GREEN ROOFS: CHARACTERISTICS, TYPES, BENEFITS, DISADVANTAGES - ENVIRONMENT - 2024

A roof or green roof is the upper deck of a building or house where a topsoil has been established. This technique has its antecedents in the old living grass roofs of Norway and gained momentum in Germany during the 1960s of the 20th century.

The purpose of green roofs is to save energy, regulate temperature, filter the air, absorb CO2, and efficiently manage rainwater. Therefore, they are technologies with an ecological function and not simply areas of the roof where potted plants are arranged.

Green roofs in the City Hall in Chicago (United States). Source: TonyTheTiger

To make a green roof, a special preparation of the support on which the crop is to be established must be carried out. It consists of a layered system at the base of which is an additional waterproofing layer of the upper roof deck.

Subsequently, a succession of layers is placed that allow drainage, prevent the development of the roots towards the ceiling and provide a suitable substrate for the plants.

There are green roofs of various types, such as extensive roofs that are low load and low maintenance, with herbaceous or succulent plant cover. Likewise, there are intensive ones with high load and maintenance that include from grasses to trees and semi-intensive ones that are intermediate variants.

Green roofs provide various benefits such as thermal regulation, energy saving, air purification, use of water, recreation, among others. The disadvantages are the structural risks that they can imply for buildings and maintenance costs.

characteristics

Church with green roofs in Iceland. Source: Ira Goldstein

History

The antecedents of green roofs are located in Norway between the seventeenth and nineteenth centuries when the roofs were covered with earth and grass was laid. This technique was used mainly to regulate the temperature of the house.

Later, during the 19th century, settlers in the United States used this technique to solve the shortage of wood for the roofs of houses.

Also, in Germany in the same nineteenth century houses were built with roofs covered with tar as waterproofing that caused devastating fires. Because of this, roofer Koch proposed covering roofs with sand and gravel to reduce fire hazards.

This type of substrate allowed the development of herbs in a natural way that covered the entire roof and made it waterproof and very resistant. In fact, by 1980 there were still houses with these original roofs in good condition.

The modern rise of green roofs developed from initiatives in Germany in the 1960s. It is currently estimated that about 10% of roofs in Germany are green.

This trend has spread to many countries both in Europe and America where you can find important buildings with green roofs. Among these we have the Frankfurt airport (Germany), the Vancouver Public Library (Canada), the Palais Omni Sports in Paris (France) and the Santander Bank in Madrid (Spain).

Components

A green roof is made up of a layered system made up of a series of layers with defined functions. These functions are to prevent seepage, drain water and provide substrate for plants.

Waterproofing layer

A central element in green roofs is to prevent seepage since the vegetation cover retains a large proportion of water. Even when some of this moisture is consumed by plants, the surplus must be properly dislodged.

In addition, it must be ensured that the waterproofing is long-lasting since repair costs are high because the entire upper system has to be dismantled.

Drainage layer

Above the waterproofing layer should be established a layer designed to allow the drainage of excess water. It is a structural layer that allows the movement of water between the lower waterproofing layer and the upper anti-root layer.

Anti-root layer

One of the risks of green roofs is the potential damage that roots represent to the roof structure. The roots can be highly developed and can cause problems to the roof covering such as leaks or more serious structural damage.

For this reason, it is necessary to place a layer that prevents the development of the roots beyond the substrate layer.

Substrate layer

Plants require a substrate in which to take root and that supplies them with the essential nutrients for their development and survival. This substrate can consist of a soil mixture or an absorbent artificial substrate that is irrigated with nutrient solution.

Topsoil

Finally, the selected species are sown on the layer of substrate that has been established. The selection of the species to be planted will depend on factors such as climatic conditions, structural conditions of the roof and the established design criteria.

Irrigation system

Depending on the climatic conditions in the area, the green roof will likely require irrigation at least one period of the year. If required, drip irrigation is recommended for the most efficient use of water.

Types

Green roofs in France. Source: SiGarb

Extensive green roofs

They are low maintenance, are generally installed in inaccessible places and mainly include herbaceous and succulent plants. On the other hand, depending on the selected species and the geographical area, it may not be necessary to apply irrigation or fertilization.

The thickness of the vegetal substrate is 5-20 centimeters because the sown species have shallow root systems and grow horizontally. Its maximum weight completely saturated with water does not exceed 200 kg / m2 and its maturation process lasts around four to six months.

Intensive green roofs

They are generally designed to be accessible to people and can be used for recreation. Their maintenance is intensive and require constant irrigation and fertilization.

This type of green roof can include a wide range of biotypes and species from trees, shrubs of different sizes and herbaceous plants. The design possibilities are very great and foliar tones and flower colors can be combined.

It is recommended to use species that are adapted to the climatic conditions of the project site. The plant substrate layer is from thirty-five centimeters to more than one meter.

The structural load in this type of roof can range from 250 kg / m2 to 1,200 kg / m2 and its maturity can take several years.

Semi-intensive green roofs

Combine both designs and divide the load according to the structural characteristics of the property. The thickness of the substrate ranges from 12 to 30 cm and the weight of these installations can vary between 120 and 250 kg / m2.

Green roofs by purpose

Likewise, green roofs can also be classified according to their particular use. Therefore, there are green roofs for gardens, water accumulators, power generators and for recreation, among others.

How to make green roofs

Green roofs in the Faroe Islands (Denmark). Source: Erik Christensen, Porkeri (Contact at the Danish Wikipedia)

There are many material and design alternatives for the establishment of a green roof. The selection of the most appropriate depends on the structural characteristics of the property, available budget and use.

It should be taken into account that a suitable design and materials will allow its long-term enjoyment and with relatively low maintenance costs.

The most complex phases in establishing a green roof system are waterproofing, structural bearing capacity, and drainage water management. If you do not have the appropriate technical knowledge, it is best to use the services of a specialist.

Establishing a green roof step by step

1.- Define the purpose and design

The first thing is to define if the green roof will be ornamental, food or for the cultivation of medicinal plants. In addition, it must be taken into account that the size of the plants to be used is limited by the load capacity of the roof.

2.- Structural evaluation

An evaluation of the structural characteristics of the building must be carried out to know its load capacity. For this, it is best to consult a civil engineer, architect or master builder.

3.- Selection of plant species

The selection of plant species to be cultivated is determined by the use of the green roof, the structural limitations of the building and the climate of the area.

It should be taken into account that the incidence of solar rays, the temperature and the wind regime are affected by the location and height of the property. When selecting food plants, additional care may be required to control pests and diseases.

In intensive green roofs it is possible to apply the full range of landscape design possibilities combining annuals and perennials with different flowering periods.

For extensive roofs, grasses are generally used, which require more frequent watering during the summer. When it is not possible to have an irrigation system, it is recommended to use succulent plants such as cactaceae or species of Sedum, Sempervivum or Delosperma.

4.- Correction of original waterproofing and roof slope

It is essential to check the condition of the waterproofing layer of the external roof covering and to assess whether the surface is well leveled. If there are any problems, the waterproofing layer must be repaired or replaced and the necessary level corrections must be made.

5.- Special waterproofing

Establishing a green roof requires an additional waterproofing layer that is long lasting to avoid having to replace the system. For this, waterproofing systems must be installed with guarantees of up to 10 or 20 years and a useful life of 40 to 50 years.

This special waterproofing includes the installation of a layer of high-density polyvinyl chloride (PVC). When it comes to a roof (flat roof), it is essential that there is adequate leveling in the direction of drainage to avoid the accumulation of water.

6.- Anti-root layer

It is necessary to install a layer over the drainage system that prevents the roots from entering the lower layers. This prevents clogging of the drainage or damage to the waterproofing layer.

The anti-root layer is resistant and filtering since it must allow the passage of water and prevent the passage of the roots.

7.- Drainage

A drainage layer must be placed that allows the passage of water, which may consist of a corrugated sheet or corrugated plate.

Likewise, a structural mesh with a filter layer can be used that allows water from the upper substrate to pass through and disperse it to the drain outlets.

8.- Filtering layer

Above the drainage layer it is convenient to include a filter layer that prevents the passage of coarse particles of the substrate that could obstruct the water outlet.

9.- Substrate

Subsequently, on the filtering and anti-root layer, the substrate layer is deposited, whose composition will depend on the type of plants to be established. It is very important that the texture of this layer guarantees adequate moisture absorption without excessively retaining water.

The most suitable substrate is a horizon system that has a layer of sand and fine gravel at the base and on top of this a mixture of clays with fertile soil.

10.- Sowing

Once the substrate has been established, planting of the selected plant species should be carried out. For some plants such as grasses or lawns, pre-seeded rolls can be placed and quickly set to the substrate.

In the case of other plants, the seeds or seedlings obtained in the nursery can be sown directly.

10.- Maintenance

Generally green roofs require very little maintenance such as periodically checking the drains to verify that they are working properly.

On the other hand, it may be necessary to irrigate at least during the dry period to ensure proper plant development. In this case, the most suitable irrigation systems are sprinkler or drip.

Benefits

Thermal regulation and energy saving

In cities there is a great abundance of concrete and asphalt as well as air conditioning equipment and vehicular traffic that produce an environment of high temperatures. Therefore, the so-called albedo effect or urban heat island effect occurs.

Albedo is a measure of the amount of solar energy that is reflected by a surface and therefore is not absorbed as heat. Urban areas have an albedo that is 10% lower than rural areas.

In this sense, green roofs help regulate the temperature of buildings by reducing the incidence of ultraviolet rays on the exterior roof covering. It has been estimated that using green roofs can reduce the use of air conditioning devices by up to 40%.

CO2 absorption

Green roofs help reduce the greenhouse effect and reduce global warming. This is because plants are carbon sinks, since they trap the CO2 generated in the city to carry out photosynthesis.

Air purification

Plants are natural air filters as they absorb CO2 and release oxygen and thus help purify urban air. On the other hand, green roofs have been found to reduce sulfur dioxide and nitrous acid in the air by 37% and 21% respectively.

Use of rainwater

When rain falls on a conventional roof, it directly impacts the outer covering causing an erosive effect. Likewise, when faced with a smooth and uncovered surface, the flow rate and speed of movement increases.

Another problem in cities is the events of saturation of the sewage system (overflow) that produce water courses capable of carrying a large amount of waste. These wastes can end up in rivers or the sea and generate pollution.

For example, in New York City it is estimated that 50% of rainfall events end in overflows. It has been estimated that they provide 40 billion gallons of untreated water annually.

On the contrary, in a green roof the vegetation layer and the substrate cushion the impact of rainwater. In this way a part of the flow is absorbed and the drainage speed is reduced.

In addition, the green roof system protects the waterproofing layer, reduces the risk of sewer overflow and prolongs the useful life of the drainage system.

Increases the useful life of waterproofing

The external covering of a roof is subject to large temperature variations, particularly in areas with marked seasonal changes. In a study it was shown that an uncovered roof can suffer diurnal variations of up to 50ºC and with a green roof system it is reduced to only 3ºC.

Therefore, a well-managed green roof contributes to increasing the service life of the waterproofing of buildings. The vegetation cover layer moderates changes in temperature and protects against solar radiation.

Improves acoustics

The vegetal layer dampens urban noises and improves the acoustics of the area. In this way it contributes to the sound insulation of the property.

Decorative elements and spaces for recreation

Green roofs with a suitable landscape design are a relevant ornamental element. On the other hand, in the case of green roofs they become a place of recreation.

They provide food and natural medicine

In the green roofs it is possible to grow food and medicinal plants that can be used for the consumption of the inhabitants of the building. However, it should be noted that these plants require additional care to ensure effective production.

Property valuation and tax savings

According to international studies, when a green roof is placed, the capital gain of the property can increase by up to 15% in the real estate market. This is because they generate significant energy savings and improve the quality of life.

Also, the investment in the installation of green roof systems can be treated as a maintenance expense. Therefore, this investment may be tax deductible.

Disadvantages

Risks of leaks or structural damage to the building

If not placed properly, green roofs can cause moisture problems, leaks, or even structural damage to the building. Some species of plants have an aggressive root system and can reach the outer covering of the roof and cause damage.

On the other hand, the green roof system represents an additional weight on the building that must be carefully considered in order to avoid accidents.

High setup costs

The average cost of setting up a green roof can be two to three times that of a conventional roof.

Requires permanent attention

A traditional roof only requires adequate periodic supervision to assess the condition of the waterproofing mantle. In the case of intensive green roofs, maintenance must be regular to guarantee their proper functioning.

References

1.- Barcelona City Council. Living roofs and green roofs guide. BCN. 41 p.

2.- Castleton, HF, Stovin, V., Beck, SBM, & Davison, JB (2010). Green roofs: building energy savings and the potential for retrofit. Energy and Buildings 42: 1582–1591.

3.- Getter, KL, & Rowe, DB (2006). The Role of Extensive Green Roofs in Sustainable Development. HortScience 41: 1276-1285.

4.- Gómez-Velázquez JA (2014). Analytical criteria for the valuation of sustainable real estate. Green roofs and walls Presents. L National Congress of Valuation Valuation: Present, Past and Future. Guanajuato, Mexico. 34 p.

5.- Mentens, J., Raes, D., & Hermy, M. (2006). Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning 77: 217–226.

6.- Oberndorfer, E., Lundholm, J., Bass, B., Coffman, RR, Doshi, H., Dunnett, N. Rowe, B. (2007). Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services. BioScience 57: 823-833.

7.- Zielinski S, García-Collante MA and Vega-Patermina JC (2012). Green roofs. A viable tool for environmental management in the hotel sector of Rodadero, Santa Marta? Management and Environment 15: 91-104.