Green Roofs – Bringing Nature to Your Doorstep

What do you tell yourself when you are under a lot of stress? Do you ever feel like “I wish I could run away from the city and spend just a couple of days far away from everything“? Do you find yourself trapped in daily obligations and lack the time to go out in the nature?

Well, now you don’t have to wish for an Armageddon or some other excuse for the workload to stop. You can bring nature directly to your home!

Green roofs are a modern replacement for standard concrete roofs, from which they differ by presence of vegetation of different type and abundance. In some countries (Germany, Japan, Canada, Denmark) there are even laws that require construction of green roofs in the new buildings and this trend is slowly spreading worldwide. Green roof is a good solution, especially for commercial centers, as they can turn completely empty and unpleasant areas into functional spaces that, with their appearance, leave a strong impression on visitors.

60° slope intensive green roof on School of Art, Design and Media at Nanyang Technological University in Singapore.

Intensive green roof on Nanyang School of Art, Design and Media in Singapore. Image source: greenroofs.org

Advantages of green roofs

Esthetic

  • They offer an attractive alternative to standard roofs due to presence of vegetation that attracts different birds, insects, and similar animals.
  • The presence of organic materials soothers strict shapes of solid inorganic materials, while the combination of these elements makes the area relaxing, esthetically beautiful and pleasant to spend time at.

Environmental

  • Green roofs reduce “heat island” effect, creating a favorable micro-climate. If constructed and maintained properly, they can lower the concrete temperature even for about 40°C in summer days. Therefore, the ambient temperature is lower for a couple of degrees during summer months.
  • Vegetation absorbs air pollution and, therefore, air is cleaned from carbon monoxide, lead and other adverse pollutants induced by traffic, industry, etc.
  • Green roofs absorb precipitation and reduce the excess weight on sewage system during rain and snow melting, and this water is being returned to the atmosphere through evaporation. Moreover, investment into sewage system to prevent possible flooding is not necessary, so the city authority also benefit from green roofs.
  • Te reduce noise level for about 50 dB (which is at the rumor level in a restaurant).
  • Except esthetic effect, green roofs offer habitats for birds, insects and small animals, contributing to the conservation of biodiversity of these species.
  • They increase environmental awareness of local population.

Economical

  • Green roofs act as thermo-insulators so the building on which they are constructed is colder during summer and warmer during winter. This contributes to preservation of energy which would normally be spent for air conditioning and heating.
  • Buildings with this architectural solution have a higher market value so the invested funds often pay off when selling the property.
  • Green roofs protect the roof insulation and material from UV radiation and they reduce daily temperature fluctuations, which prolongs the roof system lifetime sometimes even double.

Disadvantages of green roofs

  • Initial investments are mostly higher than for the standard roof system.
  • Green roofs give excess weight on the building.
  • They require maintenance and care for vegetation health as illness of one plant can easily be spread to others.
  • Extensive green roofs have to be maintained every six months to one year depending on vegetation, while intensive roofs have to be maintained every month or two.

Types of green roofs

According to the International Green Roof Association (IGRA), green roofs are divided into three types: intensive, semi-intensive and extensive green roofs. This division is based according to the green roof construction (which depends on the depth of the growing media). Construction choice depends on several conditions such as roof slope, financial investments, maintenance possibilities, climate conditions, purpose of use, construction of the building itself, etc.

Intensive type

The depth of intensive roofs’ growing medium normally ranges from 10-50 cm but it can also be deeper. This allows for installation of vegetation that leaves the impression of real gardens, due to the presence of bushes, flowers, lawns and even small trees up to 4 meters in height, creating an ideal green oasis within urban environment. However, due to the depth of the growing medium, amount of vegetation and presence of drainage layers, these roofs are very heavy (extra weight to the building accounts for around 400 kg/m2). Therefore, it is necessary to install necessary structural elements which would support the green roof construction. A civil engineer should be consulted for these calculations. Moreover, trees have to be additionally secured as there is danger from plucking and falling during storms and high winds. This type of green roofs requires intensive maintenance (every month), irrigation, plant care, etc.

Intensive green roof in Singapore.

Intensive green roof in Singapore. Photo by ‘OffPeakClassic8’ from the Singapore forum

Extensive type

Extensive green roofs are thinner, growing medium depth is from 2-10 cm, and they allow installation of vegetation that is less demanding in maintenance and is suitable for life in extreme environmental conditions (drought, direct sunlight, etc.). these plants are moss, grass, and low growing plants of 5-25 cm in height, and succulents (plants which retain higher amounts of water in their body) are also suitable. These roofs are relatively easy to install, they are economical, easier to maintain and often do not require an additional structural support (extra weight to the building accounts for around 100 kg/m2). Despite everything, extensive green roofs still have high ecologic and energy values. Their maintenance should be done once per year.

Light extensive green roof

Light extensive green roof. Source: nachi.org

Semi-intensive type

This type of green roofs is designed with vegetation of medium height, from 25-50 cm, and growing medium depth is around 20 cm (extra weight to the building accounts for around 250 kg/m2). This type is actually a combination of intensive and extensive green roofs. Maintenance is required every 6 months and plants are usually those from reed family as well as middle-growth sedums, which are easy to maintain.

Semi-intensive green roof.

Semi-intensive green roof. Source: rooflitesoil.com

Green roof construction

Green roof construction layers on a flat roof surface..

Green roof construction layers. Source: greenestate.co.uk

In general, green roofs consist of the following components, although, depending on the green roof type, some of them can have several functions while some can be left out:

1. Vegetation
2. Growing medium
3. Filter mat
4. Drainage layer
5. Insulation
6. Protection mat
7. Root-protection barrier
8. Weatherproof membrane
9. Structural support

The following layers are other standard layers: roof thermal insulation, hydro insulation, etc. This construction applies for designing flat green roofs, where roof slope is less than 8 degrees.

1. Vegetation – the choice of plants depends on the type of a green roof, climate conditions, roof slope, surrounding vegetation, etc. in general, intensive green roofs support a bigger amount and weight of vegetation (such as perennials, bushes, smaller trees, etc.), extensive green roofs support mainly lawns, sedums, smaller perennials, etc., while semi-intensive roofs are a transitional form between the two and support bulrush, smaller bushes, flowers, lawns, etc.

2. Growing medium – the depth of this layer depends on the green roof construction type. It is made of mineral and organic components in an appropriate ratio which is suitable for plant growth. Usually, additional mineral materials are added as the soil is shallow and plants are growing in unfavorable environmental conditions. When a plant has enough food in the soil (mineral materials) its roots will not spread extensively and damage other construction layers of the green roof. Moreover, soils with too much organic material should be avoided as they encourage extra energetic plant growth. The depth of this layer ranges from minimum 2 cm for extensive roofs, to more than 10 cm for intensive ones.

3. Filter mat – prevents fine particles from being washed into the drainage layer, i.e. allows only water and a part of the roof to pass. .

4. Drainage layer – drains the water surplus from the growing media and it allows the root to breathe as the air is available in cavities of this layer. This is important in order to prevent plant roots from rotting due to excess water in the soil. Moreover, it serves to store rainwater which plants use during dry periods. It can be made of different materials: several types of plastic channels, or simply any granulated material such as gravel, brick rubble, lava stone, sand, pumice, pebbles, vermiculite, etc. a special attention should be put to drainage layer material weight and find a compromise among the price, weight, and water retention capability. Drainage layer is not necessary for roofs with slope greater than 7 degrees; moreover, in that case it is unfavorable as it leads to the growing medium drying out too quickly.

5. Insulation – depends on climate conditions, can be left out.

6. Protection mat – protects the root-protection barrier from sharp objects and retains rainwater for plants.

7. Root-protection barrier – prevents roots from penetrating the lower construction layers. This layer is usually coated with root-repelling chemicals, and it is necessary when the weather-protection membrane contains chemically organic materials such as bitumen or asphalt (which contains bitumen), which is the case with built-up roofs and some fluid-applied membranes (see section below). If the weather-protection membrane consists of single-ply rubber (EPDM or butyl rubber) or other inorganic material, root-protection barrier is not necessary.

8. Weatherproof membrane – protects the building’s structural support. There are three types of this membrane: a built-up roof (composed of layers of asphalt roofing felt), a single-ply rubber membrane (e.g. EPDM rubber), or a fluid-applied membrane (which can be applied hot or cold to form a continuous seal). Single-ply rubber membrane can be applied in a form of EPDM rubber or regular PVC rolls (of minimum 1 mm thickness). Before placing the weatherproof membrane, the roof should be cleared of all bulges, and its slope should be minimum 1 degree.

9. Structural support – serves to support the construction of green roof. It can be already built up, but it can also be constructed later to correspond the project. Support can be made of wood, steal, metal, etc.

Special attention should be given to roof drains which have to be protected from sand, plant roots and plant detritus, by using gravel and geo-textile, in order to allow undisturbed water drainage.

Comparative review

The following table shows a comparison among the three basic green roof types:

Type Extensive Semi-intensive  Intensive
Depth (growing medium + vegetation) 6 – 20 cm  12 – 25 cm 15 – 60+ cm
Excess weight 60 – 150 kg/m²  120 – 200 kg/m²  180 – 400+ kg/m²
Plants Mosses, sedums, cactus, herbs and some grasses Several perennials, sedums, ornamental grasses, herbs and small shrubs Perennials, lawns, shrubs and trees
Irrigation No, not recommended Partly, when needed Yes, automatic/manual
Maintenance Low Middle High
Construction Fast Fast Slower
Use Esthetic Diversity, habitat Garden, park
Costs Low Middle High

table source: igra-world.com; livingroofs.org