Green roofs—roofs planted with perennials or even trees—have been rising in popularity in recent years, thanks to their many benefits for both people and nature. They can significantly cool their surroundings, helping to reduce the urban heat island effect by absorbing and storing heat. Green roofs also buffer rainwater, slowing runoff and easing pressure on drainage systems. And perhaps most importantly, they provide valuable habitat for insects, especially in areas where green space is scarce. In this article, we explore the value of green roofs, their different forms, and the possibilities for implementation.
Types of Green Roofs
When greening existing roofs, we distinguish between two main types:
1. Extensive Green Roofs
These use a mix of drought-tolerant plants that thrive in a thin substrate layer. They require little to no irrigation and minimal maintenance, as the vegetation largely sustains itself. Common species include sedums, grasses, and low perennials.
Sedums, for example, are hardy and survive long dry periods without irrigation. While they provide nectar, their ecological value as shelter or nesting space is limited. Nevertheless, they remain a visually attractive and practical choice for extensive roofs—certainly more appealing than gravel or bare roofing.
2. Intensive Green Roofs
These roofs feature a thicker substrate with water-buffering capacity. The planting palette is much like that of a traditional garden, including trees, shrubs, perennials, and bulbs. Irrigation and maintenance are required, and structural load capacity must be carefully assessed before installation.
With deeper substrates or modular planting systems, intensive roofs can host diverse, drought-tolerant perennials and ornamental grasses. A fully planted roof, even with trees, not only enhances cooling but also supports biodiversity and adds strong visual appeal.
The Power of Smart Combinations
Green roofs are even more effective when combined with solar panels. Rooftops already receive plenty of sunlight, but solar panels lose efficiency as temperatures rise (negative temperature coefficient). The cooling effect of a green roof helps maintain lower panel temperatures, boosting energy output. Around 7% of green roofs today also host solar panels. Often, sloping roof sections carry panels while flat areas are planted with vegetation such as sedums—an approach sometimes called “bitumen-plus.”
From Real Estate to Living Space
When a roof is accessible to building users, transforming it into a rooftop garden creates an additional living space—green, attractive, and enjoyable. This immediately increases the property’s value, turning real estate into “living estate.”
Trees and Shrubs on Rooftops
A rooftop becomes truly spectacular when trees are planted. However, this requires specialist knowledge in design, construction, and maintenance, especially for existing buildings. Factors such as load-bearing capacity, anchoring, and irrigation must all be considered.
Not all trees are suitable. Only heat- and wind-tolerant species should be used. Multi-stemmed trees are often preferred because of their lower center of gravity, making them more stable in windy conditions and reducing pressure on the structure. Since irrigation is disconnected in winter to prevent freezing, rooftop trees must also withstand dry winter periods.
Where a roof cannot support traditional planting, a tree container system (boomveer) can be used. This cube-shaped structure suspends the root ball, allowing controlled movement to absorb wind gusts. Such systems make it possible to place trees in locations where they otherwise could not grow.
In Conclusion
Greening rooftops is a simple yet highly effective way to make cities more livable. Green roofs provide extra green space, mitigate climate change, improve air quality, and reduce energy use. They also bring numerous social, economic, and ecological benefits.
