Green Roof
Interested in learning more about this topic and more social and sustainable ways of doing architecture? Apply now for our Postgraduate!
Our exploration on alternative insulation technologies has lead us to focus on development of insulation materials upcycled from industrial waste. Until now, we have been exploring mycelium, cardboard and wool for their innovative use and low-key technologies in the building field. With our last research we are currently exploring the potentialities of a whole ancestral system that is still for us on an ongoing building process to cover the 130m2 of our production centre.
The existence of vegetal covers has gone through times in the human settlements. Its poetic origin goes back to the Babylonian hanging-gardens times in 600 B.C. but their ancient creation consisted of shelters for agricultural, dwelling or ceremonial purposes. These original green roofs were made as cave like structures or sod roofs with earth and plants protecting from external elements, insulating in winter and cooling in summer [1].
Figure 1: copy of a bas relief from the North Palace of Ashurbanipal (669–631 BC) at Nineveh shows a luxurious garden watered by an aqueduct.
Figure 3: Construction of a typical house. (from Mulloy, Bulletin 4, Easter Island Committee, International Fundfor Monuments. 1974).
The green roof as an insulation system made of natural local materials took shape in a variety of vernacular architecture. The Scandinavia Sod or Turf, mix of earth and vegetation, was rooted in a sloping wooden roof by a succession of birch barks layers [2]. This layer system started to be developed into a modern technology capable of irrigating, draining, water retention and root ingression protection, marketed on a large scale in Germany in the 70s [3]. The first world modular system, named as Brecht system, combined with a three layer system many benefits in one polystyrene box declinable in different planting depth; a water reserve, a nursery soil, a low cost production and a lightweight system. This tray system faced several issues partly caused by the lifespan of its material and was replaced by a wall-to-wall or seamless green roof system but participated in the commercialisation and industrialisation of the modern one [4].
In the last decades, this technology has gained a certain popularity for its promising answer to face the problems of urban environment: pollution, water retention, density and quality of buildings, energy efficiency and loss of biodiversity. Some noticeable arguments demonstrate the need to consider green roofs as a valuable system to at least partially resolve those issues. Its vegetation and layers composition give to cities a system that benefits its quality of life by delaying peak flood while retaining rainwater, reducing CO2 while producing oxygen and recreating biodiversity. Porto doesn’t drive out of the phenomena and has initiated a movement of green roof introduction and promotion in the city [5].
The evolution of the green roof system and the improvement of its components has expanded the variety of its types and uses. Nowadays we can mainly distinguish three types of green roof, extensive, semi-intensive and intensive; they are defined by their soil depths, their loading capacities and their functions. Those types can be applied on any type of structure; stone, metal, wood, concrete; strong enough to carry the load. This wide range of possibilities requires a series of leading choices in the whole experimentation process.
Searching on the green roof options and possibilities, the semi-intensive type caught our interest as a valuable system offering the opportunity to cultivate food. If the extensive green roof seemed the easiest option to achieve so far, experimenting a more complex green roof offering even more benefits is worthwhile for our research. In addition, the semi-intensive option allows us to use it as a proper outside space. We will as well create windows opening to light the space and develop a rainwater collecting system. This choice forces us to deal with a heavier load on the structure impacting directly the design and the construction process of our own prototype.
LAYER BY LAYER
A green roof is by essence a vegetal cover. The modern system requires the use of several layers answering to specific and necessary functions. The composition follows a general canvas that will get shaped by the choice of the green roof type and materials. Those can easily be non-natural options either because of their function or their cost affecting directly the sustainable aspect of the project. This forced us to look further for alternatives to the common solutions.
Plantation layer
The vegetal cover type will highly depend on the soil depth of the green roof type. From the simple grass layer of the extensive roof, the variety of the plant species increases with the (semi)-intensive category offering even the opportunity to plant trees. Considering this roof as a new garden itself; basic rules and knowledge such as solar exposure, quality of soil, irrigation requirement and even plant associations should drive carefully the choice of the species. Keeping in mind the biodiversity, the green roof can therefore contribute more to the urban ecosystem with a thicker and varied substrate on which more species can live on it [6].
| Vegetation class | Characteristics | Comparable to green roof type |
| Muscinal stratum | Composed of non-vascular plants, lichens, fungi and small herbaceous plants, such as creeping succulents (e.g., Sedum). | Extensive |
| Herbaceous stratum | Dominated by non-woody herbaceous plants such as grasses and flowering plants that can exceed 1 m in height at maturity. | Semi-intensive |
| Arbustive stratum | Shrubs, bushes, and young trees from 1 to 7 m high. | Semi-intensive |
| Arboreous stratum | Large trees (over 7 m). | Intensive |
Growth medium layer
The green roof recreates a new permeable soil, but the plantations need to grow from a medium from which the quality, richness and thickness depend from the vegetation species. If the (semi-)intensive roof demands more nutrients to respond to the needs of the wider variety of plants than an extensive, the growth medium must be generally composed to answer to specific needs. The substrate should be carefully thought to reduce the weight but be good enough to give stability for the plants, retain water and drain the layer by using a right amount of organic (SOM of 4% or less) and mineral proportions. The composition has to create a substrate opened and porous where smaller particles can fit in its structure. The proportions contributes to the amount of water that the mix is able to retain by capillarity in the particles of the substrate. Higher the proportion of fine particles is, more the water can be retain [7]. A granular system can then be quiet interesting for its lightweight and the aeration and water retention offered by its shape [8]. Regarding the mineral contribution, its source can come from several components; crushed recycled bricks, rubble, roof tiles, lava, rockwool, perlite, scoria, ash, pumice, sand, coir, pine bark, etc.
Filter layer
To support the soil of the growth medium and to protect the drainage layer, a permeable filter is required in the green roof composition. This layer is usually made from a geotextile, a fabric that prevents fine particles in the growing medium to enter and clog the drain at the same time that it acts as a capillary wick [9].
Drainage layer
Capturing rainfall and reducing extreme precipitations effect, the green roof drainage layer is fundamental for its hydraulic dynamic and providing optimal plant growth conditions. Different types of drainage exists. The ideal option to use should provide a good water circulation and evacuation while offering some water supply. These supplies provide a stock to water the plants, flush toilets, etc. if needed while preserving the roof from an root overgrowth and their potential damages. The optimization of the drainage can slightly increase with the slope of the roof. From a minimum of 2% it can rise to a maximum of 20% over which it might slip [10]. The thickness of the drainage layer can therefore be optimized as well thanks to the water circulation already provided by the incline.
The modern drainage system preferably use rigid and open mesh structures plastic sheet or boards [11]. Some alternative solution upcycling old tires or using clay can be considered as long as this material is not contaminated in case of reusing rainwater for your irrigation system. Finally, a perimetrical gravel zone and a drain tape surrounding the roof and the windows must be added to maximise the water drainage and the soil filtering in those areas and protect them from the infiltrations and the humidity and avoiding damages on the structure.
D. Nigel and K. Noel, (2004 in: LandArchy Malaysia, n.d.).
Root barrier and waterproof layer
To protect the roof structure from the root growth, the use of a root barrier is fundamental. Its solidity and thickness depends though on the type of species planted, therefore their roots, on the roof. In addition, the choice of the product for the root barrier needs to consider that some can be attracted by the roots such as asphalt and increase then the potential damages. To play efficiently enough its multiple functions, different criterias must be taken in account to select a qualitative membrane. Considering its environmental properties as much as its technical and economical one, an EPDM “Ethylene Propylene Diene Monomer”, a synthetic rubber roofing membrane, seems for us the best option in spite of its un-renewable nature; though some companies are nowadays producing products from recycled material such as car tires. Lasting way over 25 years, this membrane is less influenced by the thermal changes affecting directly the lifespan and the sealing of a roof [12].
Insulation layer
The green roof is often claimed as a good insulation system. Though, on the insulation topic, the green roof acts more as a good thermal mass system than as a good insulation. This slight difference is fundamental basic to understand in therms of energy efficiency. Insulation and thermal mass have both the characteristic of slowing down the heat movement. However, those two acts quite differently. A thermal mass, brought by the density and the conductivity of a material, has the ability to absorb, store then release and radiate the heat. An insulating material, for its part, will slow down and control the heat transfer between an external and internal big difference of temperature [13]. Therefore, the green roof has a particular interest to stabilize the internal temperature by retaining a considerable amount of heat in hotter season and climate thanks to the saturated growing medium and the vegetation. In the winter, an external isolation system would be needed to be really efficient against the cold and avoid the heat loss of the internal space.
Load-bearing structure
The load structure can be made from different type of support. Working on a renovation case, a good evaluation of the existing load-bearing system is fundamental to check its ability to carry the weight of the installation. Considering the overweight of an intensive green roof type, it is rarely possible to set it up on the existing one, the foundations and the structure of the building need to be properly calculated and sized according to the load.
Extensive | Semi- intensive | Intensive | |
| Soil depth | 10-20 cm (lawn and low vegetation) | 20-45 cm (bushes etc) | 45-120 cm (higher vegetation, trees) |
| Weight (kg/m2) | 50-150 | 120-200 | 180 – 500 |
Installation cost (€/m2) | 40 – 70 | medium | 80 – 150 |
| Irrigation | autonomous system | yes | necessary |
Inclination of the roof | 0-20% | 0-20% | 0-5% |
Summary of criteria for designing a green roof
Along with our goal to explore cost-effective insulation alternatives for socially relevant spaces, our own green roof project has been lead with the intention of developing a prototype that would contribute to the urban environmental benefits. In the same time this self-building project will explore alternatives and face challenges on a sustainability level. The Co-lateral green roof project: covering the 130m2 of the existing production center; multiplies the constraints. The prototype is experimented in a renovation process with the amount of exiting unforeseen situations and adjustments that it involves. This green roof is a real exciting journey, a step by step full time learning process that we are looking forward sharing and reflecting on.
Sources:
[1] Green Roof Technology (n.d.). “History”. [Online] available at: http://www.greenrooftechnology.com/history-of-green-roofs.
[2] Wikipedia (n.d.). “Sod roof”. [Online] available at: https://en.wikipedia.org/wiki/Sod_roof.
[3] See [1].
[4] Green Roof Technology (2017). “World’s First Modular Green Roof System”. [Online] available at: http://www.greenrooftechnology.com/green-roof-blog/world-s-first-modular-green-roof-system.
[5] ANCV| Portuguese National Association of Green Roof (n.d.). “About us”. [Online] available at: http://www.greenroofs.pt/en/about-us.
[6] Urban Biology (n.d.). “The impact of Green Roofs on the Urban Ecosystem”. [Online] available at: http://www.projects.science.uu.nl/urbanbiology/articlepageroofs.html.
[7] Growing Green Guide (n.d.). “Substrate Properties”. [Online] available at: http://www.growinggreenguide.org/technical-guide/construction-and-installation/green-roofs/substrate-properties/.
[8] Green Roof Technology (n.d.). “Green Roof Systems”. [Online] available at: http://www.greenrooftechnology.com/Default.aspx?PageID=9096713&A=SearchResult&SearchID=31657342&ObjectID=9096713&ObjectType=1.
[9] Wikipedia (n.d.). “Geotextile”. [Online] available at: https://en.wikipedia.org/wiki/Geotextile.
[10] Growing Green Guide (n.d.). “Guide to DIY Green Roofs”. Retrieved from: http://www.greenroofguide.co.uk/media/en/applications/GRC_DIY_Guide_small.pdf.
[11] Growing Green Guide (n.d.). “Drainage layers”. [Online] available at: http://www.growinggreenguide.org/technical-guide/construction-and-installation/green-roofs/drainage-layers/.
[12] IBGE|Institute Bruxellois pour la Gestion de l’ Environment (2010). “GUIDE PRATIQUE POUR LA CONSTRUCTION ET LA RENOVATION DURABLES DE PETITS BATIMENTS” (in French). Retrieved from: http://app.bruxellesenvironnement.be/guide_batiment_durable/docs/MAT03_FR.pdf.
Useful guides to DIY Green Roofs:
See [11]; [12].
Figures:
Figure 1: Wikipedia (n.d.). “Hanging Gardens of Babylon”. [Online] taken from: https://en.wikipedia.org/wiki/Hanging_Gardens_of_Babylon.
Figure 2: Wikipedia (n.d.). “Orongo”. [Online] taken from: https://en.wikipedia.org/wiki/Orongo.
Figure 3: Koll, R.R. (1994). “Orongo Cerimonial Center: Past, Present, Future”. Taken from: http://islandheritage.org/wordpress/wp-content/uploads/2010/06/RNJ_8_2_Koll.pdf.
Figure 4: Wikipedia (n.d.). “Sod roof”. [Online] taken from: https://en.wikipedia.org/wiki/Sod_roof.
Figure 5: Wikipedia (n.d.). “Sod roof”. [Online] taken from: https://en.wikipedia.org/wiki/Sod_roof.
Figure 6: Green Roof Technology (2017). “World’s First Modular Green Roof System”. [Online] taken from: http://www.greenrooftechnology.com/green-roof-blog/world-s-first-modular-green-roof-system.
Figure 7: Green Roof Technology (2017). “World’s First Modular Green Roof System”. [Online] taken from: http://www.greenrooftechnology.com/green-roof-blog/world-s-first-modular-green-roof-system
Figure 8: Catalanoa,C., Laudicinab, V.A., Badaluccob, L., Guarino, R. (2018). “Some European green roof norms and guidelines through the lens of biodiversity: Do ecoregions and plant traits also matter?”. Ecological Engineering, Vol.115, pp.15–26. Retrieved from: https://www.researchgate.net/figure/Extensive-green-roof-standard-technical-section-according-to-FLL-guideline-and-UNI_fig1_323258059.
Figure 9: ANCV|Portuguese National Association of Green Roof (n.d.). “Trinidade Metro Station”. [Online] taken from: http://www.greenroofs.pt/en/project/trindade-metro-station
Figure 10: ANCV (n.d.). “Lisbon Plaza”. [Online] taken from: http://www.greenroofs.pt/en/project/lisbon-plaza.
Figure 11: ANCV (n.d.). Map. [Online] taken from: http://www.greenroofs.pt/en/map.
Figure 12: Green Roof Plan (2010). “Intensive Green Roofs: A Primer”. [Online] taken from: https://www.greenroofplan.com/intensive-green-roofs-a-primer/.
Figure 13: Urban Biology (n.d.). “The impact of Green Roofs on the Urban Ecosystem”. [Online] taken from: http://www.projects.science.uu.nl/urbanbiology/articlepageroofs.html.
Figure 14: Conservation Technology (n.d.). “Green Roof Components”. [Online] taken from: http://www.conservationtechnology.com/greenroof_components.html.
Figure 15: LandArchy Malaysia (2008). BlogPost. [Online] taken from: http://landarchymalaysia.blogspot.com/2008/09/roof-garden-system.html.
Figure 16: Row, M. (n.d.). “Knowing the Difference Between Thermal Mass and Insulation”. InsulationShop.co. [Online] taken from: https://www.insulationshop.co/Knowing-the-Difference-Between-Thermal-Mass-and-Insulation.
