Understanding green roofs and their uses in energy retrofits
A green roof is an assembly laid over waterproofing, with drainage, substrate and plants. In an energy retrofit, it doesn't replace insulation, but it can improve summer comfort, protect the waterproofing from UV, and buffer part of the rainwater. It suits flat roofs and low-slope roofs best, after checking loads and upstands.
The different systems: extensive, semi-intensive, intensive
Extensive: shallow depth, sedums, limited maintenance, reduced weight. Semi-intensive: more substrate, diverse vegetation, more regular watering and maintenance. Intensive: significant depth, shrubs or even an accessible garden, high loads, near-systematic irrigation. The choice depends on the load-bearing structure, maintenance access and objectives, in particular summer comfort and stormwater management.
Boosting insulation with a green roof: what you can actually gain
Summer comfort, thermal mass and winter limits: points of attention on site
A green roof mainly delivers summer comfort. The substrate and vegetation act as a buffer. They limit heating of the waterproofing, delay heat peaks and reduce the thermal swing under the roof. The result is a less stifling loft space and a membrane better protected against thermal shock.
In winter, the insulation gain remains modest. It depends heavily on the thickness and moisture content of the substrate. On site, secure the essentials first: continuity of the insulation, a suitable vapour barrier, waterproofing upstands, root barrier, drainage and water evacuation. Also check permanent loads, and plan ahead for maintenance access, especially in renovation.
Stormwater management: retention, evacuation and protecting the waterproofing
The key layers: drainage, filter, water reservoir and waterproofing upstands
On a green roof, water must be retained a little, then guided without hesitation toward the drains. The drainage-plus-overflow pairing ensures good flow even during heavy downpours. Also plan sterile gravel zones around outlets, roof lights and penetrations to keep control points visible.
The most common build-up on a warm roof: a drainage layer (mats or dimpled panels) to create channels. Filter geotextile to stop the substrate from clogging. A retention layer (mats, cells) to store a few litres per m² and limit runoff peaks. Then comes the substrate and vegetation.
The sensitive point remains the waterproofing. Protect it with a suitable root barrier and take care with the waterproofing upstands. Keep these upstands clear, with no substrate in direct contact, and regularly check that the stormwater outlets are clean.
Biodiversity and durability: plants, maintenance and long-term performance
Choosing plants based on exposure, slope and climate
On a green roof, durability starts with the right plant choice. Full sun and wind: favour sedums, low-growing perennials and understated grasses. In shade or a humid climate, mix ferns, sedges and tolerant ground covers. For a marked slope, choose species with dense root systems and plan an anti-slip device.
To keep stable coverage over time, rely on local species and a diversity of blooms. This feeds pollinators and limits bare patches. For the first 12 months, establishment watering can be useful. After that, aim for simple annual maintenance: weeding, checking the drains, removing woody growth and checking the edges. A well-monitored roof ages like good insulation. It settles a little, but stays effective.
- Adapt the substrate. 6 to 12 cm for extensive systems. More if you want taller perennials.
- Avoid invasive species and rich fertilisers, which favour weeds.
2026 framework: DTU, insurance, safety and available aid depending on the project
Compliance, RGE and responsibilities: what you must define with the client
In 2026, a green roof is managed like a complete roofing package. You must define the execution framework (waterproofing DTU, the system's technical approval, manufacturer's requirements) and the exact scope. Who does the waterproofing, insulation, protection, planting. Everything must be in writing, otherwise responsibilities get mixed up.
On the insurance side, check before signing that your ten-year liability cover (décennale) properly covers the declared activity, in particular flat-roof waterproofing and work at height. For safety, plan access and collective protections. Compliant scaffolding, guardrails, lifelines, PPE, and instructions for coordinating with other trades.
- Aid. MaPrimeRénov' and CEE mainly fund insulation, not the planted assembly. The client must understand what is eligible.
- RGE. To access the aid, the RGE certification must match the work carried out. Keep evidence — photos, technical data sheets, CEE sworn statement.
Key figures
50 to 80%
Stormwater retention
-3 to -5°C
Overheating reduction
0.10 to 0.15 m²·K/W
R of 10 cm substrate
Frequently asked questions
Have a structural engineering firm validate load-bearing capacity, factoring in permanent loads "at water saturation" (assembly + retained water + snow) and live loads if the roof is accessible. For the rules of the trade, refer to the waterproofing DTUs (series 43 depending on the substrate) and the professional rules for green roofs and terraces (ADIVET) for the build-up, sterile zones and upstands.
Louis Meneteau
CPO of Argile
