Green Roof Design: State of the Art on Technology and Materials
Abstract
:1. Introduction
2. History and Modern Applications
3. Green Roof Benefits
- Shading: Vegetation provides an additional layer that shades the substrate and the roof, blocking part of the incoming solar radiation;
- Evapotranspiration: Plant transpiration and soil evaporation cool the surface of the plants, decreasing the heat flux toward the interior of the building and the urban heat island effect;
- Thermal inertia: The substrate increases the roof thermal mass, delaying and reducing incoming heat fluxes;
- Thermal insulation: The substrate and drainage layers increase the heat resistance of the roof by providing an additional thermal layer.
4. Technology Classification
5. Materials and Components
5.1. Waterproof Membrane
- Elastomeric membranes: Characterized by an elastomeric polymer mixed with bitumen, which gives flexibility at low temperatures and excellent elasticity;
- Plastomeric membranes: Characterized by a plastomeric polymer mixed with bitumen, which gives stability at high temperatures and offers high resistance to UV exposure;
- Elasto-Plastomeric membranes: Combines the characteristics of the two membranes above-described.
5.2. Anti-Root Membrane
5.3. Protection Layer
5.4. Water Storage and Drainage Layer
- Granular materials: These have a minimum thickness of 6 cm and a minimum density of 150 kg/m3. If porous, they are also used as water storage. The main aggregates used in green roofs are pozzolana, pumice, lapilli, expanded clay, expanded pearlite, expanded slate, and crushed bricks;
- Modular panels: These have a thickness between 2.5–12 cm and a weight of about 20 kg/m2. These panels are produced with high-strength synthetic or plastic materials (polyethylene or polystyrene) and cavities to store water while still allowing the removal of surplus water.
5.5. Filter Layer
- Granular materials, such as pozzolana, pumice, lapillus, expanded clay, expanded perlite, expanded slate, and crushed bricks, characterized by a water permeability greater than 0.3 m/s;
- Non-woven geotextiles with water permeability greater than 0.3 cm/sl × 10−3 m/s, able to absorb 1.5 L/m2 of water.
5.6. Substrate
- Physical parameters, such as density, particle size, water permeability, maximum water volume, and maximum air volume in saturated conditions;
- Chemical parameters, such as pH index, electrical conductivity, and quantity of organic matter.
5.6.1. Performance
- High hydraulic conductivity and water retention capacity;
- High aeration and flow attributes;
- Poor leaching and high sorption capacity;
- Lightweight, locally available, and cost effective;
- Stability of the physical and chemical structure in severe climate conditions;
- Minimum organic content;
- Wide variety of vegetation;
- Improved water quality.
5.6.2. Composition
5.7. Vegetation
- 0–5 cm: Sedum, mosses, and lichens;
- 5–10 cm: Short wildflower meadows, long-growing, drought-tolerance, perennials, grasses, alpines, and small bulbs;
- 10–20 cm: A mixture of low or medium perennials, grasses, bulbs and annuals from dry habitats, wildflowers, and hardy sub-shrubs.
5.7.1. Performance
5.7.2. Sedum Species
5.7.3. Other Possible Plant Species
5.8. Green Roof vs. Conventional/Traditional Roofs
6. Design Optimization for Mediterranean Climate
6.1. Influence of Climate Conditions
6.2. Possible Material Selection
6.3. Comparison with Tropical Climate
- Vegetation: Peanut has been found to perform significantly better than Sedum;
- Substrate: A 5 cm layer of soil composed of completely decomposed granite amended with 20% fully mature compost and slow-release fertilizer was suitable for Peanut growth;
- Rockwool layer: The rockwool layer had the benefit of lightweight and exceptionally high-water storage capacity which can enhance water supply to plants.
7. Examples of Products Available in the International Market
- Extensive: They are lightweight and have a shallow build-up height. Suitable plants include various Sedum species, herbs and some grasses. After the establishment of the vegetation, the maintenance is limited to one or two inspections a year.
- Intensive: They are usually multifunctional and accessible. They require more weight and a deeper system build-up. The maintenance is regular and depends on the landscape design and the chosen plant material. Anything is possible from lawns, perennials, shrubs, trees, including other landscape options, such as ponds, pergolas and patios.
- Pedestrian/Vehicular: During the installation of the different build-up layers the waterproofing has to be protected from damage. It is possible to install a protection mat or a drainage layer which functions as a protective layer as well. Driveways on rooftops require both a stable construction and adequate load-bearing capacity. Additionally, to the self-weight and imposed loads on driveways, horizontal forces and torsional movements may occur through acceleration, steering or breaking.
- Photovoltaic-green roof: The panels are covered with a prescribed amount of growing medium and the desired vegetation is then planted. The combined weight of the growing media and plants provides the ballast required by the solar energy system to deal with wind loads. Thanks to this ballast principle, roof membrane penetrations that would normally be necessary for anchoring standard solar energy systems are not required.
- Sloped: The plant selection has to be well adapted to the extreme conditions of steep pitched green roofs, where the solar radiation is the highest on the south facing roof side and the water runoff is much faster compared to a flat roof.
- Lightweight: It comprises mature sedum on 20 mm of extensive substrate and incorporating multifunctional water retention and filter layer. The system is suitable for both new build construction and retrofit refurbishment projects. In most instances an additional drainage layer is not required though on roofs up to 2° or in areas of high rainfall, its inclusion may be necessary.
8. Irrigation Systems in Mediterranean Climate
9. Recycled Materials for Green Roof Layers
10. Discussion
11. Conclusions
Funding
Conflicts of Interest
References
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Target Country | Green Roof Total m2 (2014) | Green Roofs New/Year m2 | Ratio Extensive | Ratio Intensive | Yearly Sales Figures € |
---|---|---|---|---|---|
Austria | 4,500,000 | 500,000 | 73% | 27% | 27,350,000 |
Germany | 86,000,000 | 8,000,000 | 85% | 15% | 254,000,000 |
Hungary | 1,250,000 | 100,000 | 35% | 65% | 5,662,500 |
Scandinavia | - | 600,000 | 85% | 15% | 16,050,000 |
Switzerland | - | 1,800,000 | 95% | 5% | 51,300,000 |
United Kingdom | 3,700,000 | 250,000 | 80% | 20% | 28,000,000 |
Main Characteristics | Intensive | Extensive |
---|---|---|
Maintenance | High | Low |
Irrigation | Periodically | Regularly |
Plant diversity | Sedum-Herb-Moss-Grass | Lawn-Perennial-Shrub-Tree |
Cost | Low | High |
Weight | Lightweight (60–150 kg/m2) | Heavy (180–500 kg/m2) |
Thickness | 60–200 mm | 140–400 mm |
Use | Accessible | Inaccessible |
Extensive | Intensive | Pedestrian Vehicular | PV | Sloped | Lightweight | |
---|---|---|---|---|---|---|
Zinco | X | X | X | X | ||
Bauder | X | X | X | X | ||
Daku | X | X | ||||
Perlite | X | X | ||||
Harpo | X | X | ||||
Climagrun | X | X | X | |||
Optigrun | X | X | X | X |
Anti-Root Membrane | |||||||
---|---|---|---|---|---|---|---|
Parameters | N. 1 | N. 2 | |||||
Thickness (mm) | 1.1 | 0.36 | |||||
Surface mass (g/m2) | 1130 | 310 | |||||
Breaking strength (N/5cm) | 80 | 20-47 | |||||
Breaking expansion (%) | >20 | >400 | |||||
Drainage layer | |||||||
Parameters | N. 1 | N. 2 | N. 3 | N. 4 | N. 5 | N. 6 | N. 7 |
Height (mm) | 45 | 19 | 25 | 40 | 60 | 75 | 25 |
Surface mass (kg/m2) | 2.0 | 19 | 1.7 | 2.0 | 2.2 | 1.0 | 5.0 |
Resistance (kN/m2) | 138 | 400 | 200 | 170-250 | 40-533 | 55 | 460 |
Water storage (l/m2) | 17 | - | 3.0 | 6.0 | 13 | 3.0 | - |
Runoff 1% slope (l/(s × m)) | - | 0.34 | 0.59 | 1.5 | 1.1 | 1.54 | 1.0 |
Runoff 2% slope (l/(s × m)) | - | 0.47 | 0.85 | 2.1 | 1.6 | 2.21 | 1.5 |
Runoff 3% slope (l/(s × m)) | - | 0.57 | 1.05 | 2.6 | 2.0 | - | 1.9 |
Filter layer | |||||||
parameters | N. 1 | N. 2 | N. 3 | N. 4 | N. 5 | N. 6 | N. 7 |
Thickness (mm) | 7.0 | 17-20 | 5.0 | 6.0 | 0.6 | 1.7 | 1.0 |
Surface mass (g/m2) | 650 | 1500 | 470 | 850 | 100 | >300 | >150 |
Water storage (l/m2) | 7.0 | 12 | 5.0 | 4.0 | - | - | - |
Breakthrough force (N) | - | 2300 | >2000 | >3500 | 1100 | 4300 | 2250 |
Substrate | |||||||
Parameters | N. 1 | N. 2 | N. 3 | N. 4 | N. 5 | ||
Dry Volumetric weight (g/l) | 1000 | 1000 | 950 | 1000 | 1120 | ||
Saturated Volumetric weight (g/l) | 1500 | 1500 | 1400 | 1400 | 1400 | ||
Maximum water capacity (%) | 50 | 50 | 45 | 40 | 28 | ||
Permeability (mm/min) | 0.3–30 | 0.3–30 | 0.3–30 | 0.6–70 | 60–400 | ||
pH (CaCl2) | 6.5–8.0 | 6.5–8.0 | 6.5–8.0 | 6.5–8.0 | 7.0–8.5 | ||
Saline content (g/l) | <2 | <2 | <2.5 | <2.5 | <2.5 | ||
Organic matter (g/l) | <90 | <90 | <90 | <65 | <40 | ||
Compacting factor | 1.3 | 1.25 | 1.25 | 1.2 | 1.12 |
Authors | Reference | Recycled Material Used | Main Findings |
---|---|---|---|
Bisceglie et al. | [106] | Waste of granular Autoclaved Aerated Concrete | The pH value of the water extract was of 7.23; the organic matter was less than 4.08; the apparent density was 459.2 kg/m2; the demand for high water retention capacity was completely satisfied by the value of 222.62% of the mass of water absorbed relative to the mass of the dry sample. |
Chen et al. | [105] | Recycled glass | It performed well in the neutralization of acid rain, but did not significantly reduce the levels of other pollutants. |
Matlock and Rowe | [107] | Crushed porcelain and foamed glass | Substrate volumetric moisture content was generally greater in shale than in foamed glass or porcelain. |
Eksi and Rowe | [108] | Crushed porcelain and foamed glass | Total plant coverage in both porcelain and foamed glass was equivalent to expanded shale on five of the six dates measured over two growing seasons. Substrate moisture and temperature were observed during the second season. The moisture content of both the porcelain and foamed glass was either equivalent to or greater than that of the expanded shale throughout the season. Subsurface temperatures were cooler in the porcelain and foamed glass than the expanded shale during the daytime for the majority of the second season. Variation in daily temperatures in the porcelain was significantly lower than the expanded shale when plant coverage was below 50%. |
Molineux et al. | [109] | Inert construction waste material | Some of the alternative substrates are comparable to the widely used crushed red brick aggregate (predominantly found in commercial green roof growing substrate) for supporting plant establishment. For some materials, such as clay pellets, there was increased plant coverage and a higher number of plant species than in any other substrate. |
Bates et al. | [110] | Crushed brick, crushed demolition aggregate, and solid municipal waste incinerator ash aggregate | Treatments with a high proportion of crushed brick in the growth substrate supported richer assemblages, with more species able to seed, and a smaller amount of Sedum acre. |
Mickovski et al. | [111] | Inert construction waste material | The substrate mix containing recycled construction waste materials was adequate in supporting plant growth, was resistant to erosion and slippage and capable of providing good drainage. |
Molineux et al. | [112] | Clay and sewage sludge, paper ash, and carbonated limestone | Particle density and loose bulk density results have shown all substrates to be classed as lightweight aggregates and leaching analysis has confirmed that all substrates perform within legal leachate limits for drinking water. |
Farias et al. | [113] | Sieved waste | The new aggregate had low bulk density and increased water absorption and porosity. The thermographic camera results provided evidence that new aggregates had significant insulating properties and were suitable for use on green roofs. |
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Cascone, S. Green Roof Design: State of the Art on Technology and Materials. Sustainability 2019, 11, 3020. https://doi.org/10.3390/su11113020
Cascone S. Green Roof Design: State of the Art on Technology and Materials. Sustainability. 2019; 11(11):3020. https://doi.org/10.3390/su11113020
Chicago/Turabian StyleCascone, Stefano. 2019. "Green Roof Design: State of the Art on Technology and Materials" Sustainability 11, no. 11: 3020. https://doi.org/10.3390/su11113020