Green roofs: a solution for regulating rainwater

A property operated by the Jardin sur le toit, an association which grows fruit and vegetables in the city several metres high, on green roofs, using rainwater.

"We use it directly when it falls, or via water collectors," explains Jean-Julien, 68, a volunteer with the association, whose shared garden on the roof of a gymnasium allows individuals to grow fruit and vegetables in the heart of Paris's 20th arrondissement.

This green oasis contrasts with the impermeable surfaces (tar, concrete) omnipresent in the city, preventing water from infiltrating into the ground.

Consequences: the streets are warmer, less humid and the risk of flooding increases.

In a city like Paris, the wastewater drainage network runs alongside the rainwater drainage network. In the event of severe weather, the drainage system becomes saturated and risks overflowing.

Storing water

"The green roof is above all a way of storing water" explains Hervé Andrieu, retired hydrologist, "the thicker it is, the more water we store".

Green roofs are divided into three categories. Extensive ones with a very shallow depth of 10 cm, which accommodate succulents and do not require maintenance.

Semi-intensive where you can grow grass and shrubs up to 30 cm thick.

And the intensive ones, the deepest, between 30 cm and 1 m where even trees can grow and on which one can move freely.

A green roof can absorb "50 to 70% of the annual rainfall on average for substrates of 3 to 15 cm", explains David Ramier, hydrologist at Cerema.

Once stored, the water will experience a "mini-cycle". Falling from the sky, it will be captured by the plants and the soil on the roof, then evaporates, returning to the atmosphere. This last step, evapotranspiration, allows you to feel a sensation of freshness near the roof.

Protect the building

"The fact that water is stored in the greening system is not a risk" for the roof, assures Raphaël Dupont, green roof installer, who notably participated in the construction of the Arena Porte de la Chapelle.

On the contrary, the green roof helps to better preserve the waterproofing of the roof, "provided that the work is done well".

Made of synthetic or bituminous material, these protections are generally covered with gravel or slabs on pads and need to be changed every 20 to 25 years.

"Green roofs act in two ways to better protect these surfaces, through a gravity effect and through an insulating effect, which will limit temperature differences and better preserve the integrity of the waterproofing which will be able to last a few more years," adds the roofer.

But to be able to install vegetation on your roof, you must ensure that the building can support its weight and have certified waterproofing against the risk of being pierced by roots.

“Today, many protections already have this certification, even without vegetation,” assures Raphaël.

While the cost remains similar to that of a conventional roof in the case of construction, it is higher by 20 to 38% on average, depending on the type of green roof chosen, when converting a traditional roof according to several professionals.

Financial aid exists to help with investment and is more or less significant depending on the department.

Although green roofs represent an effective solution for managing runoff water, "they are not the only solution" to fully address the problem of water management in cities, David Ramier cautions.