'Darling, fetch some broccoli from the roof, would you?' This could be a reality in the future. Ecologist Pengxuan Xie investigated whether rooftop farming can contribute to food production in cities. This is important now traditional food supply chains are vulnerable to disruptions.
Extreme weather, geopolitical tensions and logistics bottlenecks are making food chains increasingly vulnerable. Rooftop vegetable gardens can strengthen part of that chain, says Xie, who will defend his thesis on 8 April. They would provide fresh produce close to home and increase food security. At the same time, they would also help biodiversity, combat urban heat islands and collect rainwater.
369 hectares of suitable rooftops
Xie wanted to find out how many rooftops are suitable for growing vegetables and how they could contribute to food production. He began his research in Amsterdam. Using a Geographic Information System (GIS), which allows you to view and analyse data on a map, he looked at how many roofs would be suitable for growing vegetables.
He considered factors including roof slope, building height, rooftop size and unoccupied area on the roof, and concluded that Amsterdam has 396 hectares of suitable roofs: 15.8 per cent of the total roof area.
Simulation model
Using a simulation model, Xie then investigated the urban food self-sufficiency (the ratio between food production and food demand) from these roofs. By carefully determining which crops would be grown where, he calculated that the roofs could provide 12.4 per cent of Amsterdam's vegetable consumption. Vegetable consumption is based on the World Health Organization's recommended daily intake of fruit and vegetables (400 grammes).
Suitable crops include onions, lettuce, leeks, cauliflower, cabbage and broccoli. 'I deliberately chose vegetables that are eaten locally,' says Xie. 'And the soil layer on a roof is usually no thicker than 20 centimetres, which makes shallow-rooted crops particularly suited to rooftop farming.'
Different results for different cities
After examining 626 other European cities, Xie discovered that food-producing roofs would not yield the same results everywhere. 'The urban food self-sufficiency depends on the local climate, population density, the size of the city and how many suitable roofs there are,' he explains. On average, green roofs - with additional watering - can provide between 30 and 40 per cent of the vegetables needed in a European city.
Gap between simulations and reality
Xie notes that there is still a gap between his simulations and real-world practice. For instance, clear rules and policies on the large-scale implementation of green roofs are often lacking. Nor is there much information about how much weight roofs can bear, when this is crucial for safety. A layer of soil and vegetation makes a roof heavier, and not all buildings are suitable for this.
But there are already some good examples, says Xie. In Basel, the targets set by the European Federation of Green Roofs and Walls - such as 5 square metres of green roof per inhabitant by 2030 - have been achieved to some extent. 'That shows it is possible, but that further steps are needed before green roofs become a reality on a large scale.
These steps would include testing the simulation in pilot projects on real rooftops, conducting further research on technical constraints such as roof load-bearing capacity and informing policymakers and urban planners of the findings. This would help bridge the gap between simulation and real-world implementation.'
