The zinc covering the roofs of central Paris has given the French capital's skyline its distinctive grey hue for almost two centuries.

Now the roofs and the workers who create and care for them are aiming to enter a select heritage club to showcase a profession adapting to the challenges of climate change.

The French culture ministry has chosen the zinc roofers as the country's entry for the UNESCO list of Intangible Cultural Heritage to be decided at the UN body's session in Paraguayan capital Asuncion in December.

The craftsmanship of roofers and other ornamentalists who have sculpted the capital's skyline will be among 67 candidacies vying to join other iconic heritage sites such as India's Taj Mahal.

According to the city's urbanism agency Apur, Paris has 128,000 roofs covering a surface area of 32 million square metres, of which 21.4 million are of the traditional zinc-covered variety.

"Paris seen from above, it's obvious you're not in another city," said an enthused Delphine Burkli, mayor of the capital's ninth district.

Burkli helped initiate the French bid and first proposed in 2014 to add the roofs to UNESCO's heritage list.

But the plan has since changed as it is "very complicated", said Gilles Mermet, coordinator of the bid.

The campaign to etch the roofs into the prestigious books of world heritage stumbled when Paris town hall withdrew its support. 

Mermet said it was "afraid" of "no longer being able to build in Paris without the agreement of UNESCO".

"In the end, it was more interesting to showcase the profession itself" -- which struggles to recruit -- more than the roofs as such, to protect the beauty of the urban landscape, he added.

Feeling the heat

Every morning, Paris faces a shortage of about 500 roofers to complete the work needed, according to Meriadec Aulanier of the trade union bringing together companies in the plumbing and climate engineering industries.

The French candidacy aims to encourage thinking about the future of the city and a craft forced to adapt in the face of climate change, adds Burkli.

The zinc the covers almost 80 percent of Parisian roofs has come under criticism for its role in overheating buildings.

Darker roofs absorb more energy from sunrays -- and that is bad news when summer heatwaves are becoming longer, more frequent and more intense as the planet warms up.

An Apur study in 2022 found that 42 percent of roofs in Paris had a weak reflection capacity, meaning they absorbed more heat. 

These insulate poorly and "contribute to the rise in temperatures in homes", according to an assessment carried out by the Council of Paris in 2022 titled "Paris at 50C".

Lack of insulation

Roofscapes, a French start-up launched at the Massachusetts Institute of Technology, put zinc under the spotlight in a test during the summer of 2023.

The experiment used an eight-storey building covered in zinc with a technical void in the attic that acted as a thermic buffer zone.

The study showed that a zinc roof heated the surrounding air during the day, up to 10 degrees Celsius above the home's temperature and 7C warmer than the day's weather.

Nightfall brought little relief: the homes under the roofs heated 6C more than the temperatures on the lower floors.

"At night, the zinc at the surface cools down. On the other hand, the heat continues to penetrate inside and that's where there's overheating in the homes," explained Eytan Levi, an architect and co-founder of Roofscapes.

But Mermet is adamant that the zinc itself is not the problem, rather the absence of insulation in old buildings.

Training schools now teach budding roofers to pose the insulation, and the old zinc is recycled, he said.

The "Paris at 50C" study raised the possibility of repainting the existing zinc roofs with a lighter-coloured paint to reflect the heat without harming their heritage value.

Mermet, though, was unimpressed. Such an idea has "no interest" for him because "it will increase the price of restoring the roofs".

"With the rain, your paint will end up burning and going to the sea."