08.05.2026

Disadvantages of Green Roofs: Principles, Planning, and Examples

Building design

Aerial view of an urban development—a bird's-eye view of established urban structures. Photo: chuttersnap/Unsplash

Green roofs are considered a prime example of sustainable urban development: they cool the city, store rainwater, promote biodiversity, and improve the urban climate. However, anyone who plans, builds, or operates green roofs inevitably encounters a number of constraints, technical risks, and economic realities that are often underestimated in public discourse. The disadvantages of green roofs are not an argument against the concept, but they are a planning consideration that anyone who wants to implement green roofs seriously and on a long-term basis must be aware of.

What green roofs entail from a technical perspective and the different types
What structural requirements and load issues green roofs entail
Why waterproofing and root protection are among the most critical weak points
What costs are actually incurred during planning, construction, and maintenance
How waterlogging, drought, and substrate failure endanger the vegetation
What restrictions apply to roof pitch, building type, and use
Why care and maintenance are often underestimated and what mistakes can result
How the disadvantages of green roofs can be managed during the planning phase

Green Roofs: Definition, Types, and Planning Framework

A green roof, also known as a vegetated roof, refers to the systematic planting of flat roofs, sloped roofs, or roof terraces with a multi-layered structure consisting of a vegetation base layer (substrate), filter fabric, a drainage layer, a protective layer, and a root barrier over the roof waterproofing. The Technical Guidelines for Green Roofs, published by the Research Association for Landscape Development and Landscape Construction (FLL), is the authoritative set of standards in German-speaking countries and distinguishes between two basic types: extensive green roofs and intensive green roofs.

Extensive green roofs use thin substrate layers, typically ranging from three to twelve centimeters, have a low weight of approximately fifty to a maximum of one hundred fifty kilograms per square meter when water-saturated, and feature low-maintenance plant communities consisting of sedum species, mosses, herbs, and grasses. It is the most commonly used form and is considered low-maintenance, though not maintenance-free. Intensive greening, on the other hand, corresponds to a rooftop garden with substrate thicknesses ranging from twenty centimeters to over one meter; it allows for trees, shrubs, lawns, and perennials, but requires correspondingly high load-bearing capacities, a complex irrigation infrastructure, and regular maintenance similar to that of a ground-level garden. Between these two types lies what is known as “simple intensive greening,” which serves as an intermediate stage.

The disadvantages of green roofs are common to both types but vary in severity depending on the specific design. Those who are aware of the system’s weaknesses can address them during the planning phase. Those who ignore them risk structural damage, vegetation failure, and economic losses that permanently call into question the ecological benefits of the green roof.

Structural Engineering and Weight: The Underestimated Prerequisite

The most significant disadvantage of green roofs—in the literal sense—is the additional load that any green roof places on the building’s supporting structure. Even a simple extensive green roof with ten centimeters of substrate exerts a surface load of around one hundred to one hundred fifty kilograms per square meter when saturated with water. An intensive green roof with fifty centimeters of substrate, trees, and paved paths can easily reach five hundred to over one thousand kilograms per square meter. These loads must be safely supported by the roof structure, the floors, the walls, and the foundation.

In new construction, the supporting structure can be designed from the outset to accommodate the planned green roof; while this incurs additional costs, it poses no technical challenges. The real problem arises when retrofitting existing buildings. Many flat roofs on older buildings, particularly those from the postwar period, were designed for significantly lower live loads. In such cases, a structural analysis by a structural engineer is absolutely essential before even a single square meter of substrate is installed. It is not uncommon for this assessment to conclude that intensive greening is not feasible and that even extensive greening is possible only with structural reinforcements, which can significantly impair the project’s cost-effectiveness.

In addition, the load distribution on the roof can be uneven: substrate that becomes saturated due to irrigation or heavy rain, snow loads on the green roof, and localized loads from trees or structures must be taken into account in the calculation. Inadequate structural design is one of the most common causes of damage to green roofs and ranks among the most serious drawbacks of green roofing, as it can, in the worst-case scenario, jeopardize the safety of the building.

Waterproofing and Root Protection: Where Green Roofs Most Often Fail

The most technically sensitive element of any green roof is the roof waterproofing. It must be permanently watertight and, at the same time, root-resistant, because plant roots are capable of penetrating and widening even the smallest cracks and seams in waterproofing membranes. The FLL technical standard stipulates that either a root-resistant waterproofing membrane must be used or a separate root barrier membrane must be installed. Nevertheless, damage to the waterproofing caused by root penetration is among the most common and costly types of damage to green roofs, especially when trees, shrubs, or aggressive rhizome plants are used.

The problem lies not only in the plants’ root strength but also in quality control during installation. Seams, connections at roof penetrations, parapets, and drainage inlets are weak points that must be carefully executed and inspected during installation. In practice, however, it often happens that waterproofing and greening work are carried out by different contractors without sufficient coordination. If a leak occurs, locating the source of the damage under a green roof is time-consuming and expensive: the substrate must be removed, at least in certain areas, in order to inspect and repair the waterproofing.

To make matters worse, leaks in green roofs are often not noticed until much later. The substrate and drainage layer can temporarily store water and release it gradually, allowing moisture to seep into the roof structure long before damage becomes visible on the interior ceiling. Regular inspections of the roof waterproofing, especially at connections and penetrations, are therefore an indispensable part of green roof maintenance. Anyone who neglects these inspections risks hidden structural damage that can negate the entire economic benefit of the green roof.

Drainage and the Risk of Water Logging

Another critical issue is drainage. Green roofs are designed to store rainwater and release it gradually, a feature known as water retention capacity. At the same time, the substrate must not remain permanently water-saturated, because waterlogging damages most green roof plants and causes the roots to die. The drainage layer must therefore be dimensioned so that excess water is reliably drained away without completely losing the retention effect. Achieving this balance is challenging during planning and prone to errors during construction.

Clogged roof drains are a common problem that has particularly serious consequences for green roofs. Leaves, substrate particles, and plant material can block inlets, causing water to pool on the roof. Excessive water pressure strains the waterproofing and, if the construction is inadequate, can lead to overflows or moisture penetration. The FLL technical guideline therefore recommends protective strips of gravel or other mineral materials around roof drains, which must be kept clear on a regular basis.

Costs in Planning, Construction, and Operation

The disadvantages of green roofs are directly reflected in the cost analysis. Depending on the structure, substrate thickness, and plant selection, the construction costs of an extensive green roof are significantly higher than those of a conventional flat gravel roof. For a simple extensive green roof, additional costs of thirty to sixty euros per square meter compared to a non-green roof waterproofing system are realistic; for intensive green roofs, the additional costs can be many times higher. Added to this may be costs for structural reinforcements, irrigation systems, substrate delivery, and crane work for material transport.

Operating costs are often underestimated in public discourse. Even an extensive green roof requires at least one to two maintenance inspections per year, during which weeds are removed, drains are checked, gaps in the vegetation are replanted, and the condition of the waterproofing connections is inspected. For intensive green roofs, the maintenance effort is comparable to that of a soil-based garden, including pruning, fertilization, irrigation, and pest control. Anyone who fails to include these ongoing costs in their cost-benefit analysis will be surprised by the actual expense.

On the other hand, a green roof waterproofing system installed by professionals significantly extends its service life compared to an unprotected roof waterproofing system because it is protected from UV radiation, temperature fluctuations, and mechanical stress. This advantage offsets the additional costs in the long term, but it requires that the vegetation actually functions sustainably and that the waterproofing is not damaged by roots or maintenance errors. The cost-effectiveness of a green roof therefore depends to a large extent on the quality of its installation and the consistency of its maintenance.

Vegetation, Substrate, and Site Constraints

Green roofs are extreme environments. The plants, growing on a flat or sloped surface, are exposed to sunlight, wind, frost, and drought without the buffering effect of deep soil. Substrate layers just a few centimeters thick can store only limited amounts of water and heat up in the summer to temperatures that can far exceed fifty degrees Celsius. These conditions significantly limit plant selection and make vegetation loss a real risk, especially during dry periods without irrigation.

For extensive green roofs, sedum species are the most commonly used plants because they tolerate drought, heat, and nutrient-poor conditions. However, even sedum is not invulnerable: Prolonged drought without any rainfall can damage even these hardy plants, especially in regions with a continental climate and long summers. Substrate failure due to erosion, compaction, or leaching of fine particles can destabilize the vegetation cover in the long term. On steeply sloped roofs, there is a risk that the substrate and plants will slide off if adequate retaining structures are not provided.

Roof pitch is one of the most important limitations on the applicability of green roofs. Flat roofs and gently sloped roofs up to about five degrees are well suited for extensive green roofs. Steeper slopes require special retention systems, cross-braces, or cassette structures, which complicate installation and increase costs. At pitches of approximately twenty-five to thirty degrees or steeper, conventional green roof systems are hardly economically viable. While green roofs on sloped roofs do exist, they are technically challenging and rare in practice.

Biodiversity: Between Aspiration and Reality

Green roofs are often promoted as a contribution to urban biodiversity, and while this contribution is real, it is limited. Extensive green roofs with sedum monostructures do provide habitat for some specialized insects and spiders, but they are less species-rich than ground-based ruderal areas or near-natural open spaces. The ecological value increases significantly when substrates with varying grain sizes, deadwood elements, sand lenses, and a diverse selection of native plant species are used. However, these more sophisticated systems require more maintenance and are more expensive. Anyone who categorically classifies green roofs as a biodiversity measure without ensuring the quality of their implementation underestimates the disadvantages of green roofing in this regard.

Planning Constraints and Common Mistakes in Practice

In addition to the technical and economic disadvantages of green roofs, there are planning constraints that make this approach difficult or impossible in certain contexts. Buildings with complex roof geometries, numerous penetrations for ventilation systems, skylights, or roof structures make it difficult to install a seamless waterproofing membrane and increase the risk of leaks. Technical roof areas that must be accessed regularly for maintenance work on building services systems are poorly suited for extensive greening, which is damaged by frequent foot traffic.

A common design flaw is insufficient coordination between roof waterproofing, greening, and drainage. When roof waterproofers, landscape architects, and building services engineers fail to coordinate their work, interface problems arise that later manifest as damage. The FLL technical guideline recommends early, cross-disciplinary planning that treats all layers of the green roof structure as a single system. In practice, this recommendation is not always followed, especially in small projects with tight budgets.

Errors in substrate selection are another common problem. Green roof substrates must be lightweight, water-retentive, low in nutrients, and structurally stable over the long term. Regular garden soil is unsuitable as a green roof substrate because it is too heavy, becomes compacted, leaches nutrients, and loses its drainage capacity. Nevertheless, in practice, people repeatedly try to use less expensive soil mixtures, which in the long term leads to vegetation loss, substrate failure, and increased loads. Substrates that comply with FLL standards are more expensive but technically necessary.

Finally, accessibility for maintenance and care is often not given sufficient consideration in the planning phase. A green roof that is accessible only through a narrow roof hatch can hardly be maintained properly. The absence of walkways, inadequate fall protection, and a lack of access for maintenance equipment are design flaws that increase the maintenance workload and endanger the safety of maintenance personnel. Especially with intensive green roofs, which require regular use of machinery and personnel, accessibility must be considered from the very beginning.

Disadvantages of Green Roofs in Context: Weighing the Pros and Cons Rather Than Rejecting the Idea

The disadvantages of green roofs are real, technically describable, and manageable through proper planning. They do not justify a blanket rejection of the concept, but they do require an honest, expert examination—one that is sometimes neglected in the politically and communicatively charged debate surrounding green infrastructure. Green roofs are not a “set-it-and-forget-it” solution that functions maintenance-free once completed. They are a technical structure that requires ongoing attention, expert maintenance, and a solid foundation in structural engineering, waterproofing, and substrate.

For landscape architects, urban planners, and building owners, this means that the decision to install a green roof should be based on a comprehensive cost-benefit analysis that includes construction costs, operating costs, structural requirements, waterproofing risks, and maintenance requirements, as well as the ecological and climatic benefits. Where this assessment yields a positive result, the green roof is a valuable component of urban blue and green infrastructure. Where the assessment is negative, there are often more sensible alternatives, such as ground-level greening, green facades, or unpaved open spaces, which provide the same ecological benefits with lower technical risk.

The quality of a green roof is not determined at the moment of completion, but over decades of operation. A green roof that is professionally planned, installed, and maintained can have a lifespan of forty years or more, making a genuine contribution to stormwater management, cooling the urban climate, and promoting biodiversity. A poorly planned or neglected green roof, on the other hand, becomes a source of damage that undermines confidence in the concept as a whole. Addressing the drawbacks of green roofing is therefore not a minor issue, but rather the foundation for ensuring that green roofs deliver on their promises.

22.01.2025

Project

Co-creation instead of participation

Building design

Management

Participation versus co-creation: Would the Superkilen work better today if citizens had been integrated into the process from the outset?

Superkilen in Copenhagen has failed. The citizens as a whole use the square more as a place to pass through than to linger, even though they were integrated into the planning process through participation – according to a long-term analysis by students at the Danish Institute for Study Abroad. During the design and development process, the Superkilen planners asked citizens to suggest artifacts from their countries of origin for the new city square. Holistic planning with co-creation and prototypes would have been one way of increasing acceptance of the square.

Co-creation

Co-creation originally comes from the business world, which began to involve consumers in the product development process in the 1990s. The Leading Cities research group defines co-creation in urban development as “the active flow of ideas and information between five sectors: government, business, academia, non-profit organizations and citizens that promotes participation, engagement and development.”

What is the difference to citizen participation? Public participation is a means for planners to learn more about the attitudes and opinions of local residents and to develop new ideas. They increase acceptance for a project. Co-creation, on the other hand, involves the most important sectors of society on an equal footing from the outset.

It therefore offers citizens and citizens’ organizations a better opportunity to be truly heard and gain more influence in planning processes. Co-creation views users as proactive citizens rather than consumers, and focuses on long-term cultural change and the whole community rather than individual user groups. Where citizen participation seeks to integrate citizens’ opinions into an already prescribed program, co-creation helps future users to shape and enforce their own decisions. Co-creation is an endless process in the best sense of the word, with regular exchanges taking place between those involved.

Co-creation has a number of advantages. The public provides input and feels equal, citizens feel they are taken seriously. They become much more aware of their living environment. In return, the government has to be much more responsive and responsive to citizens and other sectors. In return, however, it saves costs because only what is actually needed and used is implemented and the risk of a new project is correspondingly low.

Prototypes

In large design projects such as Superkilen, it is extremely important to test ideas in advance before they become a permanent part of the landscape or city. It is a long way from the abstract idea, analysis and public participation to the concrete installation. The prototype should be seen as a learning tool that can be optimized and changed in the next step. Prototypes are a democratic way of dealing with public space, they also reduce the risk of a failed project and unnecessary expenditure.

The holistic approach

Combining co-creation with the prototyping approach makes it easier to explore the potential of a space. This holistic approach leads to more sustainable and vibrant places in the city. However, the Superkilen started with an assumption that co-creation and prototyping would have led to a different design. Users would accept the space better and make it more their own. Perhaps a more critical examination of the initial idea would also have been achieved.

You can read Bianca Hermannsen’s current analysis of the square here.
Robert Schäfer visited Superkilen in 2012, directly after the opening. You can read his review of the new city square in Copenhagen here.
Watch the video here to see how BIG travels with local residents to their countries of origin to select artifacts for Superkilen.

19.12.2024

Design

Heavyweight with a message

Building design

The Globo Uovo sculpture is an impressive work by stone sculptor Marc Reist and is currently on display at the Dürrenmatt Center in Neuchâtel, Switzerland. The artist made the 6.5-ton “global egg” out of white marble. In an interview with STEIN, the Swiss artist explains what message he wants to convey with his sculpture, why he only uses white marble and […]

The Globo Uovo sculpture is an impressive work bystone sculptorMarc Reist and is currently on display at the Dürrenmatt Center in Neuchâtel, Switzerland. The artist made the 6.5-ton “global egg” out of white marble. In an interview with STEIN, the Swiss artist explains the message hewants to conveywith hissculpture, why he only works with white stone and why he launched an appeal to collect eggshells before the opening.

Marc Reist: It’s a logical consequence of my artistic work and how I reflect on my surroundings by observing them. I notice how certain rules and mechanisms are similar on both a small and large scale. I never consciously came up with the topic, it just developed. I think it started in 2005.

The Globo Uovo symbolizes the world and the origin of life. Was there a specific reason for this idea?

Yes, that was in 2011. I was invited by a newspaper in Switzerland to write a few columns. I started to write about resources and food in these texts and about what bothers me: the way we deal with food, the way we deal with our environment. And during this phase, I designed the globe and then the egg. The eggs actually always resonate with me. Regularly for my wife – I only mention this in passing – for her birthday, for Mother’s Day, there are always drawings of chicken and egg. I either start with the chicken or the egg, but the egg always resonates.

Do the many openings in the sculpture also have a meaning?

The openings were created by the lines of longitude and latitude. And I found them very special as soon as I was able to work in the egg. There is a feeling of protection when you are in your human egg and you can see the outside world through these openings. At the same time, it could also be a prison. These feelings arose in me when I was working in the egg.

From a professional point of view – I trained as a stone sculptor and also took the master craftsman’s examination – I know almost all the materials. But even so, I used to only work with black stone for my shapes. And now, for years, I’ve only been working in white. For the small sculptures I make, I use a bright white marble. There is no other solution for me and for my forms. I have never used any materials with textures. They are beautiful, but not for my shapes.

But the marble for the egg has a certain texture. Was a block of 55 tons in pure white not available?

For the Globo Uovo, the stone is a little more marbled. Because this size in pure white – that would have been almost impossible and would have meant such a long wait. And the egg in this size in pure white would almost be a little too beautiful(laughs), almost baroque. That’s impossible. Visually too, it’s almost not rock anymore.

So the egg also looks much more “alive”.

Exactly, that’s what I mean. This methamorphosis of the rock, this mountain, that has to resonate.

How did the change from black to white rock come about?

That’s an interesting question. I have to go back to that. I wanted to make a really big sculpture in the 1990s. I chose a large block in Carrara without knowing what I was going to do with it. I wanted to be inspired by the material, by the block. I normally proceed differently. I have a drawing or a model and then work on the stone. But now I wanted to be influenced by the block. At the time, it was 20 tons in size. And that put an enormous strain on me(laughs). I suddenly realized that the further north the block was transported from Italy, the greater the strain in my head became. I was very blocked! And suddenly I found the solution. I then worked out a light cloth from this block. That was probably my solution, to release this heaviness in me in relation to the block. And since then I’ve only worked with white stone.

Are there purely visual reasons why you like to work with marble a lot, or is it also because of its properties?

Both, actually. My sculptures are also a matter of light. Not only with the large egg, but also with the other sculptures. The way the light passes through the opening and the edges is what makes it so good. And the properties for working are a pleasant side effect. It is easy to work with because it is so even, so homogeneous. But I mainly use marble because light and shadow are important for my sculptures.

You also want to send out a message with your art objects.

Yes, absolutely. I think that’s the greatest task of an artist. That his work is seen and heard, as the case may be. Communication is important. It makes no sense at all if I make an egg like this just to have another beautiful sculpture in a park somewhere. That wouldn’t be enough. It needs a message.

What message is that?

We have great difficulties with resources in general. This is most sensitive when it comes to food. And I really want to draw attention to the fact that people need to deal with the issue of food and resources. That a small train of thought changes.

Your sculpture is currently on display at the Dürrenmatt Center in Neuchâtel, Switzerland. Before the exhibition began, you launched an appeal to collect eggshells. What was the reason?

We had planned a performance with a dancer for the vernissage. I needed a lot of eggshells for that. Over the course of a year, people collected around 35,000 eggshells, washed them and brought them to me. Something you would normally throw away. For the exhibition opening, we laid them out on the floor of the museum and cleared paths, like a labyrinth. The dancer danced her way through it. But every now and then she made missteps on the eggshells, causing them to crack. And at the same time, with every misstep, a part of her body died. Because the shells represent our resources and if they are destroyed, the person dies too. And so she danced through the labyrinth and gradually died a dramatic death after three or four missteps.

Find out more about the project and the natural stone work in the February issue of STEIN. You can also find more information at www.stein-magazin.de/skulptur-des-monats-globo-uovo.