On the roof of the Institut de Ciència I Environmental Technology (ICTA-UAB), located on the campus of the Autonomous University of Barcelona, next to the AP-7, there is a greenhouse that, until a few weeks ago, had basils and lettuce. The leaves were collected at Christmas and distributed among the 200 building workers. Now the tomato season has started. On this green roof, which has been operating for ten years, a square meter is equivalent to 17 kilos of tomatoes.
With that data, scientists Diego Macall, agronomist and magister in agricultural economy; Xavier Gabarrell, Bachelor of Chemical Sciences and Doctor of Biotechnology; and Sergio Villamayor Tomás, a specialist in ecological economy, began to calculate. How many roofs are needed to meet the demand for tomatoes from the city? They asked. They took as reference the 437 municipal buildings, 65 hectares of roofs.
In the simulation, the mathematical models showed a production of 8,866 tons of tomatoes. Barcelona has 59,718 buildings, so 437 represent 0.64% of the total area. On average, and according to data from the Ministry of Agriculture, Fisheries and Food, the inhabitants of Barcelona consume, collectively, 28,549 tons of tomatoes a year.
“That is, using only public roofs we could supply 31% of the demand for tomatoes of the 1.6 million inhabitants of Barcelona, which, frankly, is surprising,” Macall admits. Clarifies that it is a “conservative” estimate, because in research, pending to be published in the scientific journal Citiesa production of 14 kilos per square meter was taken as a reference, three less than the average harvest in the ICTA-UAB building.
Tomatoes, lettuce, cucumber
The study focused on tomato, the cultivation of greater consumption in Barcelona, but the results are replicated for two other vegetables, lettuce and cucumber, which are also part of the Mediterranean diet. Macall explains that to obtain these yields, the roof gardens need at least a “simple plastic structure” for temperature and humidity control, and to protect birds from birds and rodents.
“The figures will change if we study in other cities in Spain. There are variables that will not be the same, such as temperature, slopes of buildings or urban morphology. But this research reveals that many cities in the country could use their roofs to produce large -scale food, ”says the researcher.
The work, he adds, seeks to make visible the potential of green roofs in a global crop reduction context due to climate change. “Food production will decrease in the coming years due to global warming, that is a fact. Therefore, we are obliged to be more creative and resilient. Cultivating on roofs is a very good urban adaptation strategy, ”says Macall.
According to the data panel of the United Nations Food and Agriculture Organization (FAO), Spanish tomato production has decreased almost 19% in a decade. In 2014, 4,888 million kilos were collected. In 2023, the figure descended to 3,968.46 million. Market variables explain part of this decrease, but climatic factors – almost chronic droughts in producing areas and extreme temperatures – have also influenced, according to this international organism.
Food production will decrease in the coming years due to global warming, that is a fact. Therefore, we are obliged to be more creative and resilient. Cultivating on roofs is a very good urban adaptation strategy
Diego Macall
– Agricultural Engineer at the Institut de Ciència I Environmental Technology
Most of the tomatoes that are consumed in Catalonia come from the greenhouses of Almería, suffocated in the last campaigns due to the lack of rainfall. Farmers are using desalinated water to water, which has increased production costs and has generated a price increase.
For Macall, the production of food on the roofs “is a business niche for the private sector, an opportunity for municipalities and a social reinsurance for citizenship, which will have a local production and lower prices,” he explains. “Local adaptation policies to climate change will be increasingly necessary. Food reduction seems still distant, but it will arrive. And the more resilient we are, the less impacts we will suffer, ”he concludes.
His colleague, Xavier Gabarrell, adds that renaturing the roofs in cities, which are planned based on concrete, has multiple benefits, such as mitigating the ‘heat island’ effect and retention of rainwater before episodes of torrential rainfall. Most green roofs in Spain are “unproductive roofs of ornamental vegetation.” Very few communities of neighbors bet on “edible varieties.”
It is very difficult to knock down buildings to make parks or green spaces. What are the surfaces that are very badly used? The roofs.
“We need the roofs not only to be green, but also recover the functions that nature has lost due to its waterproofing, such as food production. It is very difficult to knock down buildings to make parks or green spaces. What are the surfaces that are very badly used? The roofs. If we have removed a part of the soil for nature to perform its functions, because we have built on top, perhaps it is time to return the last layer of our buildings: the roofs, ”he says.
The garden garden of Manuel de Falla Street
Both Macall and Gabarrell encourage the neighbors of the big cities to build roof gardens, that this renaturalization includes the cultivation of fruits, vegetables and vegetables. “It is worth trying only for social function. When a ceiling without utility becomes food producer, a new dynamic is generated among the neighbors, a new corner of meeting and reception appears, all experiences confirm this, ”they explain.
Until 2020, the owners of the 12 homes of the Manuel de Falla 34 street building, located in the Barrià neighborhood of Barcelona, had almost no relationship. In a meeting of owners, a neighbor convinced the rest of presenting himself in the second edition of the Green Coverty Contest, a call launched by the City Council in 2017 in private ownership.
The five -story building was one of the ten winners. The works transformed a cover of 236 square meters of ceramic tile on a roof plagued with Mediterranean vegetation. Last year, the Municipal Urban Landscape Institute published a report with the “social benefits” derived from the implementation of this plan.
In the case of this building, due to the demographic characterization of their families (many children), a self -managed urban garden was built with biological vegetables for the realization of recreational and pedagogical activities.
The technicians who carried out the inspection for the report observed that the garden had connected families, which was “the axis” of renaturalization. “The main activities carried out are the care of the garden, considered the key element of the roof, and the neighborhood meetings and celebrations. In fact, it is considered that the roof has been an important improvement in the quality of life of neighbors ”, is detailed in the document.
For Gabarrell, the challenge is that the roof of Manuel de Falla street ceases to be the exception and transforms into “the rule” in the coming decades. “15 years ago, energy communities looked like a utopia. And today these communities advance and are consolidated. “What better for the owners of a building than to go from the cold and fleeting encounter in an elevator, to a warm and pleasant interaction next to the garden of a roof”?
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