The Mall of Tripla in Helsinki, Finland, combines a shopping center, a railway station, a hotel, and several office blocks. Approximately one hectare of the building's upper area is a green roof, where Leca® aggregate was used as a growth medium for the plants.

The project stands out because much of the area to be developed is located on the building's roof. "The goal was to create an open, light-filled green space that harmonized with the scale of the built environment in terms of design and rhythm. The planters form a cohesive whole, with various elements evoking a welcoming and relaxed atmosphere, unlike the usual," explains the landscape architect responsible for the project, Niilo Tenkanen.

According to the architect, the idea of using lightweight Leca® aggregate as a plant substrate came when the team began planning the different phases of the green roof construction in more detail. 

"Since the roof structure was designed to withstand heavy vehicle traffic during construction, there was no need to use lightweight aggregate for the roof base. Our focus was on creating optimal conditions for plant growth. The chosen solution is economical and provides an airy growth medium. If we had only used soil, it would have become too dense," explains the architect.

The layer of lightweight Leca® aggregate used in the Mall of Tripla’s rooftop planters varies between 10 and 70 centimeters, serving both as a lower layer and a substrate. "It's essential to choose the growth medium based on the goal and define the desired nutrient concentrations for each planter. The desired acidity also influences the selection process. The more plant species, the better it is for the green roof," says Juha Liukkonen from Tieluiska Oy, the company that supplied the substrate for the roof. Designing and constructing green roofs requires expertise in structural engineering, as well as knowledge of water management in the planting area and the adaptability of plants to climate and growth changes. The main technical motivation for the increasing popularity of green roofs is their ability to manage stormwater runoff.