasing nutrients to ensure the development and evolution of the ecosystem over time, since the thickness of a green roof’s plant substrate is much smaller than the soil layer in which these species usually grow. To further compensate for potential deficiencies, a mix of mycorrhizal fungi was added, which live symbiotically with local species, aiding root proliferation, nitrogen fixation, water and nutrient retention in the soil, and also increasing the drought resistance of the plants. The mix was complemented with small amounts of clay and peat to improve water retention in the soil and prevent evaporation.

The substrate thus created was spread over the roof surface in thicknesses ranging from 10 to 20 cm, depending on the plant species to be planted or sown.

The roof’s ecosystem was created from a mixture of Bauder seeds for extensive green roofs, mainly composed of small perennial herbaceous flowering plants. Additionally, as previously mentioned, a mix of seeds from meadow plants characteristic of the hilly areas of the Subcarpathians, collected from the sites near the castle, and alpine meadows from Harghita County, were added. Because these mixes have slow germination and sowing was done in July, these plants will only become visible at the beginning of the following spring. To ensure the roof was still green during the summer, a small percentage of fast-germinating grass was added, which over time will be replaced by the more aggressive local species. In wetter areas, created by the small DSE 40 dams, species from the nearby Porumbacu Riverbed were planted, such as Mentha piperita, Menta Aquatica, and Lythrum salicaria.

After planting, the perimeter gutters of the roof were cosmetically finished using halved birch branches, creating a natural parapet perfectly integrated into the landscape. The restaurant is neighbored by a birch grove planted in the spring to visually block an unsightly building on the opposite bank of the river. Since the roof slopes significantly toward the back of the restaurant and the inside of the gutters was visible from certain angles, they were masked with a layer of gravel for drainage, applied over a filter sheet, thus creating a type of channel that fulfills a drainage function while also having a decorative role by alternating textures: vegetal-mineral.

The entire project lasted 3 weeks, during which Odu specialists collaborated with volunteers from the “Ion Mincu” University of Architecture and Urbanism in Bucharest, the local team, and the ever-present member Azorică, who was extremely delighted with the new viewpoint created. Overall, it can be said that this roof is not just an installation, but a page in the beautiful ongoing story of the Castle, written by everyone who visits it.

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