The green roof
First introduced in France in the 90s, green roofs now play an important role in thearchitecture, urban planning and development of our cities.
As the building’s 5th façade, the roof, which had long been left to technical equipment, has become an increasingly valued space.
In addition to its aesthetic appeal, rooftop vegetation helps to create islands of coolness, by limiting the impact of solar radiation on the building envelope. It helps to reconstitute the “green grid” within cities, fostering biodiversity and contributing to the “blue grid” through rainwater management.
Future-proof solutions for your buildings
Get inspired by the green roofs we’ve already built for our customers
There are a wide variety of possibilities for green roofs. Whether simply for insulation or access, they can also provide services such as vegetable gardening or rooftop gardening. Our expertise lies in guiding you towards the most appropriate solution, based on technical and climatic constraints, your wishes and your budget.
What is a green roof?
A green roof, also known as a vegetated roof, green roof, eco-roof or green roof, is a green installation made up of materials and plants installed on top of a building. The vegetation is designed to be self-sufficient, thanks to a selection of plants capable of developing in a stable ecosystem. Green roofs are used in particular to combat the heat islands that develop in cities. Green roofs are part of a sustainable development approach, offering natural insulation in urban areas.
The elements of a green roof
Delimited by a separation device and a sterile zone, the green roof is made up of 8 superimposed layers.
The choice of vegetation complex must be adapted to the load the building’s load-bearing element can support.
The green roof can be installed on any load-bearing substrate (concrete, TAN, wood). Drainage evacuates excess rainwater into storm drainage systems. The filter prevents clogging between the substrate and the drainage layer.
Layers 4 to 8 form the greening complex. The roof must be equipped with a water supply sized according to the planted surface area, to avoid excessive plant dieback in the event of prolonged drought.
There are 3 vegetation systems
1. Extensive green roof system
A green roof with an extensive solution is, by definition, a thin, light, natural-looking plant mat that can be installed on all types of elements: concrete supports, ribbed sheet steel (TAN), wood and wood products. Extensive systems integrate a wide variety of flowering, decorative, aromatic or medicinal perennials right from the outset, creating an elaborate, landscaped plant palette.
The advantage of this system over an intensive or semi-intensive roof is its reduced weight and limited installation costs, which means it can be used on new projects as well as in rehabilitation (renovation and existing roofs).
Another advantage, and by no means the least, is its low maintenance. Although watering of extensive roofs is not mandatory in all regions, it is nevertheless recommended for best results.
2. Green roof semi-intensive system
The semi-intensive green roofing technique makes it possible to create a landscaped look on a terrace within the weight constraints of a traditional terrace. A semi-intensive vegetated roof can support a load of up to 500 kg per square metre. The roof slope should not be too steep, ≤ 20%.
Designed for non-accessible roofs, the semi-intensive system requires a medium-thick cultivation layer of around 15 to 30 cm. It allows for a wider range of plants, including perennials, small woody plants and shrubs, requiring essential watering and moderate but regular maintenance.
Depending on the choice of plants and the region, it may be necessary to install a drip irrigation system.
3. Intensive green roof system
Intensive green roofing or garden terrace roofing involves transposing the elements of a traditional garden from the ground to the roof. Intensive vegetation consists of a very thick layer of substrate (30 cm or more), with large plants, lawns, shrubs and trees. Watering is essential to ensure the longevity of the vegetation.
Heavy overloading (in excess of 500 kg/m²), substrate or lightened topsoil with a high-performance drainage system must be considered at the design stage of the building, when calculating the foundations and structure. This intensive vegetation, governed by standard NF P 84-204 (ref. DTU 43.1), can only be installed on concrete substrates with a slope of less than 5%.
Unlike extensive or semi-intensive vegetation, an intensive system is accessible. The roof can be used for recreational purposes (growing fruit and vegetables, landscaping, etc.). Maintenance is identical to that of a ground-based garden, to which are added the particular constraints of waste disposal.
Comparison of different green roofs
| Criteria | Extensive vegetation | Semi-extensive vegetation | Intensive vegetation (roof garden) |
| Load-bearing element |    |
|
Maximum 5% slope |
| Choice of vegetation | Sedums, perennials | Perennials, small shrubs, lawns | Shrubs, trees, lawns |
| Substrate thickness (cm) | 4 à 15 | 12 à 30 | 30 and more |
| Weight of complete system (Kg/m², CME weight) | 60 à 180 | 180 à 500 | 500 à 2000 |
| Watering | No* | Yes | Yes |
| Maintenance |  |
|
|
| Cost of roofing | € | €€€ | €€€€ |
| Accessibility | No | Limited | Yes |
The different planting techniques Le Prieuré
HYDROPACK®
The ready-to-install “all-in-one” greening complex
It comprises all the layers of a vegetation system (drain, filter, substrate and plants) grouped together in a cassette (also known as a slab or tray). Very easy to install, the pre-vegetated tray is a quick and easy solution! Le Prieuré designed and developed the 1st pre-cultivated planter with water reserve in 2005: the HYDROPACK® planter. To date, it’s the only all-in-one pre-cultivated module to have received TECHNICAL ADVICE. It is suitable for flat roofs and pitched roofs up to 200%.
L.P. MAT
Pre-cultivated vegetation complex with sedum matting
The matting system consists of a field-grown roller, which is unrolled and laid directly onto the substrate layer and the usual complex layers (drain, filter). Pre-cultivated mats enable immediate roof greening and almost year-round installation. Le Prieuré grows pre-vegetated mats on its nursery in the Centre region of France. L.P. MAT mats are suitable for flat roofs and pitched roofs up to 100%.
L.P. MOTTES
The planting complex with plugs or cups
Planting rootballs or buckets on roofs can diversify the green roof’s plant palette. The installation of this greening complex is only possible in spring or autumn, to optimize plant rooting in the substrate. Plant coverage reaches 80% after 12 to 24 months. Le Prieuré grows L.P. MOTTES sedums and perennials at its nursery in the Centre region of France.
L.P. FRAGMENTS
The vegetation complex with fragment sowing
The vegetation complex with fragment sowing is particularly suitable for large-scale projects (over 1000 m²). Easy and quick to install, the vegetation cover rate reaches 80% after a period of 18 to 36 months. Le Prieuré grows L.P. FRAGMENTS sedum and perennial fragments at its nursery in the Centre region of France.
Why integrate a green roof into your real estate project?
Rooftop greening offers many advantages, both in terms of sustainable development and in terms of protecting biodiversity and the urban environment.
Rainwater management
The increasing urbanization of cities is creating ever larger waterproofed surfaces, which do not absorb rainwater. Green roofs make it possible to reduce and regulate the quantity of rainwater reaching a building and then discharged into the public drainage system. Part of the rainwater is absorbed by the vegetation complex (substrate and vegetation) until it reaches saturation point, and is then consumed by the plants through evapotranspiration. Once saturated, the vegetation complex slows down the evacuation of water into the drainage system. The green roof absorbs rainfall peaks (“delay effect”), thus limiting sewer clogging during violent storms.
Thermal comfort
The various layers of the green roof complex (drainage, filter, substrate and vegetation) provide an additional layer of thermal insulation on the roof, protecting the building from the cold in winter and combating the urban heat island effect in summer. The evapotranspiration, shading and reflection mechanisms of the plants making up the green roof can reduce the temperature of a building in hot weather.
Environment Canada’s 2004 study of Toronto showed that greening 6.5 km2 of the city’s roofs would :
- lower room temperature by 1 to 2°C in the hottest periods
- eliminating 2.18 tonnes of greenhouse gases and 30 tonnes of greenhouse gases per year.
Acoustic comfort
The plant complex’s layers of substrate and vegetation dampen impact noise (rain, hailstones, etc.) and reduce the perceived volume of airborne noise.
Extends the life of roofs
By protecting the waterproofing membrane against thermal shock and aggression (UV, punching), green roofs help extend the life of roofs.
Improves quality of life for city dwellers
A veritable “green lung” in the city, the green roof helps combat pollution: plants release oxygen, absorb carbon dioxide and bind dust in the air. Bringing nature back into the urban environment also means that buildings blend in better with their surroundings: the city is more beautiful and the living environment more pleasant for city-dwellers.
Promotes biodiversity
By reintegrating green spaces into the urban environment, green roofs restore ecological functions degraded by urbanization. A layer of substrate, diversified planting adapted to the local climate and the creation of refuge zones encourage the return and development of urban flora and fauna.
Do you still have questions about green roofs?
Is a green roof suitable for all climates and orientations?
In France, sedums grow wild in the most diverse regions and climates: seashores, mountains, southern regions. They can therefore be used for roof greening in a wide variety of contexts.
However, it is imperative to adapt the vegetation complex (drainage, substrate and vegetation) to each location and each solution. That’s why a personalized study must be carried out for each project. This will enable us to choose the right vegetation complex, and thus highlight the precautions that must be taken,…
Does a green roof always have to be installed on a concrete slab?
The overload of a green roof generally varies from 75kg/m² to several tonnes (when saturated with water). These figures vary according to the location of the project, the general context (slope, rainwater management, etc.) and the green roofing complex installed.
The solutions can be adapted to suit the load-bearing element. Our solutions are also adaptable to timber frame and TAN.
We recommend that you consult our engineering office or sales department for the best advice.
Is it necessary to water a green roof?
Plants selected for extensive to semi-intensive green roofs generally live in poor environments (rocky soils, etc.), and require very little water.
For flat roofs, watering systems are sometimes envisaged, depending on the characteristics of the project (tree planting, sloping roofs, etc.).
On the other hand, a water supply is mandatory on all green roofs, to enable punctual watering when the vegetation is being installed or during difficult climatic conditions. While it’s preferable not to use mains water, vegetation needs water to survive, especially during heatwaves.
The Le Prieuré teams are available to advise you on the choice of automatic watering system best suited to your project.
Is it safe to walk on a green roof?
Extensive green roofing is reserved for inaccessible roofs.
Sedums do not tolerate intensive trampling; however, one or two passes per year for maintenance will not alter the plant cover.
When regular access to the roof is required, a pathway is recommended. This can be a Japanese-step path, for example, which blends in perfectly with the green roof.
Garden roofs are transitable. They can be fitted with a variety of coverings (decking, slabs on blocks).
Are sterile zones mandatory on a green roof?
The sterile zone is a space created around the periphery of the vegetation, whose purpose is to facilitate the inspection of waterproofing and rainwater drainage systems. It is not considered to be an accessible area, nor is it a path for the maintenance of any equipment.
This perimeter band is imposed by DTU 43 and included in the Professional Rules published by CSFE (Chambre Syndicale Française de l’Etanchéité).
In some cases, the sterile zone is not mandatory:
- Where vegetation is planted using HYDROPACK® or the OASIS rainwater management solution.
- In some cases, vegetation is planted using multi-layer systems.
The Le Prieuré teams will be happy to advise you.
Won't the vegetation complex damage the waterproofing?
The waterproofing under the green roofing complex must be resistant to root penetration over the entire surface of the roof (including sterile areas and waterproofing upstands).
Green roofing is an excellent technique for improving the durability of roofing membranes. It reduces the risk of membrane deterioration by :
- protecting the membrane from solar radiation
- to limit the risk of punching
- reducing and limiting thermal shock
The installation of a green roof in no way alters the ten-year warranty attached to the waterproofing membrane.
Green roofing with LP MAT pre-cultivated mats is a simple process that requires no special horticultural skills. Once the drainage system is in place, simply spread the substrate and position the mats, which roll out “like carpet”.
What is the aesthetic appeal of vegetation on a green roof?
The aesthetic appearance of plant cover changes with the seasons, and will differ depending on the planting solution used.
Sedums generally look green in autumn and spring. The color changes to red in summer and winter. Flowering (white, yellow and pink) usually takes place in June, during the summer or in September. Over the years, certain varieties can take over from others, changing the overall look of the green roof.
On garden roofs, the plant palette can be chosen according to the different flowering periods.
The location of the project, its exposure, the solution implemented and the maintenance of the fittings all have a major impact on the appearance of the plants and the overall look of the roof.
When do you really water a green roof?
Watering, whether automatic or manual, is mainly used for :
- during the period of reinforcement of the vegetation roof until the plants have fully taken root,
- during routine maintenance and prolonged dry spells.
How long does a green roof last?
A green roof is a system designed to be self-sufficient and continuously self-regenerating.
A well-designed and regularly maintained green roof complex can last for several decades. Regular fertilization will maximize the lifespan of your green roof!
Do sedums thrive in shady situations?
Sedums are plants designed for sunny situations, and cannot tolerate permanent shade. For this reason, sedums should not be planted on roofs where sunlight is limited (e.g., where several facades overlook the green roof, or in patio-type configurations). Le Prieuré offers plant palettes specifically adapted to shaded roofs.
Do Le Prieuré green roofing processes have a Technical Approval?
Le Prieuré roofs and the Le Prieuré HYDROPACK® tray are validated by an Avis Technique issued by the CSTB in 2011.
Avis Technique 5.2/18-2599_V2 is valid for flat and sloping roofs up to 20%, on any waterproofing and any substrate.
Le Prieuré green roofing solutions comply with the Règles Professionnelles established by the Chambre Syndicale Française de l’Etanchéité (CSFE).
