That day, in Nantes, the tiles cut from thick pine planks passed a watertightness test in the wind tunnel of the Scientific and Technical Building Center (CSTB), one of the few in Europe capable of testing real-size building elements to confront them with life-size weather and the effects of climate change.
"We are testing the water resistance of the roof", explains Nicolas Couillaud, project manager at the Jules Verne wind tunnel of the CSTB in Nantes, a public establishment (EPIC) which works for both industrialists and public authorities.
The test lasts 90 minutes in total, at the rate of 200 ml of water per hour, "a good shower". "No rain should come inside," he warns.
"Rain tester"
Equipped with a raincoat and boots, the "rain tester" enters the shed. No drop of water pearls, no flow on the interior face of the tiles. “This roof is rainproof,” says Mr. Couillaud.
A Polynesian industrialist requested these tests (for a fee). He wants to produce tiles from Caribbean pine, which is locally abundant and can be harvested sustainably. Its goal: to reduce imports of building materials in Polynesia and thus CO2 emissions.
"Climate change forces us to look for new solutions, to seek to promote innovative local, biosourced or recycled materials", explains Mr. Couillaud. Provided that they respect the strict standards of construction in France.
The Nantes CSTB evaluates many roofs. Other sites test facades or insulation. Still others assess low-carbon cements or concretes before they are authorized to be placed on the market.
The center works on mitigating the effects of climate change. The aim is to determine how the building sector, which alone is responsible for 18% of French greenhouse gas emissions, can participate in reducing global emissions.
Technicians are also working on adaptation to global warming, for example improving the insulation of homes against summer heat waves.
Heat islands
And there is urgency. According to a recent parliamentary report, the worsening frequency and intensity of droughts is costing insurance companies more and more in France: 1 billion euros per year since 2016, against 500 million on average between 2000 and 2015.
Clay soils settle during periods of drought and swell with humidity, a phenomenon that can cause cracks to appear on buildings with shallow foundations, typically individual houses, resulting in very expensive work.
Of the 19,4 million houses in France, 10,1 million are exposed to clay risk, including 3,1 million highly, according to the report which recommends making builders pay for cracks appearing in the 10 years following the construction of houses.
At the CSTB in Nantes, "with 2 MW of cooling power available, we can vary the temperatures and reach extremes, from -32 degrees to +55 degrees, for our tests", explains Julien Rogé, director of the center.
In terms of wind, the wind tunnel reaches up to 280 km / hour, "well beyond all the major storms we have experienced in recent years which were around 200 km / h", such as that of 1999, he raises.
In all possible climates, the center tests street lamps, street furniture, wind turbines, pergolas, photovoltaic panels, green roof terraces or simple tiles.
Besides rain, snow, hail, sand or dust storms are simulated.
For the attenuation chapter, the Nantes CSTB now plans to launch studies on urban heat islands, i.e. the reflections of heat accumulated by bitumen and concrete, which accentuate the rise in temperatures in the city in summer. It will be a question of evaluating their interaction with the vegetated islands, even the urban forests, a concept dear in particular to the town hall of Paris.