Strictly speaking it is not a "couverture" which is sloped, but "étanchéité" (flat roofing). I can't see a garden on a pitched roof!

IMO "roof garden" conveys the message. Anyone who knows about these things will know it requires a concrete slab of suitable strength to support the depth of earth required, the water that earth can contain, the weight of the trees, etc.

I did a long translation earlier this year on flat roofs, and "toitures végétalisées" of different kinds took up a dozen pages. Some quotes:

Toiture végétalisée Eco-roof, turf roof, grass roof [Web], planted roof, green roof [Barbour91p472]; can be intensive (higher plants) or extensive (grasses; may be a brown roof), sedum roof

• Roof gardens: flat roofs covered with earth (at least 30 cm deep) and plants and shrubs (intensive green roofs). Zero fall is permitted; if the roof has walkways, the 1% fall required for water runoff can be obtained with the walkway material.

• Eco-roofs: flat roofs covered with earth (up to 10 cm deep) and grass or ground-cover plants (extensive green roofs). Falls can range from 0 to 25%.

 Roof gardens and eco-roofs
• Roof gardens
 Roof gardens generally receive foot traffic and look like traditional gardens.
 They can be built only on concrete roof slabs. The depth of soil can vary from 0.30 to 1.00 m.

• Eco-roofs
 Eco-roofs are not trafficable. Access is restricted to the purposes of maintenance of the waterproofing and plants.
 Plants are chosen to require less maintenance and watering than roof gardens. They are hardy ground-cover plants (25 cm high at most) with low nutritional requirements that reproduce and spread well. Mixing several carefully selected varieties achieves a multicoloured effect that changes with the seasons.
 The soil depth is small (3 to 20 cm) and therefore implies low imposed loads on the structure (115 to 135 kg/m²). Eco-roofs can therefore be built on concrete, steel or wooden loadbearing members.
• The special features of roof gardens and eco-roofs are as follows
(see illustrations overleaf).
 A waterproofing system treated with a root-repellent; the product's data sheet should be consulted.
 A drain layer conveying water to outlets and enabling roots to breathe; it consists of expanded clay aggregate, pozzolane, pebbles, gravel, or sheets of ribbed cellular polystyrene; it is between 6 and 10 cm thick.
 A filter layer holding back fines of the topsoil to prevent them clogging the drainage layer; it must be watertight and rotproof, and resist tearing and punching (perforation); it usually consists of layers of rockwool or non-woven synthetics.
 A layer of soil, between 3 and 20 cm deep for eco-roofs and 30 cm and 1 m deep for roof gardens.
 There should be a sterile zone or edging about 40 cm wide around the perimeter of the roof to facilitate inspection of upstands, rainwater drainage, and the height of upstands.
 This sterile zone must be separated from the eco-roof area by perforated metal strips or concrete or brick kerbs holding back the earth but allowing water to drain out.