(Roof)top solutions for taming the rain

This green oasis is not only beautiful to look at; it is a fine example of the impact green roofs can have on their immediate surroundings. The roof garden is freely accessible and offers visitors a unique spot of nature within the city – a place to relax on lush lawns in the shade of the trees, listening to the buzzing insects, busy pollinating the plants. What most people don’t see beyond this idyllic scene is the sophisticated rainwater management system laid out among almost 23,000 plants.

Uwe Harzmann explains that sealed surfaces in Hamburg's city centre turn into veritable heat islands, especially in the summer months. The city heats up and causes air temperatures to rise sharply, the hot air then rises rapidly, dragging moist air from the surrounding area with it. In higher air layers, this moist air cools down quickly, leading to sudden and heavy downpours. These masses of water overload the sewer system and frequently result in flooding.

That is why the green roof on the bunker is equipped with so-called retention roofs. These act like a sponge by absorbing a large part of the precipitation and preventing rainwater from running directly into the sewers.

For this purpose, there are boxes under the substrate in which rainwater can collect. They only drain as much water as necessary. If imminent rain were to overload the roof, the water is drained into underground storage tanks or the sewer system. This can be the decisive factor, especially in the case of short but heavy rain showers and can prevent flooding in the vicinity of the bunker. Harzmann explains that the whole thing is a smart system with sensors that measure the water level, and a weather forecast just for the building.

He also emphasises that the planting of the bunker is not only perfectly prepared for rain; the collected rainwater can be used to irrigate the plants during dry phases. This saves money on drinking water and evaporation has a cooling effect on the immediate environment. In other words, green roofs can significantly improve the microclimate in Hamburg and other cities, especially if many are installed.

Not all green roofs are the same

Uwe Harzmann also knows that not every roof can be greened as spectacularly as the bunker in Hamburg. However, the positive effects can still be achieved with a simpler green roof, the so-called extensive green roof.  This solution uses low-growing plants such as mosses and grasses, which can largely sustain themselves. The planting acts as air conditioning, dust filter and habitat for insects at the same time. Such a structure is between 5 and 15 centimetres high and possible on roofs up to an inclination of 45 degrees. For the roof, this means an additional weight of 50 to 170 kilograms per square metre. This type of greening is often found on schools, warehouses or businesses of all sizes due to the low maintenance required.

The Green Bunker is a particularly vivid example of intensive green roofs. It clearly shows how planting perennials, lawns and even trees can create a natural retreat in the middle of the city. Flat roofs are almost exclusively suitable for such greening. The structure is usually between 25 and 100 cm high and loads the roof by an additional 300 to 1,200 kg/m².

Air conditioning and living space at the same time

Green roofs of all kinds are a habitat for birds and insects – and when it buzzes and chirps, cities come alive. And the extra greenery is also a clear benefit for all city dwellers – who wouldn’t be happy spending their lunch break on a planted roof terrace or doing a round of yoga in the fresh air? 

“A good green roof really is the perfect place for any lunch break“ Uwe Harzmann

The plants provide relief for the entire building. They act like a natural air conditioning system, insulating the building in winter and protecting against the heat in the summer. This saves money on energy costs. In addition, the plants filter fine dust from the air and significantly reduce sound reflection due to their mass.

But that's not all. Harzmann says that a professional green roof also significantly extends the service life of a roof. The plants protect the roof surface from extreme temperature fluctuations and mechanical damage, which can almost double the service life.

Green roofs – the cure for all?

Although there are hardly any convincing arguments against green roofs, not all roofs are greened. One main reason for this is that many existing buildings can only carry additional weight to a limited extent. This, Harzmann says, is why it is so important for all future builds to add green roofs from the planning stage.

In the future, every square metre really counts and the combination of all options – we want to get everything out of the buildings. The more roofs are greened, the better!Uwe Harzmann

In addition to the obvious advantages such as promoting biodiversity and protecting against flooding, green roofs and, above all, the associated rainwater management are good preparation for adhering to future legal regulations. In more and more regions, there are regulations such as water discharge restrictions, guidelines on the water balance, runoff values or even specifications on the amount of evaporation, especially for new buildings.

Plants and solar panels - how much space is there on the roof?

In times of rising energy prices and the urgent need for climate-friendly solutions, many people are looking at solar panels.  But there is only so much space on any roof. Will people have to weigh up whether to focus on keeping energy costs low (solar) or conserving nature (green roofs)? Developers at Optigrün have found a way to combine the two with an innovative combination of photovoltaic systems and green roofs that creates a symbiosis of sustainable water management, greater biodiversity and environmentally friendly power generation.

In recent years, the demand for this combination has grown and it is very clearly possible! Basically, in the future, every larger roof will have to be covered with greenery and a photovoltaic system.Uwe Harzmann

A solar green roof makes optimal use of the advantages of both technologies. The green roof not only ensures improved water retention and evaporation, which helps to relieve the burden on urban wastewater systems, but also increases the efficiency of the solar modules. This is because these are cooled by the water evaporating on the plants, which achieves higher electricity yields than modules located on non-green roofs.