Turning Notre Dame Cathedral green is an emerging trend in the French government's competition to design a new roof for the fire-damaged historic structure. First, Paris–based Studio NAB proposed replacing the damaged roof with a massive greenhouse, and now fellow Parisian and self-described “archibiotect” Vincent Callebaut has revealed a plan to turn the roof into a flourishing urban farm.

Callebaut's firm, Vincent Callebaut Architects, specializes in conceptual ecological buildings that fight against global warming. The firm's past project proposals include vertical farms for New York City's Roosevelt Island and floating cities for climate refugees.

Applying this ethos to his city’s iconic cathedral, Callebaut has developed a spire covered with crystal solar panels that would serve as a “new symbol of spiritual aspiration” while turning Notre Dame into an energy-positive building.

The new spire could harvest solar energy and store it in hydrogen fuel cells, ensuring that the cathedral has power to share. Underneath the spire, an urban farm could grow food through aquaponics and permaculture to feed Paris’s homeless. The amount of food would be so plentiful, posits Callebaut, that the cathedral could hold a weekly farmers’ market.

In his proposed design, four gables mimic the original geometry of the 32-foot-high attic and converge over 55-degree pitched roofs that gradually stretch to create a vertical spire. The gables would be constructed with cross-laminated timber pre-stressed with carbon fiber slats, to minimize the amount of material used and maintain a low carbon footprint.

Callebaut also proposes honoring the cathedral's past architecture by incorporating the original statues (which survived the fire as they had been removed for cleaning) in the new roof, as well as crowning the spire with the famous rooster retrieved from the rubble. “In this position, it should remain the ‘spiritual lightning rod’ and protector of the faithful,” says Callebaut.