The advantages that the installation of a green roof brings suggest that this is a trend that turns towards overcrowding; however, ignorance on the subject does not allow this practice to spread as expected.
by María Cecilia Hernández Ocampo
Although it was born in the 60s, the trend of covering the roofs of buildings with vegetation has been taking flight with greater force in the last decade.
Green roofs offer great benefits for the operation of the building, for example, in issues such as the control of the microclimatization of the same, the saving of energy and money, as well as the valuable contribution it offers to the environment and the improvement of the architectural appearance of the property.
According to the expert in natural construction and bioconstruction, the German Gernot Minke, this type of roofs have many characteristics that bring excellent benefits to a property, one of the most important is its useful life: while a traditional roof has a duration of between 20 and 25 years, a green roof can have a useful life of 100 years.
Among the conditions that must be met for a green roof to be truly effective, there are three requirements that must be present. Edgardo Suárez, architect and member of the Nuestra Córdoba Network in Argentina, an association created to think about the city in environmental terms, indicates that the roof must be able to drain water, prevent the roots from affecting the slab and not overload the structure to the point of damaging it.
Being living plant species, what may be, perhaps, the only disadvantage of this type of roof is presented: they must be maintained as if it were a garden; mow the lawn and, in some cases, water it.
However, the advantages are many more and contain in themselves a burden of invaluable benefits.
Individual and plural advantages
The expert Federico Cena, owner of the company En Altura, specialized in this type of facilities, comments that the advantages are counted from the particular to the general; that is, from benefits for each property that has one of these covers, to benefits for the area where the installation is located.
Among the main values that have to do specifically with the structure that the green roof has, is the regulation of temperature. According to Cena, through water evaporation, photosynthesis and the ability to store heat in its own water, the plant extracts heat from the environment, producing cooling. At night through condensation recovers heat. It therefore reduces thermal oscillations in the day and night.
The expert adds that a green roof also protects and increases the life of the waterproof membrane. "The duration of conventional roofs (asphalt membrane, sheet metal, tiles, mortar, etc.) is influenced by climatic effects, temperature variations, wind, hail, UV rays. On the other hand, with a living roof, the water-repellent insulation is completely protected."
Some additional advantages are:
- Thermal insulation: vegetation produces insulation by the air contained in plants and soil, also reduces the effect of wind.
- Increased thermal inertia: The earth provides great inertia keeping the temperature cool at night during the day and vice versa.
- Acoustic insulation: the floor has a great capacity to reduce ambient noise (up to 40db) and also the noise of footsteps on the ceiling.
- Aesthetic and psychological effects: green sight prevents depressive states and encourages the spirit.
For the common benefit
As far as the positive effects on a collective scale are concerned, living roofs purify the air by producing oxygen and reducing CO2. Clean the air as the plants filter the dust.
"This type of installation reduces the swirling effect of dust, the surface of a concrete roof can reach 80 degrees Celsius producing an upward movement of the air that moves the dirt particles that we then breathe, when there is vegetation this effect does not occur because the temperature remains at values lower than the environment", Cena explains.
In this way, the expert continues, the temperature is regulated and the heat island effect is avoided, that is, the concentration of high temperatures in certain areas, for example, in the center of cities where the difference in heat sensation can be between four and 11 degrees Celsius higher than in peripheral areas.
Also, plants have the ability to regulate humidity, in dry air seasons they can evaporate water and on wet days they capture water in the form of dew. They can also retain 70% of rainwater during the summer, and 40% during the winter, which translates into a significant decrease in flooding problems.
Recently the National Research Council of Canada reported that green roofs have the property of reducing by more than 75% the energy that is used in a house of about 400 square meters.
Supporting this result, the Canada Environment Agency stated that if at least 6% of Toronto's roofs were transformed into green surfaces, that is, if 6.5 million square meters of green terraces were planted, it would be possible to reduce the temperature of the entire city by two degrees during the summer, and consequently energy consumption could be lowered by means of air conditioning devices.
On the other hand, the invited experts assure that the plants and the layers that this system needs serve as acoustic insulation. The layer corresponding to the growing medium blocks the low-frequency waves, while the plants block the high-frequency waves. A green roof with a growing medium 12 centimeters thick can reduce sound by 40 decibels, and one with 20 centimeters will reduce sound by 46 to 50 decibels.
According to Edgardo Suárez, although the application of this type of roof is increasing, it is still not massive enough as would be necessary to achieve the desired environmental impact.
Components and recommendations
The web portal of the Colombian Association of Architects, breaks down in detail the characteristics and recommendations of the main components that a green roof should have.
- The waterproofing membrane can be made from bituminous materials, with a certain recycled content, such as products from polyethylene or synthetic rubber EPDM. The use of PVC, due to its environmental impact, would not be recommended. It is advisable that they are monolithic membranes, so that there are no joints where it is possible for leaks to occur when there are conditions of constant humidity or even stagnant water.
Sometimes waterproof sheets incorporate an anti-root treatment that can be a membrane of high density polyethylene or modified asphalt reinforced with polyester and ceramic granules.
- The insulation is usually made of extruded polystyrene, which in addition to its insulating properties has a great resistance to water penetration.
- Water retention and drainage components are specially designed systems to allow water retention for times when it does not rain, while ensuring good drainage and aeration, so as to ensure a balance between water and air. The system can be made with recycled polypropylene fibers or with recycled polyethylene panels.
- A filter prevents the soil from falling and plugging the drainage layer. The filter can be a geotextile material made from polyester fibers.
- The substrate has little thickness, and is a growth medium designed to achieve optimal water retention, permeability, aeration capacity and erosion resistance, as well as being the support of vegetation throughout the surface of the roof. It must provide the essential nutrients to the plants (nitrogen, phosphorus, calcium, magnesium ...) and have a correct structure and pH.
In extensive roofs, a proportion of 75% mineral soil and 25% organic soil is recommended, and in intensive ones 55% mineral soil and 45% organic soil.
- The plants chosen for the roof should be resistant to the climatic conditions of the place, act as upholstery and should not need much maintenance, especially in extensive systems. It is convenient that they are perennial to maintain the green cover all year round. In general, annuals and perennials (which remain part of the year underground) are not favorable, because they must reproduce every year, it would be subject every year to whether there is a good germination or not, and for half a year nothing would grow on the cover.
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