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Geosynthetics in green roofs

The role of geosynthetics in effective, safe and reliable installation of green rooftops.

Features | August 1, 2022 | By: Asenet Lozano

To talk about geosynthetics on green rooftops is to talk about an opportunity to change and make an environmental improvement in the world. The growing need to create awareness of an ecological responsibility and the environment has allowed this concept to spread more successfully.

FIGURE 1 8,611 square foot (800 m2) extensive green roof central Mexico City.

The hanging gardens of Babylon, built in the late eleventh-century B.C., are considered the first that existed. Since then, this trend has grown for the better. The first green rooftop was developed in 1960 in Germany. Then it spread to other European countries and eventually the whole world.

But what are green rooftops? 

We are talking about natural vegetated gardens located on the roofs of houses or buildings, which aim at sustainable urban development. They can also create spaces for recreation and leisure that contribute to improving the life quality and environment of those who can enjoy their benefits.

There are three types of green roof covers: extensive, semi-intensive and intensive. They can be differentiated by the amount of vegetation used (thickness of substrate density and size of vegetation). An extensive green roof can support up to 25 pounds per square foot (122 kg/m2); an intensive green roof can hold up to 150 pounds per square foot (732 kg/m2). 

The type of maintenance can be another factor in differentiating green rooftops. An intensive roof takes much more work in terms of managing the different types of plants separately and maintaining them. On the other hand, an extensive green roof is allowed to grow naturally without restrictions. 

The intensive green roof allows you to choose the vegetation, taking into account style and aesthetics. An extensive green roof is designed to be maintained only once a year, with slightly more “wild” vegetation.

Green roofs provide several environmental benefits: 

  • Air quality: 11 square feet (1 m2) of vegetation cover generates the oxygen required by a person throughout the year. Vegetation, both on a green roof and on any other surface, produces oxygen and consumes carbon dioxide through photosynthesis, which improves the air quality we breathe.
  • Biological corridor: As the green roof concept spreads, flora and fauna will once again be present throughout the city, including urban areas that have experienced deforestation. 
  • Fight the heat island phenomenon: Green rooftops are spaces that can absorb the sun’s rays and counteract the phenomenon of heat islands—hot air domes that form when the sun’s rays bounce against the concrete and paved surfaces that have replaced areas of vegetation.
  • A more comfortable local climate: Green rooftops create a thermal system that reduces temperatures up to 47°F (8°C) on the roof and from 36°F to 43°F (2°C to 6°C) on the floor immediately below the green roof of the property in warm months and that helps retain heat generated inside the property in the cold months.

Green roofs also generate economic benefits:

  • Reduction of operating expenses: Direct savings are experienced in electrical energy due to the decreased use of air conditioning.
  • Increase in the capital gain of the property: Green rooftops increase commercial property values by an estimated 15%.
  • Permanent waterproofing: Having a green roof eliminates the need to change waterproof barriers.
  • LEED: Green rooftops provide points that can be applied to LEED and many other certifications.

Design standards

The following factors should be considered when designing and placing a green roof:

  1. Objective. Being clear about the purpose of placing a green roof is of utmost importance since everything starts from how you are going to use it. Once you have established a goal, you can start choosing the type of vegetation and the design for your green roof. 
  2. Current type of construction. It is necessary to ensure that the slab on which the green roof will be placed is in optimal condition. 
  3. Location. The place where the green roof will be located helps determine the type of vegetation you should use due to climatic aspects, wind speed, sun exposure, temperature and relative humidity, among other factors. 
  4. Geosynthetics. A green roof can generate multiple benefits. However, if the materials used are incorrect or the green roof is poorly executed, it could be counterproductive, creating many problems such as the growth of fungi and bacteria, the appearance of dark spots on ceilings and walls due to moisture and saltpeter, high humidity, breathing problems for occupants, corrosion and rust of metals, rooftop flooding and high repair costs.

To prevent such problems, it is essential to consider using geosynthetics for waterproofing instead of a traditional system or material. Geosynthetics play an extremely important role in green rooftop projects, making installation easier and extending the project’s useful life, so it is essential to have high-quality materials. In addition, professional installation can help avoid future problems. 

FIGURE 2 2,260 square foot (210 m2) intensive green roof north Mexico City

Geosynthetics used in green rooftops

Geosynthetics are a fundamental part of green rooftop projects that offer many benefits compared to traditional solutions and materials.

Geomembranes

Using a geomembrane for green rooftop projects is more efficient and a safer alternative, especially in the long term, than traditional waterproofing materials, which do not always solve potential problems. These can include small fissures in the slab caused by soil settlements on the foundation, excess weight or even earthquakes; such microfractures are almost impossible to perceive, but they are water filtration points that can cause serious problems.

Using a geomembrane as a waterproofing system, independent to the slab (floated system, not adhered), will prevent water or some fluid leaking. This is because geomembranes are synthetic waterproof monolithic sheets. In addition, geomembranes work as an anti-root barrier, preventing roots from creating cracks or structural damage, something that traditional waterproofing typically cannot achieve. 

The waterproofing system with geomembrane is considered permanent, since its correct installation can be corroborated with a system for electrical conductivity tests:

  • Electric charge, capable of detecting imperceptible perforations
  • Negative pressure, performed in extrusion welds
  • Pressured air channel, in thermofusion welds
FIGURE 3 10,441 square foot (970 m2) extensive green roof on a corporate building

Geocomposites

Geocomposites are used on green rooftops as drainage systems. Their function is to efficiently evacuate fluids, since they create a ratio of voids greater than that found within the soil. They are designed to be able to handle large pressure loads without affecting their draining capacity. 

There are several advantages to using geocomposites as drainage systems on rooftops. They significantly reduce the costs of storage and placement of materials, prevent flooding, do not require specialized labor for their proper installation, and reduce the time of placement compared to other drainage systems. In addition, geocomposites are a very light material made from polymers, avoiding the traditional and heavy stone system for drainage. Geocomposites allow water to run quickly and efficiently while retaining the fine soil particles, so that the drainage network that goes in the middle does not clog.

Geotextiles

Geotextiles work as a soil separator to maintain integrity and proper functioning between the different layers of the soil. It also serves as a filter for existing fluids by allowing the passage of fluids but not solid particles.

Geotextiles are materials that help prevent the migration of fine soils within the granular and this contributes to maintaining its function, such as drainage, structural or finishing. 

To choose the right geotextile, it is important to consider various factors: the mechanical and physical characteristics of the terrain and the filling material, size of the aggregate, bulk density, internal friction angle and cohesion.

Geotextiles on green rooftops can fulfill several functions such as serving as anti-vegetative layers and preventing the growth of unwanted weeds, and as a protective layer of the geomembrane against granular soils that could cause damage to the geomembrane by punching. They can also be used for vegetation separation according to the terms of the design. Geotextiles are versatile and flexible; they can adapt to the irregularities of the surfaces on which they are installed.

FIGURE 4 (ABOVE) 6,598 square foot (613 m2) extensive green roof for meetings on a corporate building

Geocells

Geocells are usually used on roofs when the slabs have a slope greater than 15 and up to 60 degrees, since they can confine the filling material, performing as a surface erosion control system. The best performance from geocells is achieved on gabled roofs or when the design involves volumes on the roofs for a natural effect or an organic design. They also allow hydroseeding, protecting the roots. 

Using this system provides several benefits, beyond the reduction of costs and installation work time, compared to other types of systems. Its transportation is extremely simple, it offers a better aesthetic finish, the installation does not require specialized labor, it generates surface stability and erosion control, and it minimizes the excess accumulation of water, avoiding overweight.

Geogrids

Geogrids can be used in green roofs as soil reinforcement to improve their mechanical properties, especially in cases when the substrate layer is thick, since geogrids have openings that allow an adequate interlock between the soil particles and the geogrid. Their resistance to stresses and to biological and chemical conditions of the soils makes geogrids the best option for these cases.

Regarding placement, these are tended horizontally between layers of soil in order to form an embankment or redistribution of the loads on the structure, absorbing the stresses that the soil does not take.

FIGURE 5 (BELOW) 979 square foot (91 m2) intensive green roof on a residence

Installation of geosynthetics on green roofs

  1. Preparation of the cover

It is important to take the following into account before starting any type of procedure or placement of any material:

  • Although the geomembrane will act as a waterproofing system, it is recommended that the area has a traditional waterproof system prior to installation. This is to prevent direct condensation on the slab from the thermal gradient.
  • The slope on the surface has to be at least 2%.
  • At least one 4-inch (10 cm) diameter rainwater runoff is required every 538 square feet (50 m2).

2. Geomembrane placement

The geomembrane is used as a waterproofing and anti-root system, since it has a high resistance to chemicals and organics as the components of the different substrates, in addition to being a material of long useful life, which is prolonged even more by being covered without exposure to UV rays and the outside.

3. Geomembrane sealing verification process

To ensure the efficiency of the geomembrane and its installation, the following airtightness tests need to be carried out:

  • By electric charge, capable of detecting perforations imperceptible to the human eye
  • By negative pressure, performed on extrusion welds 
  • By pressure channel, on thermofusion welds

4. Placement of drainage system

The drainage system helps ensure the constant, efficient conduction of water, either from irrigation or storms, to rainwater runoff, to avoid stagnation. 

5. Fixing system

It is important to perform a mechanical fixation on the perimeter to ensure the proper functioning of the waterproofing system. Otherwise the water could have access and stagnate under the waterproofing membrane.

6. Substrate layer

A layer of topsoil, substrate or prepared soil is placed, according to each of the species chosen in the plant design.

7. Vegetation layer

It is essential that professionals in the field determine the vegetable selection, since it must meet certain characteristics for better development and performance according to the objective of each green roof project and to guarantee the optimal functioning of the installed geosynthetics materials. There are many types of plants that can damage the geosynthetics, putting at risk the degree of impermeability that a job of this type must have, since water infiltration to the property can cause serious damage.

FIGURE 6 8,632 square foot (802 m2) extensive green roof on a corporate building

8. Maintenance of vegetation

It should not be forgotten that living beings are being placed on the roofs and it is mandatory to take care of them to stay in optimal conditions. Without proper maintenance, there could be a saturation of unwanted vegetation or the extinction of it over time.

Every day we are looking to develop new alternatives to help fight the climatic problems that the world is currently facing. Green rooftops are part of the alternative solutions, since their appearance in large cities could bring many benefits, not only for direct users, but for everyone if their use becomes widespread. 

It is important to have good execution in this type of project, taking into account the disadvantages that would develop if an improper installation is made or the wrong materials are placed. 

Therefore, geosynthetics are the best elements for a great diversity of civil works projects, improving the quality of the infrastructure that humanity demands. 

Architect Asenet Lozano works at Azoteas Verdes in projects and design. She has also participated in the design and architectural projection of governmental and hospital projects. She likes to discover and learn new ways in which green architecture can help humanity. All figures courtesy of the author.

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