Advantages and disadvantages of roof greening

Sira Huwiler-Flamm

Green roofs offer many advantages for homeowners. Our examples show how roof greening can contribute to biodiversity and why it saves you money.

There is a house with a green roof behind a meadow and a fence.
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In this article

More and more greenery on Swiss roofs

They cool the property down in the summer, insulate it in the winter, filter exhaust gas emissions, and provide bees, butterflies and birds with additional habitat in cities: green roofs. Switzerland offers ample potential for roof greening. “In Switzerland, the surface area of flat roofs alone represents nearly 50 square kilometers,” estimates Dr. Stephan Brenneisen, head of the Urban Ecology Research Group at the Zurich University of Applied Sciences (ZHAW). “However, roofs with a pitch of up to 20 degrees can also be planted by homeowners creatively and to suit their tastes.”

The trend has already arrived in Basel: around 40 percent of the flat roofs in the city are greened. And the city of Zurich also reports a rise in the number of green roofs. But Brenneisen knows that there is still potential for growth, despite the fact that since the 1990s, it has been compulsory to green new buildings above a certain size in larger cities.

Yet the idea of roof greening is nothing new. The artist Friedensreich Hundertwasser was already convinced by the concept of the green roof, as shown in “The Green Museum” at the Kunst Haus Wien. He imagined an architecture that was in harmony with nature and people, and even dreamed of lush “roof forests”. The Krawina House built in Vienna in his honor in the mid-1980s is one of the first examples of green roofs and facades in Europe.

The Swiss architect Le Corbusier is also said to have asked, about 100 years ago: “Is it not against all logic when the upper surface of a whole town remains unused and reserved exclusively for a dialogue between the tiles and the stars?” He was perfectly right even then because the advantages clearly outweigh the disadvantages.

Several terraced houses with green roofs form part of an estate.

A green roof is extremely durable: experts estimate that greening will double its life from 20 to up to 40 years.

The advantages of a green roof

1. Natural air conditioning

Since stored rainwater evaporates in summer, the green roof cools the building down on hot days – especially on the upper floors.

2. Thermal insulation

From a height of twelve centimeters and above, the additional layer provides extra insulation and thermal insulation in the winter. A green roof thereby even offers energy saving potential. A retrofitted roof garden can also be financially worthwhile, as older houses in particular don’t have an insulation layer, or only a layer around ten centimeters thick.

3. Save precipitation water (costs)

Given that rainwater evaporates instead of entering the sewage system, a green roof relieves sewage treatment plants and urban drainage systems of around 50 to 80 percent of precipitation amounts – which is not only good for the environment, but also offers potential for savings. Many municipalities and cities (such as Basel and Bern) grant a reduction of ten to 50 percent on precipitation water fees – i.e. ten to 45 centimes per square meter of green roof per year.

4. Protection for the roof

When professionally installed by a roofer, the green seal protects the roof from UV rays, damage and major fluctuations in temperature – the Swiss Green Roof Association (SFG) estimates that the life of a flat roof doubles on average from 20 years to up to 40 years if the roof is greened.

5. Biological air filter

Green roofs filter fine dust particles as well as pollutants and nitrogen from the air. As a result, not only the residents of the greened house benefit from better air quality, but also the entire neighborhood.

6. Oasis for birds and insects

Flowering roofs provide food for insects and breeding grounds for birds. The fact that green roofs also represent a habitat for rare plants and animals is shown, for example, by the flowering orchid meadows on the roof of the Seewasserwerk Moos in Zurich-Wollishofen – where biologists have discovered twenty species of spider. Lapwings threatened with extinction were repeatedly sighted on the roof of the Migros operations center in Gossau in the canton of St. Gallen. And the innovative beekeepers of the “Wabe 3” start-up even make city honey in the middle of Zurich: all their bee colonies are located on green flat roofs.

Close-up of a green roof with trees in the background.

Good for the environment: rainwater from green roofs evaporates instead of encumbering sewage treatment plants and drainage systems.

The disadvantages of roof greening

1. Care and maintenance

To ensure that the drains remain functional and that the roof does not become rife with unwanted plants, experts recommend maintenance twice a year. An extensive roof greening with succulents, mosses, grasses and Sedum does not need much care, apart from regular weeding. If the drains remain unclogged and all the layers are laid professionally, the risk of water damage from the roof as a result of greening is very low, says expert Stephan Brenneisen.

2. Investment costs

No matter whether you want to build a new house or green your old roof, garage, garden shed or carport, the investment costs are high. In addition to the expense of the plants themselves, you will also need to allow for root protection film, a drainage layer, a filter fleece and a plant substrate. Expert Stephan Brenneisen estimates the total price at around 20 francs per square meter of green roof. However, you can benefit from savings in many areas thanks to your roof garden, for example due to the longer roof life and better insulation. You will often also enjoy lower rainwater costs. Consequently, the initial costs pay off in the long term.

In some larger cities like Zurich, it is even compulsory to green new buildings above a certain size.

Green roofs on a high-rise housing estate; more houses can be seen in the background, and there are trees standing in the foreground.

Green oases on the roof for a better quality of life

Apart from their practical advantages, green roofs also create a pleasant atmosphere in the city – Hundertwasser would certainly have shared the opinion of Le Corbusier: “The fact that even the uncompromisingly practically-minded Le Corbusier and idealistic aesthete Hundertwasser are in agreement on the subject of roof greening shows that we should adopt the idea and implement it for ourselves,” says urban ecologist Stephan Brenneisen. “Green roofs have enormous potential to restore a part of nature and give us humans a better quality of life.”

If we have sparked your interest in green roofs, you will find some great ideas here for creating your own green oasis on – instead of next to – your house.

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