Green Roofs

A stunning "green roof" designed by Canadian, Martine Brisson, creates and environment that defies the fact that it is actually on a rooftop. Photo: Martine Brisson.
A stunning “green roof” designed by Canadian, Martine Brisson, creates and environment that defies the fact that it is actually on a rooftop. Photo: Martine Brisson.

Green Roofs are Energy Efficient and Environmentally Friendly

One of the most exciting emerging roof technologies that is gaining popularity worldwide is the green roof, an energy efficient concept that is guaranteed to keep your home cool when the weather is hot, and warm when temperatures plummet.

And it doesn’t end there. Green roofs are attractive and they really are environmentally friendly. Added value in an urban environment is that they increase green space, and depending on the design and construction methods used, can reduce and slow down the runoff from storm-water and also filter pollutants out of rainwater.

While it is true that green roofs are currently more expensive to construct than conventional roofs, this extra cost may be offset by reduced energy costs as well as potential savings in storm-water management.

What is a Green Roof?

In simple terms, a green roof is simply a roof that is created by growing a layer of vegetation on a rooftop.

They are nothing new. Both turf and moss coverings have been used for roofs in Iceland and Scandinavian countries for centuries. This was largely due to a lack of other suitable building materials, and the fact that sods and moss on roofs – and walls – were (and still are) excellent insulators.

The vegetation provides shade and removes heat through a process of “evapotranspiration” (where plants absorb moisture through their root systems and then emit this moisture through their leaves). Further, the surface of the roof is cooler than the ambient air, while conventional roof surfaces can become as much as 90 °F/ 50 °C hotter than the air.

Contemporary green roofs generally incorporate a damp-proof membrane and drainage layers.

Types of Green Roofs

Generally, green roofs are described as being either intensive or extensive.

Extensive green roofs are lightweight and similar to the traditional alpine types. They are also very low maintenance and don’t generally require any form of permanent irrigation to keep the vegetation alive and growing. They are generally established on sloping roofs from 9.5 degrees to about 30 degrees and are not expected to be accessible.

Extensive roofs are also referred to as low profile green roofs.

Intensive green roofs are similar in many ways to roof gardens, where any type of plants may be grown, including trees and shrubs. They are considerably heavier in structure, so are more complicated to build, and require a lot more maintenance than either extensive green roofs or conventional roofs.

Intensive roofs are also referred to as deep or high profile green roofs.

The Benefits of Green Roofs

While certainly “alternative”, this is one of the most energy efficient roofing types available. Green roofs have a number of related benefits that reduce energy usage:

  • Heating and cooling of the buildings they shelter, reducing energy costs.
  • Absorption and storage of relatively large percentages of heat when the roof is wet, leading to reduced fluctuations in temperature.
  • Insulation of buildings when the roof is dry, decreasing the amount of heat that flows through the roof, and reducing the energy needed to cool the interior in summer.
  • Insulation of buildings in winter that reduces the heat lost through the roof.
  • Reduction of surface temperatures so that the roof is cooler than the ambient air in the heat of summer.

Energy Savings of Green Roofs

The increasing interest in green roofs has led to a number of studies to estimate just how much energy can be saved.

A Canadian study of a 32,000 sq ft/ 2,980 sq m green roof on a single story commercial building in Toronto estimated that energy savings for heating were about 10%, and 6% for cooling. It calculated that if the roof had been on a building in Santa Barbara, California, the savings for cooling would have been as high as 10%.

A study undertaken in Florida measured the annual energy savings of a green roof compared to an adjacent light-colored conventional roof. It found that the average heat transfer rate in summer through the green roof was 40% less than the next-door roof. In winter, this percentage rose to 50% for the green roof.

A study of green roofs undertaken at the University of Michigan in 2008 compared the cost of a green roof with conventional roofs. It also analyzed the benefits, including storm-water management and reduced pollution that led to improved public health. The cost figures this study cited were $464,000 for the green roof compared to $335,000 for a similar sized 21,000 sq ft/ 1,950 sq m conventional roof. But over time, the study stated, the green roof would save as much as $200,000; about $130,000 of which would be saved in reduced energy costs.

A Green Roof in South Africa

The Greenhouse Project in Joubert Park, Johannesburg, has created a sod roof as part of their Potting Shed Project, that involved experimenting with sustainable building methods – or if you will, “green building”. It involved revamping an old potting shed, and entailed a variety of processes including water-saving technologies and energy conservation strategies.

Part of the project involved construction of a sod roof made with earth and plants, that provides insulation above the bathroom, kitchen and a storage area. According to The Greenhouse Project’s booklet, Green Building: Turning an old potting shed into a living, breathing, people-centred office (Johannesburg, 2004),  the building is protected from the wet earth above by a layer of bitumen and aluminum waterproof paint, as well as biddum – a geo-fabric commonly used in the building industry. The biddum was laid on top of the painted concrete and then this was covered with gravel. Drainage was provided via gargoyles made from half-clay pipes, that slope downwards, directing the water away from the windows and walls of the shed.

Green roofs
How the sod roof was constructed. Drawing by Vanessa Black.

While the biddum works well, they state that if they were to do the project over again, they would prefer to use alternative materials that have since become available, including hemp material, or “bio jute” soil blankets.

Roof plants on the sod roof include drought-resistant Senecio macroglossis and bulbinella species. 

The “sod roof was a complex and expensive exercise for the small amount of garden it yields. Its contribution to energy efficiency is limited by the fact that heat is gained and lost through three skylights. However, the garden covering does prevent the hot sun from drying out the waterproofing which typically results in failure through cracks”.

About Penny

Penny Swift is a highly regarded journalist and author of books relating to homes and construction.

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