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Tuesday, 29 November 2016 10:51

Green roofs: Taking cues from nature

Over the last decade, the uptake of vegetated roofs has increased in South Africa, with architects and building owners looking for ways to create greener and cooler (better insulated) buildings, all while ensuring long-term sustainability and energy efficiency.

The vegetation on planted roofs slows down heat transfer through a process of evapotranspiration, where moisture absorbed by the plants’ roots is emitted by the leaves. The result is that the surface of the roof is even cooler than the ambient air, which helps to slow down heat transfer in and out of a building.

But apart from being cooler and more energy efficient, vegetated roofs are interesting and attractive, and depending on the design, may also reduce and slow down stormwater runoff and filter pollutants out of rainwater. In urban environments, they do much to reduce the urban heat island effect and cool the surrounding air while boosting biodiversity.

The push for green roofs
The popularity of green roofs is growing globally, with some countries offering incentives or even making it a mandatory practice.

Last year, France announced a law stating that the rooftops of all new commercial buildings must be covered with either plants or solar panels, at least in part. Almost a decade ago, Toronto in Canada adopted a by-law that requires green roofs on new commercial, institutional and residential developments with a minimum gross floor area of 2 000m2. Also in Copenhagen, as part of its overall strategy to become a carbon-neutral city by 2025, the city has adopted a policy that mandates a vegetated covering on all new buildings with roof slopes of less than 30 degrees.
Local industry specialist, Clive Greenstone from Green Roof Designs, notes that in South Africa, although there are no formal financial incentives or policies that guide or coerce the installation of green roofs (at the moment the only real encouragement is the Green Star building credits), homeowners and businesses are open to the idea, especially on new builds. While their decisions seem to be mainly aesthetically-driven, the end-product comes with all its benefits.

And although green roofs may be more expensive to put up than constructing conventional roofs, the extra cost may be offset by reduced energy costs and potential savings in stormwater management.

How much energy can be saved?
While local research is still lacking, an experimental Spanish study* done on an extensive green roof in a Mediterranean coastal climate zone over a period of three years shows that a green roof with high vegetation density acts as a passive cooling system when the roof is highly insulated (U-value = 0,24W/(m2 K)). In these conditions, the incoming thermal gain is about 60% lower than when the roof has no vegetation.

Another study** that compared the annual energy savings of a green roof to that of an adjacent light-coloured conventional roof in Florida, USA, found that the average heat transfer rate in the summer was 40% less through the green roof than the conventional one. In the winter, the number increased to 50% for the green roof.

* F. Olivieria, C. Di Pernab, M. D’Orazioc, L. Olivierid, J. Neilaa (2013) “Experimental measurements and numerical model for the summer performance assessment of extensive green roofs in a Mediterranean coastal climate” Energy and Buildings [Electronic] vol 63, August, pp. 1-14, Elsevier B.V. Available: www.sciencedirect.com/science/article/pii/S0378778813002235

** Cummings, J., C. Withers, J. Sonne, D. Parker, and R. Vieira. 2007. “UCF Recommissioning, Green Roofing Technology, and Building Science Training; Final Report” Available: www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1718-07.pdf [Referenced by sans10400.co.za/green-roofs/]

Four things to consider
Before designing a living green roof, Greenstone points out that there are certain factors that need to be addressed in order to ensure a successful installation and longevity:

1.    Structure
Most importantly, the building needs to be able to support the roof structure. When a green roof is planned for a new build, it is easy to plan and design for the extra weight of the vegetation and soil, but when an existing roof is retrofitted into a planted roof, assessing the structural loading capacity of the roof is incredibly important to ensure the safety of the building occupants.

2.    Slope
The design for a flat green roof will differ from that of a sloped roof. Although some may consider flat roofs as the best suited for a planted roof, water may collect and pool, seeing plant root systems damaging roof materials to get to the water.

3.    Setting
A building’s location plays a significant role with regards to plant selection. Especially in a dense urban environment, buildings can shade each other for big parts of the day. Wind strength and patterns also affect plant growth. In addition, designs on roofs that are very visible would typically be neater than those that are not in the public eye.

Identifying micro-climatic zones on a rooftop:
Element    Description
Regional climate    
•    The general climate of the region must be considered when choosing plants for a green roof habitat. For example, coastal vs inland.
•    Sloped roofs which face south or west experience less direct sunlight, and are therefore cooler and wetter.
•    Sloped roofs which face north or east experience more direct sunlight, and are therefore warmer and drier.
•    Plants in exposed areas of a rooftop experience higher wind influence. Wind stresses plants by increasing evaporation off their leaves, and damaging foliage and branches.
•    Some areas of the rooftop may be permanently or periodically shaded by surrounding buildings.
Source: Creating space for biodiversity in Durban: Guideline for designing green roof habitats.

4.    Plant selection
Selecting the wrong types of plants, such as grasses and other non-succulents, in areas that experience times with no rainfall can become problematic to the upkeep of the roof.

Types of green roofs
Turf and moss coverings have been used on roofs for centuries, but modern-day green roofs feature a wide range of plants and even trees. Rooftop gardens are also created to provide people with food or recreational spaces.

Generally, planted roofs fall in two main categories, intensive or extensive roofs, and incorporate a damp-proof membrane, drainage layers, a growing substrate and a variety of plants.

Intensive green roofs (intricate and lush) are closer to roof gardens, and may include flowery shrubs, trees and more in a thicker substrate, which makes them considerably heavier. They are normally created on flat, accessible roofs that provide amenity space and require significant maintenance.

Extensive roofs (self-sustaining ecological roofs) are normally established on sloping roofs from 9,5 degrees to about 30 degrees, and have no or limited human access. Self-regenerative plants, which are tolerant to heat, drought, wind and frost, are selected. They are typically lightweight with a shallow substrate and are low maintenance without any form of permanent irrigation.

Modular systems such as individual trays set in a grid on top of a watertight roof are a flexible option for the creation of a green roof since they are easily movable, they can be pre-planted and they do not require as many preparation layers. Another lightweight choice is thin mats that are fully vegetated with low-maintenance sedums and moss plants, which are rolled up and installed like sod.

What doesn’t work?
For the successful creation of an extensive, low-maintenance green roof, Greenstone stresses the importance of consulting a green roof specialist as well as a structural engineer, whether the plan is to create a green roof on a new build or a retrofit.

“A green roof requires proper drainage layers, the correct soil medium, appropriate plants that are naturally found within a 50km radius of the building and should not require any irrigation if done correctly,” he states. “I have green roofs that I have not watered or maintained for about four years,” he adds.

“Also, don’t try to cut corners. Not all landscapers understand what a successful green roof entails – it is a specialised installation. Therefore take the advice of a green roof specialist and make sure the installation is done by the correct person or company.”

Full thanks and acknowledgment are given to Green Roof Designs, sans10400.co.za/green-roofs, www.rooftoplandscapes.co.za and www.sciencedirect.com for the information given to write this article.

Key takeaways:
-    The vegetation on planted roofs slows down heat transfer through a process of evapotranspiration.
-    Green roofs can be even cooler than the ambient air.
-    Some countries offer incentives for green roofs.
-    The cost for green roofs may be offset by reduced energy costs and potential savings in storm water management.
-    Choose carefully - there are several kinds of green roofs, differing in aesthetics, weight and maintenance requirements.
-    Get a specialist’s input to ensure a successful installation.