For the most part, Europe has been steadily advancing towards a smarter, more efficient power grid, heavily based on renewable energy as opposed to coal or nuclear power plants. There’s enough reliance on solar power, in fact, that solar eclipses have the potential to cause gigawatt-scale power fluctuations. France, however, is still primarily dependent on nuclear power, which provided over 80 percent of its power in 2012. In an effort to rebalance its energy mix, the French parliament has approved a law mandating that all new commercial buildings feature roofs that are at least partially covered in either solar panels or plants.
This new law applies only to new construction of buildings in commercial zones in France. Originally, environmental groups had lobbied for the law to apply to all new buildings, and for the requirement to be total rooftop coverage with greenery, but it’s been scaled back to allow partial coverage or solar panels, whichever the building owner prefers.
The benefits of solar panels are straightforward: when the sun is out, they generate electricity, which can be either used on-site or fed back into the grid. Green roofs, which are covered in dirt and living plants, are a little bit more complicated. They’re much more difficult to construct, since you usually have to plan for them in the design phase of your building. All those plants, the soil they live in, and the water that the system retains weighs a lot, with the most extensive, self-sustaining green roof ecosystems weighing up to 700 kg/m2. Without planning for that sort of structural overhead over your head, it’s sometimes impossible to greenify a roof of a building that wasn’t designed for it, an designing for it from the beginning can as much as double the roof’s cost.
In one respect, green roofs are similar to rooftop solar panels. There’s a significant up-front expense, but over time, there are also significant savings that (should) more than cover the cost of construction and upkeep. For example, a green roof protects against structural damage, and should last between two and three times as long as a conventional roof. Green roofs act as insulators, reducing summer cooling and winter heating needs by about 25 percent.
On a national scale, both green roofs and rooftop solar panels tackle several problems simultaneously. By either saving energy or generating electricity, they help to reduce power demands and stabilize the national grid, especially when demand peaks due to high temperatures. Green roofs also absorb water, helping minimize runoff during heavy rains. And when it’s hot out, they can temper the urban heat island effect, where all the dark and impermeable surfaces in cities (like streets, sidewalks, and rooftops) cause localized temperature spikes of between 1 and 5 degrees C. Furthermore, green roofs can help deal with air pollution, they provide an urban habitat for birds, and they’re also a very pleasant place for humans to enjoy, especially if they're used to grow plants that can be made into salads.
Since 2009, Toronto Canada has had a similar mandatory green roof law in place, requiring green roofs on new buildings. Preliminary studies suggested that the city could save hundreds of millions of dollars in energy costs while reducing ambient temperatures by several degrees Celsius. Changes like these don't happen overnight on the scale of a city, and it may take decades for a nation the size of France to achieve measurable benefits, although on a per building basis, the benefits are immediate, and continuous. France is making an investment in energy independence, efficiency, and stability, and when it eventually pays off, the rest of us will be regretting not having started greening our roofs sooner.
Evan Ackerman is a senior editor at IEEE Spectrum. Since 2007, he has written over 6,000 articles on robotics and technology. He has a degree in Martian geology and is excellent at playing bagpipes.