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October 2009
guest feature article

A French Green Roof Research & Development Program

By Maxime Darnis and Philippe Faucon
Photos Courtesy Maxime Darnis

CRITT Horticole logo

Compared to Germany, green roof total surface in France is small but the annual market is getting more and more important.  In 2008, more than half a million square meters of rooftops were vegetated here. This shows the increasing interest for this technique, but market expansion is still slowed down by lack of knowledge: contactors are still questioning green roof benefits and techniques.  To give answers and fill the lack of research, we launched a research program in 2007.

The CRITT Horticole is a regional center of innovation and technology transfer specialized in horticulture approved by the French Ministry of Research and Universities. Our team of 10 engineers is located on the West Coast of France (oceanic climate), and works on several subjects related to the Horticulture Industry: greenhouse engineering, market analysis, plant death research activity and technology transfer to industrial, environmental and energy diagnostics for horticultural companies, horticultural activity support… and more recently: extensive green roofs.

We focused our research program on extensive vegetated rooftops with low maintenance because we think this is the most sustainable way to “green” buildings.  Now, the main topics are:

- Widen the range of species used on green roofs (plant diversification)
- Understand and model green roof thermal behavior in order to quantify the benefits
- Get references on commercialized systems (costs, environmental performance…)

Here is a short description of our green roof research and development program, partially financed by the European Community (EC):

Plant Diversification

In France, the majority of extensive green roof projects rely on a small number of species, principally Sedum species. Selection of species in French green roof companies is limited and mostly based on German research carried out in the nineties.  According to the literature, 300 taxa could be used on green roofs, hence the CRITT Horticole started the plant research program.

Widening the range of plant species used for green roofs could help to improve green roof functional performance (e.g. thermal efficiency, water management), improve aesthetics, reduce maintenance costs and enhance conservation and biodiversity.

We are looking for physiologically and morphologically adapted species to withstand rooftops' extreme environmental conditions.  We try to identify some native species to improve the green roof relationship with the local environment, facilitate biodiversity development on the roof and also promote local identity.  Moreover, a review of plant species associations is conducted to create new design patterns and more sustainable vegetation covers.  The use of special plant communities with sustainable and self sufficient vegetation can reduce maintenance and resource input on green roofs.  For instance, Fabaceae species, by fixing nitrogen, could enrich the soil locally and help other plants' development.

A 200 square meter (2,153 sf) green roof platform was initially installed at CRITT Horticole, and we have recently extended the trials for a total area of 300 square meters.  It reproduces a double layer extensive green roof system with different substrate depths.  Two hundred plant species have been tested here since 2009.  Two hundred more species or cultivars are going to be planted.

A part of the 200 square meter green roof system for plant resistance tests.

Figure 1:  A part of the 200 square meter green roof system for plant resistance tests.

Plant material comes from nurseries, botanical collections, seed companies or natural ecosystems.  They were planted as plugs or cuttings.  For each species, morphology, aesthetic characteristics, flowering, survival rate, multiplication, development, and coverage are evaluated.  This year was particularly selective because of a six week dry period this summer: around 20% of the species tested died.  Among the survival species, some are very promising and showed high drought resistance level, good coverage abilities, nice visual appearance and large flowering period.

A second step consists in studying production protocols of the most interesting species in partnership with a horticultural young plant company.  Our green roof industry needs cheap plants, easy to grow (from seedlings or cuttings) with a high survival rate after planting.

A very promising Sedum cultivar, colorful!

Figure 2:  A very promising Sedum cultivar, colorful!


Another Sedum variety, changing with seasons, very drought tolerant.

Figure 3:  Another Sedum variety, changing with seasons, very drought tolerant.


An Erigeron species blooming generously.

Figure 4:  An Erigeron species blooming generously.


A very drought tolerant and carpeting Artemisia species growing into 8 centimeters substrate depth.

Figure 5:  A very drought tolerant and carpeting Artemisia species growing into 8 centimeters substrate depth.

Thermal Performance Studies

Concerning thermal benefits (in summer and winter), an international review shows that many studies have been conducted worldwide (North America, Asia, Greece, for example) and gives different results.  Nevertheless, no data are available under French climatic conditions.

Therefore, we are developing an energy balance model of a vegetated roof and integrating it in a building energy simulation program (TRNSYS) in partnership with the LEPTIAB (Université de La Rochelle).  At the same time, in order to understand precisely the phenomenon, we study and quantify thermal and mass fluxes.  The roles of growing media, vegetation and drainage layer characteristics on thermal behavior are evaluated experimentally on a green roof modular platform.  The experimental results will be usable for model validation.

Green Roof Modules Platform

Figure 6:  Green Roof Modules Platform.

The platform consists of 30 green roof modules (1 square meter or just under 11 sf each).  These are made in stainless steel covered with root-proof membrane and filled with different green roof systems (single layer with 2-substrate depth, multilayer with different drainage layer types).  Vegetation structure and density may also influence thermal efficiency of green roof.  Therefore, two plant types were planted: grassy forms (Festuca glauca) and carpeting plants (Sedum acre and Sedum album).  The green roof modules are in a greenhouse in order to control climatic conditions but will be transported outside for other experiments.

A Green Roof Module

Figure 7:  A Green Roof Module

A data logging system has been installed and records: temperatures (air and substrate at different points under the surface), solar radiations (incoming and reflected), relative humidity, wind velocity, and weight of the module (the green roof modules are suspended from load cells).

Green roof modules suspended from load cells.

Figure 8:  Green roof modules suspended from load cells.

Energy conduction through the vegetated roof varies in response to thermal growing medium properties (thermal conductivity, specific heat capacity, albedo).  They vary in response to the moisture content of the soil.  Soil moisture depends on precipitation, soil water storage capacity and evapotranspiration.  Therefore, a greenhouse experiment was conducted to determine evapotranspiration on a green roof and adapt the theoretical formula of Penman-Monteith.  Our weighing lysimeters will be used for other experiments (comparison of growing medium water content evolutions between green roof systems…).

We have also explored variation of thermal conductivity with soil moisture for different green roof substrates.  The data will be used in the green roof energy balance model.

Model validation will also use data from an in situ data recording.  The site is a three-building area next to the city of Poitiers.  Rooftops are divided and different green roof systems will be installed.  This site is exceptional for green roof thermal benefits studies and will be equipped next year with data logging systems.

Get References on Commercialized Systems (Costs, Environmental Performance…)

Our 30-module platform is also used to study the performance of the most common vegetation systems.  So we conduct measurements and observations on coverage rate, vegetation health and development, drought tolerance, weed invasion, and maintenance needs.

This platform is the only one in France and could be used as a reference to explore many subjects, such as water management (quality or quantity), fertilizer management, acoustics benefits, new substrates compositions…

Summary and Perspective

Our R & D program is unique in France.  We installed our experimental platforms last year, with financial support from the EC, but it is only a middle or long term program.  We are now looking for collaborations in France or within the European Community with other institutes, universities, technology transfer centers, and greenroof suppliers involved in the building industry.

Maxime Darnis and Philippe Faucon

Publisher's Note:  See the ARRDHOR CRITT Horticole Green Roof Research profile in The Greenroof Projects Database here.  For complete information, visit the ARRDHOR CRITT Horticole website.

Maxime Darnis:

"I have studied horticulture and landscape engineering at the INH High School in Angers, France.  I have worked in tree management in cities and now in the green roof research program at the CRITT Horticole.

I am interested in the different ways to green our towns with both scientific and commercial approaches.  I am open to discussions and partnerships worldwide to contribute to “city greening” development."  Contact Maxime Darnis, ingénieur horticole, chargé d'étude toitures végétalisées at:

Philippe Faucon, CRITT Horticole

Philippe Faucon: 

"I am a horticultural engineer, with 20 years of experience in greenhouse engineering, technical, economic and organization study.  I am very interested in innovation in horticulture, and I have already worked on new containers (avoiding root spiraling), new substrates (made from pine barks), and new organic fertilizers (made from wool).

Greenroofs are an aesthetically pleasing and elegant way to put nature in the city. From my point of view, greenroofs have to be really “environnemental friendly,” and research programs may help, by testing, measuring, and innovating the greenroof industry to progress in this way."

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