All about the advantages and disadvantages of green roofs
Green roofs, sometimes also called vegetation roofs, are sloping roofs with roof vegetation. Extensive vegetation is often chosen, such as grass, moss, herbs and stonecrop. In some cases intensive vegetation, such as shrubs and trees, is also an option. Of course the structure must also allow for this.
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01General information about green roofs
In the Netherlands a green roof is certainly not yet the rule, but in the Faroe Islands, for example, they are the most normal thing in the world. Still, an increase is noticeable in the Netherlands too: in three years the number of green roofs has risen by 10%. Education (1,54% of all buildings) and healthcare (1,07% of all buildings) in particular are responsible for this increase.
Extensive green roof
An extensive green roof is a green roof with a green roof build-up of at most 15 cm thick and a vegetation height of at most 50 cm. This vegetation can more or less sustain itself. In practice it is often a combination of moss, grass and herbs. A further distinction is also made between:
Sedum roofs: succulents in combination with moss
Green roof tiles: various sedum species in ready-made tiles for flat roofs
Herb roof: herbs in combination with grass and sedum
Grass roof: the roof contains only grass and needs a growing layer of at least 12 cm
Intensive green roofs
If the green roof build-up is thicker than 15 cm and/or the vegetation height is higher than 50 cm, we speak of an intensive green roof. Trees and shrubs can also be seen on these. They often need intensive maintenance and do not sustain themselves. Lawn roofs are also counted among these. These are grass roofs that can be walked on, so residents can also relax on them. Because they can be walked on, a special structure with a thicker green roof build-up is needed.
Installing green roofs
Installing a modern green roof has in fact become a routine job. First a root-resistant and waterproof roof covering is laid, with a protective layer on top to prevent any mechanical damage. This is followed by a drainage layer that has to drain away the water. Plastic is often used for this, but minerals are also an option. On top of that is a filter layer. This filter layer prevents fine particles from washing out of the substrate layer. Next comes this substrate layer, the layer in which roots can grow, and finally the actual vegetation.
For flat roofs, but also for roofs with a limited slope (max. 15°), a simpler installation is possible. First a root-resistant layer is laid and then a protective layer. Ready-made green roof tiles that are already 95% or more covered in vegetation are then placed on top. These tiles are mostly made from recycled plastic. These tiles can be clicked together, which makes the build-up extra simple.
In general, green roofs can be installed on both flat and pitched roofs. Still, a flat roof that is not in the shade is the best option. With pitched roofs, the roof will dry out more quickly. If the pitched roof has an angle of more than 35°, special anchoring is also needed.
In any case, you have to rely on an expert to check whether the roof is suitable for installing a green roof. The underlying insulation material must, for example, be resistant to compression, which is not the case if a soft material such as rock wool has been used. In addition, the roof structure itself must also be able to bear the weight.
History of green roofs
Green roofs are certainly not new, as anyone who has ever heard of the Hanging Gardens of Babylon can probably confirm. There are also quite a few examples in our own part of the world. Take, for example, the spitkeet or sod hut that arose in the poorest areas of the Netherlands, such as the Friese Wouden or Overijssel. These huts had a roof of sods taken from the field. Since the Housing Act of 1901 it has been forbidden to live in such huts.
The green roofs that appear in the Dutch streetscape today are therefore modern versions. Some 10 million m² of Dutch roof is now said to be covered with vegetation. Of course these are modern structures with relatively lightweight vegetation. The NEN-normcommissie Begroeide Daken, among others, is relevant here.
02Advantages of green roofs
The advantages of green roofs show in different ways. For example, there are financial and energy-related advantages, but the advantages also show at the level of biodiversity or even at a societal level. The most important advantages can be found below.
Energy advantages of green roofs
Green roofs have an insulating effect. On hot summer days more solar radiation is reflected, taller vegetation can provide shade and evaporation also has a cooling effect. On a hot summer day it can be up to 4°C cooler inside than normal, making air conditioning unnecessary or making it use less energy. The total energy saving on air conditioning in homes varies from 2,7% (built from 2015) for modern homes to 22% for old and poorly insulated homes (built before 1987).
During the winter months, too, a green roof can limit energy consumption because it effectively forms an extra insulation layer, although this effect is more limited. For homes with an insulation value from 3,5 m²K/W the saving is practically nil, but with poorer insulation values the saving can even reach 12% (built before 1987 and heated with gas).
A green roof also makes solar panels deliver up to 6% more energy. This is because solar panels function optimally at a temperature of 25°C. On a classic bitumen roof the temperature can rise to as much as 80°C because of the black colour, whereas evaporation on a green roof limits the temperature to a maximum of 35°C. An additional advantage is that solar panels provide shade, which leads to more diversity on the roof. This in turn attracts bees, which brings ecological benefits.
Societal advantages of green roofs
With a classic roof, rainwater is collected and diverted to the sewer system. With a green roof, however, 45 to 70% of this water evaporates again. If all the roofs in a neighbourhood are green, the required sewer overflow volume decreases by 50%. Water treatment plants would in turn save €0,125 per year per m³ of water that falls on a green roof (at 50% evaporation).
Biodiversity advantages of green roofs
People are taking up more and more space, making things harder for butterflies, bees and insects. In principle we do not use our roofs, and we could give that space back to nature. Butterflies, bees, insects, birds and bats, among others, benefit from this. Finally, on extensive green roofs 2 kilos of fruit and vegetables can be grown per m². This brings benefits for residents, but also has ecological benefits because less fruit and vegetables need to be transported to shops.
Health and well-being advantages of green roofs
A green roof reflects sound. With an average sedum roof, 10 dB less noise will be experienced inside. With intensive green roofs this difference can be as much as 46 dB. This means green roofs certainly have a place in a metropolitan environment. In addition, adding green roofs to a grey urban environment is said to have psychological benefits as well. In such environments patients use up to 30% fewer painkillers, people stay in hospital one day less on average and are said to be more focused. Yet the benefits are not only noticeable at a psychological level. Planting filters particulate matter. It can capture up to 200 grams of particulate matter per m².
Technical and financial advantages of green roofs
The value of a property rises by 4 to 8% on average when greenery is added to the home. A green roof also lasts up to twice as long as a black bitumen-based roof. This has to do, among other things, with the protective effect on the roofing layer. Finally, a green roof results in 26% lower maintenance and installation costs.
03Disadvantages of green roofs
There are a limited number of disadvantages to green roofs. For example, green roofs attract the attention of birds. If many roofs in a neighbourhood are green, this is spread out enough to limit nuisance. If there is little greenery in a neighbourhood, birds can be attracted to the one green roof and cause a droppings nuisance. In addition, leaks in green roofs are less easy to detect and a bit more maintenance is involved, which mainly requires an investment of time. This maintenance mainly concerns intensive green roofs, although extensive green roofs are not maintenance-free.
Finally, converting a classic roof into a green roof naturally also requires a financial investment. That is why it is often recommended to build a green roof on a new-build property. In such a case, the weight is taken into account in the roof structure right away and the costs remain much more limited.
04Price and subsidy of a green roof in the Netherlands
The price of a green roof is difficult to predict because it depends on so many factors. Especially if structural work has to be carried out, the price quickly goes up. Because specialised knowledge is also needed to determine whether the roof is suitable for this at all, it is advisable to call in an expert, who will also draw up a quotation at the same time. Do always check whether the municipality grants a subsidy.
Price of a green roof
It is very difficult to make predictions about the price of a green roof. A lot depends on the layer thickness, any additional work (e.g. adapting roof edges), any necessary checks (e.g. calculating load-bearing capacity) and the like. In general, an extensive green roof costs on average 40 to 100 euros per square metre and an intensive green roof 85 to 200 euros per m². These margins are very wide, however, and moreover they do not yet take into account any other work, such as the installation of roof insulation. The only way to get real insight into the cost of a green roof is to request several quotations.
Subsidy and benefits for green roofs
In some municipalities it is possible to receive a subsidy when installing a green roof. Whether or not a subsidy is granted for a green roof differs per municipality. Each municipality can also attach different conditions to it. Below is an overview of municipalities that offer such a subsidy for a green roof (situation on 19/03/2021). Ask the municipality for current conditions or use the Groenesubsidiewijzer from Milieu Centraal. Sometimes only a limited budget is available and it is first come, first served.
05Frequently asked questions about green roofs
Green roofs offer many advantages, yet not every home has a green roof yet. There are various reasons for this. With this FAQ we answer a number of frequently asked questions about green roofs. This information can be convincing and can remove barriers.
A green-blue roof is sometimes also called a retention roof or a polder roof. Unlike green roofs, these roofs are not only covered with vegetation, but also have special underlayers that buffer rainwater and drain it away with a delay. They are sometimes computer-controlled and release water just before a heavy downpour so that they can serve as a buffer when sewers threaten to overflow. It is therefore a green alternative to buffer basins and sewer systems.
What is the difference between a green roof and a green-blue roof?
The ideal vegetation must always be adapted to the substrate, the desired maintenance and the load-bearing capacity of the roof. In addition, it is advisable to plant local native plants that also provide nectar and pollen during the foraging season. All too often cheap seed mixtures are chosen that mainly contain mass-grown exotic species that are of no use to native bees, bumblebees and hoverflies.
What is the ideal vegetation for a green roof?
Green roofs help limit the heat island effect or Urban Heat Island Effect (UHI) in cities. A test was once carried out in Toronto, and it showed that if 50% of the roofs in a city are green, this can result in a UHI reduction of 2°C.
What effect do green roofs have on the ambient temperature?
No. Green roofs have an insulating effect, but all in all this is too limited to replace classic roof insulation.
Does a green roof replace roof insulation?
Yes. Like all other planting, green roofs help convert carbon dioxide into harmless substances. For example, 1,5 m² of grass already produces enough oxygen to supply one person with oxygen for a whole year.
Do green roofs help achieve a CO2 reduction?
This depends on the type of green roof. With a green roof with trees, the roof does indeed become more susceptible to wind. This is also the case when lightweight materials are used. That is also the reason why an expert will always make a wind load calculation when determining whether a roof is suitable.
Is a green roof more susceptible to storm damage?
All about green roofs → Pros & Cons → Types & Weight →
Although the Netherlands is not yet completely filled with green roofs, it is important to know that they offer many advantages. Read all about them.
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