Green roofs have many advantages over conventional roofs: they have a more attractive appearance, improve local biodiversity and they store rainwater. But above all, they can add attractive usable space to highly densified urban areas.
Green roofs also offer a solution for the new regulations of more and more municipalities regarding local water storage in new developments and redevelopments. The requirements of 50 to 200 mm rainwater storage on your own plot in the urbanized area can often only be achieved with substantial water storage on the roofs. In the context of nature-inclusive construction, greening large parts of the plot surface is a requirement, and the roof offers a solution for this.
Green roofs are divided into extensive green roofs, which require little maintenance and management and have a light construction, such as moss-sedum-herb roofs and intensive green roofs, which are heavier and have irrigation systems and can also provide substantial water storage. Extensive green roofs can be applied earlier to existing buildings due to their lighter weight. Intensive green roofs range from watered grass-herbs roofs to walkable city parks on buildings.
Application and design
Good documentation from various manufacturers with principle details, implementation guidelines and technical data is available. Standard packages with different objectives such as rainwater retention, biodiversity and/or residential roof are available. Green roofs have now been further developed by various providers and are available as a complete package of planning, construction, maintenance and all associated guarantees. Some providers have developed software that can be used to determine the influence of the green roof on water management.
Four types of vegetated roofs are distinguished below in relation to water storage, namely:
- Lightweight vegetated roofs with a short-term delayed drainage;
- Vegetated roofs with limited, short-term water storage;
- Static green-blue retention roofs;
- Dynamically controlled green-blue retention roofs.
Furthermore, the following functions can be combined with the above roofs:
- Nature roof
- Landscape/garden/public roof
Construction of a green roof
The structure of a vegetated roof consists of:
- vegetation layer
- substrate layer
- filter layer
- drainage and protective layer
The type of vegetation partly depends on the insolation and the dimensioning of the substrate layer. A grass or herbs roof needs a thicker substrate layer than a sedum vegetation. The substrate layer consists of mineral and organic material, such as lava, coconut, etc. in accordance with the German ‘Forschungsgesellschaft Landschaftsentwicklung Landscape construction EV ‘ guidelines. The substrate layer basically serves as a growth medium for the vegetation layer and works hydrologically like a sponge. This layer can store water briefly through absorption until it is saturated and the maximum water capacity has been reached.
Below the substrate layer is a filter layer. The filter layer prevents particles from the substrate layer from clogging the drainage layer. This layer usually consists of a perforated plastic cloth or woven fleece.
The drainage layer serves to collect excess rainwater from the substrate layer via the filter layer and store it or drain it to the rainwater drainage system so that the substrate layer and the vegetation layer do not remain too wet. To prevent excessive saturation of the drainage layer, it is sloped. The drainage layer consists of porous material, which can be rock wool, for example, but there are also systems with plastic containers.
Then there is a protective layer and underneath that the architectural structure, which successively consists of: the root-resistant roofing system, thermal insulation, vapor barrier layer and the roof construction.
The different layers of the vegetated roof are dimensioned depending on the type of vegetation, the desired size of the water storage capacity, the load-bearing capacity of the roof structure and the use of the roof surface. For correct dimensioning, contact a recognized supplier of vegetated roofs.
Sloped green roofs
Green roofs can be used on roofs with a slope of 1° to 35°. Above 35°, additional provisions are necessary to prevent shearing. Steeper roofs also dry out faster due to faster rainwater runoff. If the additional anti-shear provisions pierce the roof skin, this entails additional risks of leakage. For rainwater buffering, a slope of up to 7° is most efficient.
- vegetation layer
- substrate layer
- sliding protection system
- drainage layer
- protective and filter layer
Requirements for the load-bearing capacity of the roof structure
In the Netherlands, flat roofs without a functional function are generally designed for an additional load of 100 kg/m² for the gravel layer. In existing situations, the gravel layer can almost always be replaced by an extensive green roof without additional constructive measures. A saturated green roof with a thickness of approximately 7 cm remains below this permitted load. The roof structure must comply with the rules in the Building Decree and the NEN standards mentioned therein for (wind) loads and deformations. It may be necessary to increase the load-bearing capacity by adapting the construction.
New green roofs
On new green roofs that have been professionally constructed on a roof with a reinforced supporting structure, it is easier to combine the functions of evaporation and reduction of discharge peaks. In extreme cases, these roofs can also handle very extreme downpours almost completely independently with minimal impact on the environment. The use of green-blue roofs in the context of local water challenges is particularly interesting for new construction. By making the water storage sufficiently large (around 70 mm or more) and controlling or limiting the discharge, the mandatory water storage can be achieved.
Only sedum vegetation and grass-herbs roofs are suitable for roofs without irrigation. Sedum can bridge periods of drought and grass-herbs roofs temporarily turn yellow but recover after rain. All other higher-growing species such as shrubs and trees must be provided with a watering system. Retention roofs offer the opportunity to use the stored water for irrigation.
Several green roofs
Lightweight vegetated roofs with a short-term delayed drainage
These vegetated roofs are also called sedum, grass or herbs roofs. Due to its low weight and low costs, this type of vegetated roof is often used on existing buildings, but it can also be used on new construction.
The sponge effect of lightweight vegetated roofs is limited and greatest during small to medium rain showers. During heavy and prolonged rain showers, the first peak of the shower is weakened, but the substrate and drainage layer are quickly saturated. With a vegetated roof of this type, approximately 20-30 mm of rainwater can be temporarily retained when the maximum water storage capacity has been reached. Once this maximum water storage capacity has been reached, the vegetated roof can no longer store rainwater and it must be drained to another form of storage or to the sewer.
When a lightweight vegetated roof is installed on an existing roof, it must be taken into account that in most cases the roofing construction can no longer be maintained as a loose system after replacing the gravel with the vegetated roof, because a dry vegetated roof weighs less. than the gravel and is therefore not suitable as an alternative ballast layer. The lightweight vegetated roof will have to be weighed down somewhat, for example by adding a gravel edge. This type of roof preferably has a slight slope.
Vegetated roofs with limited, short-term water storage
This type of vegetated roof has the same structure and function as the lightweight vegetated roof, but the drainage layer will be provided with additional storage capacity with extra resistance (for example through a meandering structure), allowing more rainwater to be stored and the drainage of that water to be slowed down. It is drained less quickly by the rainwater drainage system. This delayed drainage is used to reduce the flow rate from the roof, and not to reduce the water storage capacity of the sewerage system at ground level. In this case, the stored water is completely drained (delayed).
Static green-blue retention roof (long-term delay)
This type of green-blue roof has a drainage layer with extra storage capacity. In addition, the rainwater drains are provided with extra resistance by means of a static drainage control, so that more rainwater can be stored and the rainwater is drained less quickly by the rainwater drainage system. The static drain control has perforations in the drain; the cross-section of these perforations determines the discharge speed and can be adjusted per drain. And furthermore, in this system there is always an amount of water left in the drainage layer that remains available for the vegetation layer; a possible irrigation system for vegetation is therefore usually not necessary. The amount of water that remains in the drainage layer depends on the height and size of the perforations in the drainage arrangement.
The extra storage capacity in the drainage layer of the vegetated roof can be used in the context of a water task as a compensation facility for an increase in the paved surface on a plot. This is possible because the delay can be linked to a specific emptying time and the average and maximum flow rate during emptying can be indicated.
In that case, the stored water is partly (delayed) drained, which, in addition to additional delay, also results in additional water retention. The combination of the delayed drainage and the extra water storage capacity can be used to reduce the capacity of the sewerage system at ground level.
During heavy rain showers, an overflow ensures that the retained water in the drainage layer remains below the maximum height that matches the maximum load-bearing capacity of the roof.
An advantage of a static green-blue retention roof is that no slope of the roof is required; this saves money on new construction.
Dynamically controlled green-blue retention roof (long-term delay)
This type of green-blue roof has the same structure as the static green-blue retention roof, but has a dynamically controlled water drainage delay. Compared to the static water water drainage delay, the dynamically controlled water drainage delay has the advantage that the delayed discharge of water can be stopped completely. The basic principle is that the stored water is not discharged unnecessarily if there is no reason to do so. As long as the water remains in the system, it is available for the vegetation layer and reuse of the water for watering the plants will be optimized.
The dynamically controlled water delay is in real-time contact with a weather server and analyzes whether precipitation is expected and whether the predicted amount of precipitation can be stored in the roof vegetation system. If it appears that there is sufficient storage capacity in the roof vegetation system to store the precipitation, no action is required and the dynamically controlled water delay will remain closed. If it turns out that there is insufficient storage capacity in the roof vegetation system to store the precipitation, action is required and the dynamically controlled water delay will be opened 24 hours prior to the precipitation event and drain off such a large amount of water that the expected precipitation in the roof vegetation system will be reduced. can be salvaged.
As an extension of this system, the rainwater that is drained off slowly can be stored in a buffer facility, for example in the basement. If during long periods of drought the water level in the drainage layer falls below the minimum, the stored water is pumped up to replenish the drainage layer so that the vegetation layer does not dry out.
Nature roofs are roofs where strengthening (local) nature and biodiversity is central. Nature roofs have natural vegetation that matches the environment and suits the location of the roof. These can be grasses, herbs, shrubs or trees, but also sand, gravel, shells or combinations of these. The essence is that the existing nature in the vicinity of the roof is strengthened or enriched.
Nature roofs have a structure of the substrate layer of various heights, creating wetter and drier parts on the roof that contribute to growing places for various plants. Most animal species can be found in structured and varied vegetation. The most ecologically high-quality form of vegetation is extensive vegetation that is partly provided with substrate elevations and vegetation of sedum herbs and shrubs with smaller water surfaces, dead wood and gravel areas. Extensive and intensive roof vegetation provides a breeding habitat for a large number of ground-nesting bird species.
- vegetation layer
- substrate layer
- filter layer
- drainage layer
- storage mat
- protective layer
Public roof, or roof park
A public roof or roof park is aimed at usage options for city residents in combination with greenery. These can be different forms of green roof, from a simple moss-sedum-herbs roof to a park with lots of nature, shrubs and trees. A roof vegetable garden (‘garden roof’) is also possible. Functions that occur in a city park such as seating areas, sports facilities, water features and cool spots can be integrated into a roof park. Naturally, the roof construction must be suitable for this.
- vegetation layer
- intensive substrate layer
- filter layer
- drainage layer
- water drainage system
- root protection layer
- protective layer
Green roofs contribute to increasing the city’s sponge effect. The buffering effect of lightweight green roofs is greatest during small to medium rainfall. During heavier and longer rain showers, the contribution of a lightweight green roof is only minor due to the saturation of the package. To store rainwater in more substantial quantities, a retention roof can be used on new construction or if the roof of an existing building can support the weight.
Thermal insulation of buildings
The insulating effect of green roofs is mainly determined by the following three processes:
- Moisture absorbs part of the incoming solar radiation, the energy is removed via evaporation and provides cooling.
- The coverage of the plants provides shade on the roof.
- The albedo (the reflectivity) of the plants ensures reflection of incoming solar radiation. A lower number means less reflection and therefore more absorption of radiant heat: bitumen 5%, asphalt 5% to 20%, red ceramic roof tiles 10 to 30%, short grass 15% and long grass 30%.
The insulation value of a green roof is not a constant, but varies with the humidity and vegetation on the roof. As the roof structure is better insulated, the added value of heat insulation provided by a green roof decreases to negligibleSTOWA & RIONED 2015.
Cold insulation of buildings, the cooling load
Due to absorption and evaporation, shading and reflection, the temperature above a green roof is limited to approximately 35°C in the summer. This is much lower than above a traditional flat (black) roof or (red) tiled roof, where the temperature can rise to 70°C and with bitumen roofing even up to 90°C. Because the temperature of the green roof is lower, the temperature difference over the roof structure is smaller with the same thermal resistance of the structure. As a result, a building with a green roof requires less cooling in warm periods, the so-called cooling load is smaller.
By varying the type of plants, the planting density and the water storage on the roof, the temperature above the roof structure can be influenced and thus the heat flow through the roof. Studies with a similar climatic situation as in the Netherlands show that a green roof can reduce the average daily heat flow in summer (inwards) by 70 to 90% and in winter (outwards) by 10 to 30%. Naturally, the percentage depends on the insulation value (year of construction) of the roof in question. The effect is smaller on well-insulated roofs.
Yield from solar panels
The cooling effect of a green roof has a positive effect on the power and lifespan of solar panels. At higher temperatures, the energy yield of PV panels decreases, with crystalline silicon panels 0.4 to 0.5% per °C temperature increase of the PV panels. This doesn’t seem like much. But the air temperature above a gravel or bitumen roof can rise to 50 to 70°C in summer, while the ambient temperature above green roofs is usually limited to 35 °C. The few studies into this indicate a possible increase in yield to 6% to 10% on an annual basis. This percentage depends, among other things, on the climate. There is little research available that has been conducted in a climate comparable to that in the Netherlands.
The combination of solar collectors and a vegetated roof can have a beneficial effect on biodiversity on the roof because of the alternation of sun and shade, which creates different microclimates.
Ambient temperature and limit heat
Vegetated roofs help limit urban warming and thus improve the microclimate. A vegetated roof only heats up to around 35°C and a bare roof more than 70°C on a hot summer day. A vegetated roof cools the immediate environment through evaporation (as long as there is water). But the effect of individual green roofs on the ambient temperature in urban areas is negligible. A city with substantially more vegetated roofs remains cooler than a city with conventional roofs.
Green roofs have a higher albedo than black roofs or tiled roofs. Due to evaporation, the surface temperature even remains lower than with white-painted roofs. A city with more green roofs therefore remains cooler than a city with white roofsWeiler, et al. 2009.
Green roofs can reduce both the penetration of ambient noise into the building (sound insulation) and the reflection of sound. The noise reduction depends on the vegetation and the type, thickness and moisture content of the substrate. This reduction is therefore not a constant. Green roofs can reduce sound reflection by up to 3 dB. For sound penetration, research shows that this reduction is negligible for low (up to 250 Hz) and high frequencies (above 1,250 Hz). In between, the noise reduction can be up to 5 dB with an almost saturated substrate and more than 8 dB with a dry substrate. This effect may be relevant for sloping roofs with environmental noise from, for example, motorways, railways or air trafficGemeente Rotterdam 2009.
Filtering of particulate matter and gases from the air
Green filters particulate matter and gaseous contaminants from the air. Greenery on roofs also has a filtering effect on the air. Particulate matter (PM10) falls from the air onto the plants. The higher and rougher the roof vegetation is and the larger, moister, hairier and stickier the leaves are, the more particulate matter the roof captures. The particulate matter is not absorbed by the vegetation but is deposited on the surface of the vegetation. Precipitation washes it away to some extent. However, the effect on air quality in the area is limited. Exceeding air quality standards cannot be solved by greening roofs.
EPA, the US environmental agency, states that a green roof can bind 1 kg of particulate matter/m², capturing it in the soil and in the plant. Naturally, a green roof removes CO2 from the air and produces oxygen like all plants. Green roofs also filter other environmentally harmful substances such as NOxEPA, 2008.
Green roofs can enhance spatial quality and contribute to the well-being of city residents if they are visible and even better, walkable and usable as gardens.
Vegetated roofs are an important ‘stepping stone ‘ for all kinds of plants and animals in the increasingly petrified urban landscape. A varied vegetation layer with sedum, herbs, grasses and height differences with, if possible, shrubs and trees, optimizes urban biodiversity.
By using local soil material in the substrate, original species can be preserved on the roof. But many green roof plantings consist of mono- or polycultures, with species that can survive in a limited substrate thickness and dry periods. So-called biodiverse roofs can make a greater contribution to biodiversity. Most research has been and is being done on green roofs with mono- or polycultures.
Relatively many types of plants can live on green roofs. The possibilities can be further expanded through variation in microclimate (especially sunny and shaded areas) and planting, and minimal irrigation and maintenance (disturbance). In a green environment, seedlings increase the chance of natural increase in plant variety on the roof. In addition, the type of substrate influences biodiversity and the occurrence of special species. The size, slope or age of green roofs appears to have no demonstrable influence on plant variety. In addition to being a food source for birds, green roofs can provide a breeding ground for ground-nesting birds.
A PhD study at the University of Tübingen shows that the choice of plants on green roofs determines the presence of animals. More than 120 types of roof vegetation were examined for this research.
Simple extensive vegetation is usually only temporarily inhabited by flying animal species and must be reconquered every year. Intensive vegetation provides sustainable living space for animals for years to come. Soil animals such as earthworms can only survive in the long term if there are areas with a higher substrate build-up or partial substrate mounds, due to frost. Most animal species naturally occur in structured and varied vegetation. Extensive vegetation in combination with some substrate mounds and vegetation of sedum-herbs-scrub with a structural element, such as a temporary small water feature, dead wood and a gravel area, is ecologically the most valuableMann, et al. 1998.
Green roofs have a positive effect on water quality because the layered structure of green roofs facilitates water filtration. Rainwater is purified in this way. A green roof also ensures a delayed drainage of rainwater and therefore, in a mixed sewer system, fewer sewer overflows to the surface water.
However, there are several factors that influence the quality of run-off water from green roofs. An intensive green roof can cause phosphorus contaminants in the run-off water. This is due to fertilization, but more often due to the use of nutrient-rich substrate. The outflow of phosphorus is higher with new green roofs.
Depending on the structure of the green roof and the thickness of the water storage layer, a green roof contributes to the prevention of flooding from a few millimeters to a few decimetersGill, et al. 2009.
Green roofs contribute to the greening of the built-up area. A 20% higher green share of the total built-up area reduces the air temperature in that area by only around one degree C. The surface temperature of green roofs is lower than that of, for example, bitumen roofs, but this therefore has only a very limited influence on the air temperature of the environmentKluck, et al. 2020.
Green roofs contribute to the greening of the built-up area and biodiversity. Even simple moss-sedum-herbs roofs and gravel roofs provide living space for various birds and insects. More extensive Nature roofs and roofs with trees and various plants are not inferior to urban gardens in terms of biodiversity.
Quality of life
Walkable green roofs contribute to the greening of the built-up area and provide living space for city residents. This contributes to better quality of life and health.
Vegetated roofs with vegetation consisting of sedum, sedum and herbs, or sedum, herbs and grasses should be checked approximately twice a year. The maintenance work mainly consists of:
- Removal of unwanted vegetation in both the vegetation zones (seedlings) and the vegetation-free zones (overgrowth and seedlings) of the vegetated roof.
- Inspecting and, if necessary, cleaning the inspection shafts around the rainwater drains.
- Administer a long-acting fertilizer if the roof gardener deems this necessary.
Roofs with more intensive vegetation and roof gardens must be maintained in the same way as comparable ground-based vegetation. Roofs with more intensive vegetation must be equipped with a irrigation system.
The root-resistant property of the roof covering will prevent the roof covering from being damaged by root growth. Vegetation with very aggressive root growth must be kept away from the vegetated roofs at all times.
With normal maintenance, the lifespan of green roofs is 30 to 50 years. The lifespan of roof tiles is approximately 50 years, for bituminous roofing 15 to 30 years. Roof coverings that last 40 to 50 years are also possible for flat roofs. The lifespan of green roofs is therefore comparable to that of tiled roofs and up to twice as long as that of roofs with bituminous covering.
Due to the smaller temperature differences in a green roof, it is less stressed; the green layer also protects the roof skin against mechanical damage and UV radiation.
Indication of costs
According to the Climate-Resilient City Toolbox, the costs for installing a green roof are a minimum of €100 and a maximum of €150 per m². These are roofs without (controlled) water storage on which the vegetation thickness is approximately 10 cm.
The management of these green roofs amounts to an average of €6 per m² per year. These are estimates from Milieucentraal and Dakdokters, which assume a roof between 50 and 250 m². Information from the Netherlands Enterprise AgencyRVOshows that the costs for installing an extensive and/or intensive green roof are between a minimum of €2,500 and a maximum of €5,000 per home.
The RVO information shows that the costs for creating a roof garden are between a minimum of €5,000 and a maximum of €10,000 per home. The roof garden must be maintained four times a year and the costs for this are on average €6 per m² per year.
- EPA (Environmental Protection Agency); Reducing Urban Heat Islands - Compendium of Strategies; United States Environmental Protection Agency, 2008
- Gemeente Rotterdam; Rotterdam, groen van boven - Toepassing van groene daken in Rotterdam; Rotterdam, 2009
- Gill S., Goodwin C., Gowing R., Lawrence P., Pearson J. & Smith, P.; Adapting to climate change - Creating natural resilience, Technical Report; Greater London Authority, London, UK, 2009
- Kluck, J., ir. L. Klok, A. Solcerová, L. Kleerekoper, L. Wilschut, C. Jacobs, R. Loeve (2020), De hittebestendige stad, Een koele kijk op de inrichting van de buitenruimte, Hoge School van Amsterdam, ISBN: 978-94-92644-80-0
- Mann G.; Aanwezigheid en betekenis van bodemdieren (macrofauna) op dakbegroeiing afhankelijk van de vegetatievorm; Proefschrift Universiteit Tübingen, 1998
- Gastkemper, H. & Buntsma, J; Groene daken nader beschouwd – Rapport 2015-12: STOWA & Stichting RIONED, 2015.
- Weiler, Susan K. & Katrin Scholz-barth; Green Roof Systems - A guide to the planning, design, and construction of landscapes over structure; Hoboken, New Jersey: John Wiley & Sons, Inc., 2009