Advantages and disadvantages of green roofs


In recent years, green roofs have become more common. You can see them on big urban buildings, schools, farms, houses, flats and many more buildings. Green roofs are also called eco-roofs and are getting more and more popular. Green roofs have many advantages as well as several disadvantages, which will all be discussed in this article.

What is a green roof?

Before we dive into the advantages and disadvantages of green roofs, it is important to have a good understanding of what green roofs exactly are and why they are getting more popular these days. A green roof is a flat or slightly pitched roof that is partially or wholly covered with a layer of a growing medium and vegetation. The vegetation is planted over a waterproofing membrane that is installed on top of the roof.

The green roofs that men knew in ancient times are not similar to the ones that we know today. Until the late 19th century, turf roofs were most common. These roofs consisted of sloping wooden boards with several layers of birch bark and a layer of sod on top. It was very common in Scandinavia. Turf roofs provided the people with good insulation during the summer- and winter months. On the other hand, these roofs were not waterproof and could not keep out vermin. Modern green roofs are relatively new and were first developed in the mid-20th century in Germany, which is now one of the frontrunners. More than 10% of all roofs in Germany are green roofs, and this number is increasing in most other countries as well. Modern green roofs are made of manufactured layers that are placed over the conventional roof.

There are three different types of green roofs. These are intensive, semi-intensive and extensive. Intensive green roofs have deep soil and an optimal irrigation system. Because of this, a greater diversity of plants can be supported. However, it is expensive and more maintenance is required. Extensive green roofs have thin soil, which is more suitable for larger areas. Maintenance is minimal, little technical expertise is needed and it often looks more natural. But the downside is that the choice of plants is more limited.

The basic elements of a green roof are a roof barrier, irrigation system, drainage layer, soil and vegetation. The growing medium is not the same material as traditional soil, which is used for a garden of a house plant. Traditional soil is heavy and tight after rainfall. This reduces the aeration of the plant roots. Growing medium, however, is crafted from mineral aggregates and little organic material. The structure can hold water and supports good aeration.

Advantages and disadvantages

It is not random that green roofs are getting increasingly popular. Green roofs have many advantages. This includes environmental advantages as well as economic advantages. But unsurprisingly, there are also a few downsides to green roofs. Here is an overview of the most significant advantages and disadvantages of green roofs.

Advantage: improved stormwater management

A green roof helps control and reduce stormwater runoff. In most urban areas, there are few green spaces and mostly impenetrable surfaces. Stormwater runs off the roofs of buildings and houses into gutters, after which it flows into the sewer and enters the municipal water treatment facility or is deposited in lakes and rivers. Rainwater and industrial wastewater are collected in the same sewage system. When there is too much stormwater, a city’s sewage system can overflow. As a result, sewage that contains substances that are harmful to the environment, such as nitrogen and phosphorus, are discharged into lakes and rivers.

Green roofs help to prevent this from happening by reducing over 75 per cent of the water runoff during rainstorms. All this water is retained by the growing medium and vegetation of the green roof. Aside from that, the soil also functions as a filter. It retains the water’s environmentally harmful pollutants, which helps to neutralize acid rainwater and even stimulates the growth of the plants. The soil also traps dust and other particles. As the vegetation and its root system grows and matures, the retention of stormwater increases even more.

Advantage: heat reduction in urban areas

In cities and urban areas, the temperature is much higher than in surrounding rural areas. This phenomenon is known as the Urban Heat Island Effect, which is most noticeable during the summer months. It occurs because the sun’s radiation is being absorbed by the paved surfaces of cities. The radiation is re-emitted as heat, increasing the local air temperature and making cities at least 4 °C / 7.2 °F hotter than the surrounding rural areas.

Green roofs combat the Urban Heat Island effect by absorbing much more heat than conventional roofs. According to a modelling study in Toronto, the entire city can be cooled down by up to 0.8 °C / 1.4 °F by adding green roofs to half of the surface downtown.

Besides absorbing more heat, green roofs also use solar radiation for transpiring moisture from the plants and evaporating water from the soil. This process is called ‘evapotranspiration’. It lowers the temperature on the roof itself, which decreases the amount of heat that goes into the building.

Disadvantage: high weight load

A green roof is often very heavy. Not all buildings have a roof that is strong enough to support a green roof’s added weight, especially when the green roof is from the ‘intensive’ category. Therefore, many buildings would need to properly reinforce their roof before they can support the layers of a green roof. An extensive green roof weighs less and could be supported by most roofs, but on the roofs of high buildings, the chance that your seedlings and plants will be damaged due to the wind is higher for extensive green roofs than for intensive green roofs.

Advantage: ecological benefits

Green roofs have many ecological benefits. One of those is that green roofs result in fewer greenhouse gas emissions. By adding all types of vegetation in an urban area, photosynthesis increases. As a result, the level of carbon dioxide that the area emits via industries and vehicles reduces. Meanwhile, the level of oxygen in the area increases. Another ecological benefit of green roofs is that they provide the urban wilderness with natural habitats. It attracts birds, bees and many other insects. Thus, green roofs increase the biodiversity in cities. Green roofs also save energy. Because of the cooling effect that green roofs have, air conditioning is needed much less often or at least to a lesser extent. This saves energy, which in turn has a positive effect on the temperature in the cities. And last but not least, due to the reduction of the temperature on the roof, the efficiency of solar panels increases, which also saves energy. Getting a green roof for your solar panels to be more efficient is a reason why some people purchase a green roof.

Advantage: improved wellbeing

In many ways, green roofs contribute to an increased well-being of not only animals but also humans. For example, green roofs make the air in urban areas cleaner by purifying the air. Pollutants, carbon dioxide and dust are filtered out of the air. This lowers the rates of asthma and other diseases for example. Also, building green roofs increases the urban green space. It is found that green environments have a positive effect on people’s well-being. It reduces stress and instead offers a feeling of relaxation. The enjoyment and comfort of people living and working in areas where buildings have green roofs are improved.

Making the area greener also causes the so-called ‘healing environment’. It appears that patients recover much faster in a green environment, which means that patients have a shorter stay at the hospital. In addition, people’s tolerance of pain is generally higher in green environments.

Disadvantage: structural difficulties

Besides the fact that not all roofs can support the weight of a green roof, the slope of the roof also plays a big role. Only a limited amount of slope is allowed, meaning that green roofs can only be installed on slightly sloped roofs or flat roofs. The maximum allowed slope of a roof is 25 degrees. So, if you have a traditional roof that has a higher slope than 25 degrees, you cannot install a green roof.

Advantage: economical

Besides being beneficial for the biodiversity, environment and wellbeing of animals and people, green roofs are also beneficial for your wallet. Not initially, because installing a green roof usually costs more than installing a traditional roof, but many economic benefits will make up for this in the long term. The R-value, which is a measure of a material’s resistance to heat flow, is higher for green roofs. This lessens the need for using air conditioning and therefore saves energy. Besides having a positive effect on the environment, it also results in a lower energy bill. A study performed by the University of Michigan concluded that over 40 years, a green roof would reduce your energy bills by about €160.000 / $200.000.

Although the installation of a green roof is initially more expensive, it is cheaper to replace a green roof than to replace a traditional roof. The reason for this is that most of the materials that green roofs are made up of can be salvaged. As a result, the replacement costs are circa one-third of the initial costs for installing the roof. Aside from that, green roofs protect the roofing system much better from extreme heat and UV radiation, which results in fewer maintenance costs and extends the life of your roofing system. A green roof can last more than double the years before it needs to be replaced compared to a traditional roof.

To what extent the economic benefits of green roofs account for a specific roof depends on many factors that differ per roof. This includes the location, building, surrounding environment and design.

Disadvantage: leaks in the roof

To prevent the roots of the vegetation from growing through the roof, green roofs have a root barrier layer. But sometimes, it still occurs that the plant roots penetrate the green roof’s waterproof membrane. This causes leaks that could consequently lead to structural damage to your roof. You need to pay to repair the leak, but this is easier said than done. It is far from easy to locate the source of the leak in the roof, let alone to repair it. Leaks occur less often in extensive green roofs since a thin growing medium prevents plants from growing large enough to develop a deep and strong root system.

If you have a green roof, you can have an annual roof inspection done. During such an inspection, possibly problematic vegetation is removed. This reduces the chance that leaks will occur in your roof. Companies are also developing detectors that can trace a leak’s path to the source.

Other advantages

Other advantages of green roofs are:

  • Green roofs insulate sound by absorbing up to 30% of the noise pollution in urban areas.
  • Greening the buildings in cities is visually appealing and attracts the attention of people. The sustainable appearance contributes to an increase in the property’s value.
  • Plants create a fire-resistant layer on the building since plants contain a lot of moisture.
  • The vegetation of green roofs ensures that the roof’s substrate will not be blown away when there is a storm and forms an erosion-resistant layer for the roof.

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