What if every roof grew food?

Explore the idea

• Established fact

  Plants need light, water and air to grow. They can grow without soil.

  Plants make food from sunlight, water and carbon dioxide; in hydroponics their roots get water and nutrients without any soil.

  Photosynthesis needs light, water and carbon dioxide; hydroponics supplies water and mineral nutrients to roots without soil.

• An idea scientists discuss

  Every roof growing food is pretend. We use real roof farms to imagine it.

  No city grows food on every roof; this thought experiment uses real rooftop farms and published studies to imagine what would change.

  City-wide outcomes are inferred from individual farms, modeled comparisons and one city-scale estimate, not observed directly.

• Established fact

  Some farms are on roofs already.

  The U.S. Department of Agriculture lists rooftop farms, community gardens and hydroponic farms as kinds of urban agriculture: growing food in and around cities.

  The USDA cites community gardens, rooftop farms, and hydroponic, aeroponic and aquaponic facilities as examples of urban agriculture; there is no single formal definition of the term.

• Established fact

  A roof must be strong enough to hold a garden.

  Any flat roof can be used for growing food, but engineers must make sure the extra weight is safe, and the garden needs railings and emergency exits.

  Any building with a flat roof can host rooftop agriculture, but engineers must confirm the added load is safe, and emergency exits and protective railings are required.

• Established fact

  Plants can grow in water with plant food, with no soil.

  In hydroponics, plants grow without soil. Their roots sit in water that has minerals dissolved in it and is kept full of oxygen.

  Hydroponic roots are bathed in highly oxygenated, nutrient-rich water, with or without an inert medium such as gravel or perlite (New Mexico State University, 2022).

• Established fact

  Lettuce can grow in a small box of soil.

  A hydroponic system uses no soil, but it needs lots of water. Soil boxes need only about 20 centimeters (8 inches) of soil for small plants such as lettuce.

  Resource needs vary with the system: a hydroponic system uses zero soil but lots of water, while soil containers need only about 20 cm of soil for small crops such as lettuce.

• Established fact

  A water garden can grow lots of lettuce in a small space.

  Scientists calculated that a hydroponic greenhouse in Arizona could grow about 11 times more lettuce on each square meter in a year than a field.

  Barbosa and colleagues (2015) calculated hydroponic lettuce yields of about 41 kg per square meter per year versus 3.9 kg in fields: 11 times higher.

• An idea scientists discuss

  Water gardens might help roofs grow more food in a small space.

  Because hydroponics needs no soil and grows a lot in a small space, it might suit many roofs, if the building can carry the water and equipment.

  High yields per area might make hydroponics attractive for rooftops, but the water, structure and energy it needs would limit which roofs could use it.

• Established fact

  Plants use sunlight, water and air to make their food.

  In photosynthesis, plants use sunlight, water and carbon dioxide to make sugar for energy, and they give off oxygen.

  Photosynthesis converts carbon dioxide and water into glucose and oxygen, using light absorbed by chlorophyll in the chloroplasts.

• Established fact

  One rooftop greenhouse catches rain to water its plants.

  A company with rooftop greenhouses in Montreal, Canada, says they capture rainwater and reuse all of their watering water.

  The Montreal company's website (2026) says its rooftop greenhouses capture rain and meltwater and recirculate all irrigation water and nutrients in a closed loop.

• Established fact

  Scientists worked out that a water garden would use less water, but more energy.

  Scientists calculated that hydroponic lettuce in an Arizona greenhouse would use about 20 liters of water per kilogram, compared with 250 liters in fields, but about 82 times more energy.

  Modeled hydroponic lettuce: 20 ± 3.8 L/kg water and 90,000 ± 11,000 kJ/kg energy per year, versus 250 ± 25 L/kg and 1,100 ± 75 kJ/kg for field lettuce (Barbosa et al., 2015). Per square meter, water use was similar; per kilogram, hydroponics needed about 13 times less.

• Established fact

  Roofs with plants stay cooler.

  The EPA reports that roofs covered with plants can be up to 56 °F cooler on the surface than ordinary roofs, and they reduce and filter the rainwater running off.

  Green roofs can have surface temperatures 56 °F lower than conventional roofs, lower nearby air temperatures by up to 20 °F, and reduce and filter stormwater runoff (EPA).

• An idea scientists discuss

  Roof farms might save water but need more energy.

  If every roof grew food, cities might save water but need more energy for lamps, pumps, heating and cooling, depending on how the farms were built.

  City-wide rooftop farming might trade water savings for higher energy demand; the balance would depend on climate, crop, system and where the electricity came from.

• Established fact

  Roofs in one city could grow lots of vegetables, scientists think.

  A 2014 study estimated that if Bologna, Italy, grew vegetables on its flat roofs, they could produce about 77 percent of the vegetables its people need.

  Orsini and colleagues (Food Security, 2014) estimated that rooftop gardens on Bologna's flat roofs and terraces could provide more than 12,000 t of vegetables a year, 77 percent of residents' requirements.

• Established fact

  Food grown nearby does not need a long truck ride.

  Researchers who study rooftop farms say growing food in the city can reduce the pollution and cost of trucking food in from the countryside.

  Rooftop agriculture researchers note that local production can reduce the pollution and expense of trucking food in from the countryside (Appolloni and Gianquinto, 2022).

• Established fact

  Food grown in cities is not always kinder to the planet. It depends how it is grown.

  A 2024 study of 73 city farms and gardens found their food had, on average, about six times the carbon footprint of food from ordinary farms, but tomatoes did better.

  Hawes et al. (Nature Cities, 2024): 420 g versus 70 g CO₂e per serving on average across 73 urban agriculture sites; tomatoes and 25 percent of individually managed gardens outperformed conventional agriculture.

• An idea scientists discuss

  Roofs might grow lots of vegetables, but farms would still be needed.

  Even if every roof grew food, roofs might supply many of a city's vegetables but probably not all of its food, so countryside farms would still matter.

  Rooftops might cover a large share of a city's vegetable demand, as the Bologna estimate suggests, but that study covered vegetables only; staple foods such as grains would probably still come from conventional farms, so rooftop farming would add to them, not replace them.