Science and Technology 

Plant Skins

Green Roofs, from Nature to Technique

Esteban Domínguez  David Gómez 

The citizen of the 21st century demands to live in an ever more ecological and sustainable environment, where transportation is provided with vehicles that pollute less, where the air that one breathes is cleaner, where large parks and gardens abound, where less energy is consumed overall, and where the energy comes from renewable sources. The construction sector can contribute much to reaching this level of energy and environmental sustainability in the world’s cities, especially the great metropolises.

One of the simplest and most positively impactful measures that have been taken is the green roof, as much in newly constructed buildings as in renovations of already existing ones, whereby unused or underused spaces are converted into lawns or food gardens which, besides helping to improve environmental conditions, serve as meeting points or places for neighbors to get together and socialize and interact in.

Although this may seem like a new idea, or even just a passing fashion, it’s a solution that has been used successfully for many decades, and even centuries. Civilizations like those of the Vikings of Norway or Iceland, way back in the 9th century, knew about covering dwellings with a mantle of grass for purposes of thermal insulation. It is now already obligatory in some great urbs of America and Europe, and numerous municipalities have been incorporating them in their urban planning operations, even facilitating the funding, given the unquestionable environmental benefits of plant roofs. The grass roof is by this time a requirement in countries like Switzerland and cities like Copenhagen and Paris, and it is being actively encouraged in New York, Mexico City, Buenos Aires, Madrid, Barcelona, and many more. Germany alone boasts over 200 square kilometers of green roof.

Solutions of this kind have a lot of potential. Recent studies such as the one conducted by the city of Barcelona indicate that green roofs can be feasibly installed in close to 70% of buildings, so the benefits to the environment would be significant. But what exactly is a green roof? What are its pros and cons?

Simplified, we can define a green roof as the rooftop of a building covered with soil on which to plant grass or make urban fruit-and-vegetable gardens, in such a way that traditional construction materials like concrete, tiles, and flagstones are replaced or hidden underneath. Vegetation thus joins the list of materials available to architecture.

Installation and Types

Installing green roofs requires prior preparation of roofs, but such preparation is not technically complex. The first step is to ensure that the roof is adequately watertight and insulated. Then an anti-root solution is laid upon it which will protect succeeding waterproof layers against damage caused by pressure from the roots of plants, especially those of trees. This anti-root system is a sheet of flexible polyolefin (FPO) – resistant to bitumen, adaptable to low temperatures, and easy to weld – reinforced with a thread of polyester and further anti-root protection. Over this goes a draining layer, which is very important because it does the job of storing rainwater and any leftover irrigation water, and quickly and safely channels all excess water into the drains while also ensuring adequate oxygenation for the grass and other plants.

The draining layer also includes an air chamber through which surplus water accumulated on the roof can escape. Next is the substratum for the roof plantation. The choice of substratum depends on the kind of plants in mind, with special attention on their level of acidity. Sometimes, and depending on the particular climate, there can be a tank installed on the roof, and it would take care of storing rainwater and putting it to use, like a well. And finally the vegetation itself, the aesthetic part of the entire installation.

Depending on the type of vegetation planted, how the green roof will be used, and the care and maintenance required, the installations can be classified in two categories: extensive and intensive. Extensive roof gardens generally use grass or moss, with shallow substrata (less than 15 centimeters thick), are rarely passable, and do not need much maintenance and watering. As for intensive roof gardens, they are mainly used as leisure gardens or urban food gardens (and are therefore passable spaces), need more watering, and because the plants are heavier, require more maintenance.

The Pros

As for the advantages of vegetal roofs, they are of four kinds: environmental, energy-related, structural, and social. From an environmental angle, the local and global benefits are undeniable, even intuitive. First is improvement in air quality: vegetation on roofs facilitates absorption of greenhouse gases like nitrogen oxide and carbon dioxide, and the filtering of particles of dust and other harmful substances, cleaning up the oxygen around. Second is reduced sound impact: green roofs muffle the ambient noise of cities, which is especially important for topfloor living spaces.

Green roofs also help to battle overheating in cities, thanks to evaporation, photosynthesis, and a capacity to store heat in their own water (the plants absorb ambient heat, lessening temperature variations between day and night, and thus contributing to creating a livable microclimate).

Lastly, green roofs help regulate the hydrologic cycle by facilitating the collection and recyling, as well as the natural cleaning, of rainwater. They also thus contribute to regulating variations in air humidity.

From an energy angle, green roofs used as a ‘construction material’ are perfect insulators against cold and heat alike, doing much to reduce heat losses and gains and thus diminishing the need for energy for artificial heating and cooling.

As for structural behavior, studies carried out in this area conclude that green roofs prolong the useful life of roofs by minimizing the mechanical and physicochemical damages wrought by the impact of climate (heat, cold, rain, ultraviolet rays, wind, ozone, and gases released by industries) on the construction materials of conventional roofs, such as concrete, bricks or ceramic slabs, tiles, and watertight panels.

Although there are more pros than cons, we must not make little of some of the negative effects of installations of this kind. Green roofs can add weight to the building, so sometimes they are altogether unfeasible, or they are possible only with adequate prior reinforcement of the building’s structure, which can make the project very expensive. They also require more maintenance than the naked roof because it is necessary to prevent water from accumulating and seeping into the building’s structure. Moreover, botanical roofs are a ‘living material,’ and it is important to keep them from decomposing. Add to this the fact that the initial investment is greater than with the traditional roof, which can bring about some extra costs that are not easily borne in rehabilitation cases. Finally, humidity problems are likely to arise if the installation is executed poorly or if maintenance is inadequate.

Without a doubt, green roofs will be more and more present in our cities. As we have seen, the environmental advantages can be enormous in degree and number. This makes it desirable to make them obligatory in building codes, as they aleady are in other countries, in the same way hat codes currently in force call for energy-supplying solar panels. 

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