Green Roof Collaborative — Office of Sustainability

Green Roof Collaborative - Office of Sustainability

Green Roof FAQs

By Mara Gittleman

What is a green roof?

Green roofs are roofs with vegetation on them. They can be extensive, which means they have soil 2-6 inches thick and very short plants, or they can be intensive, which means they have at least 6 inches of soil and can hold a larger variety of plants.

What are the benefits of green roofs?

The benefits of green roofs in urban areas are numerous. They have been shown to retain stormwater, preventing the inundation of sewage treatment centers and water bodies. In abundance, they can lower ambient air temperature, mitigating the urban heat island effect (due to the high concentration of paved surfaces, cities can be several degrees warmer than surrounding rural areas). Green roofs can also provide a longer life for the roof itself, filter air pollution, and provide a layer of insulation for the building below, effectively lowering the building’s energy demand. Finally, green roofs can act as community green space, enhancing the wellbeing and aesthetic pleasure found in typically green-less urban areas.

Why is there a green roof on top of Tisch Library?

Colleen Butler, a PhD candidate in the Biology Department at Tufts University, is researching the ecology and physiology of currently used green roof plants as well as candidate species.  In addition, she is interested in designing green roofs to act as habitat for pollinators in addition to functioning as a stormwater best management practice.

How much weight is up there?

There are currently over 20,000 pounds of soil and plant material on top of the library.

Will that shorten the life of the roof?

No. In fact, green roofs have been shown to at least double or triple the life of a roof. This is because they shade the outer membrane of the roof, which is subject to reach temperatures above 150°F in the summer. When shaded, the roof remains at more steady temperatures and does not experience the stress of extreme warming in the day and cooling at night.

How much do they cost?

The price of green roofs varies greatly from roof to roof, depending on soil type, soil depth, plants used, whether or not the plants are irrigated, whether or not the roof is accessible, size of the roof, and many other factors. While the initial costs of green roofs tend to be high, it is important to remember that with so many benefits, green roofs may pay for themselves within a few decades.

By Mara Gittleman and Colleen Butler

Stormwater retention

One of the negative consequences of urbanization is that rain cannot percolate into the ground because of the large swaths of land covered by impervious surfaces, such as buildings, roads and parking lots.  Instead rain flows over the ground and is collected into large stormwater sewer systems.  Green roofs absorb the first few centimeters of rain, where it is taken up by the roots of the plants and released back into the atmosphere.

Insulation

The substrate and plants on a green roof reduce daily temperature fluctuations on the roof and also reduce the overall heat load on the building.  Reduced heat gain correlates directly with reduced air conditioning, lowering energy costs and preventing tons of carbon dioxide from entering the atmosphere.

Longer roof life

Green roofs protect a roof’s waterproofing from the effects of ultra-violet light and extreme temperature fluctuations, thus reducing leaks and prolonging the life of the membrane.

Urban heat-island effect

Cities tend to be warmer than their surrounding environs.  This is due to the replacement of natural ecosystems with impervious surfaces, like roads and buildings, which absorb heat.  In our experiments, we have found that on a hot summer day, the non-green section of roof frequently reaches 50 C, compared to 38 C for a section with substrate only and 27 C for a section with substrate and plants.

Community green space and aesthetics

Preservation of parkland in urban areas has historically been challenging, and housing close to parkland has typically been more expensive, limiting the amount of green space accessible to lower income areas. The installation of green roofs on residential buildings would provide healthier, aesthetically pleasing communal space to more people.

Green Roof FAQs

By Mara Gittleman

What is a green roof?

Green roofs are roofs with vegetation on them. They can be extensive, which means they have soil 2-6 inches thick and very short plants, or they can be intensive, which means they have at least 6 inches of soil and can hold a larger variety of plants.

What are the benefits of green roofs?

The benefits of green roofs in urban areas are numerous. They have been shown to retain stormwater, preventing the inundation of sewage treatment centers and water bodies. In abundance, they can lower ambient air temperature, mitigating the urban heat island effect (due to the high concentration of paved surfaces, cities can be several degrees warmer than surrounding rural areas). Green roofs can also provide a longer life for the roof itself, filter air pollution, and provide a layer of insulation for the building below, effectively lowering the building’s energy demand. Finally, green roofs can act as community green space, enhancing the wellbeing and aesthetic pleasure found in typically green-less urban areas.

Why is there a green roof on top of Tisch Library?

Colleen Butler, a PhD candidate in the Biology Department at Tufts University, is researching the ecology and physiology of currently used green roof plants as well as candidate species.  In addition, she is interested in designing green roofs to act as habitat for pollinators in addition to functioning as a stormwater best management practice.

How much weight is up there?

There are currently over 20,000 pounds of soil and plant material on top of the library.

Will that shorten the life of the roof?

No. In fact, green roofs have been shown to at least double or triple the life of a roof. This is because they shade the outer membrane of the roof, which is subject to reach temperatures above 150°F in the summer. When shaded, the roof remains at more steady temperatures and does not experience the stress of extreme warming in the day and cooling at night.

How much do they cost?

The price of green roofs varies greatly from roof to roof, depending on soil type, soil depth, plants used, whether or not the plants are irrigated, whether or not the roof is accessible, size of the roof, and many other factors. While the initial costs of green roofs tend to be high, it is important to remember that with so many benefits, green roofs may pay for themselves within a few decades.

By Mara Gittleman and Colleen Butler

Stormwater retention

One of the negative consequences of urbanization is that rain cannot percolate into the ground because of the large swaths of land covered by impervious surfaces, such as buildings, roads and parking lots.  Instead rain flows over the ground and is collected into large stormwater sewer systems.  Green roofs absorb the first few centimeters of rain, where it is taken up by the roots of the plants and released back into the atmosphere.

Insulation

The substrate and plants on a green roof reduce daily temperature fluctuations on the roof and also reduce the overall heat load on the building.  Reduced heat gain correlates directly with reduced air conditioning, lowering energy costs and preventing tons of carbon dioxide from entering the atmosphere.

Longer roof life

Green roofs protect a roof’s waterproofing from the effects of ultra-violet light and extreme temperature fluctuations, thus reducing leaks and prolonging the life of the membrane.

Urban heat-island effect

Cities tend to be warmer than their surrounding environs.  This is due to the replacement of natural ecosystems with impervious surfaces, like roads and buildings, which absorb heat.  In our experiments, we have found that on a hot summer day, the non-green section of roof frequently reaches 50 C, compared to 38 C for a section with substrate only and 27 C for a section with substrate and plants.

Community green space and aesthetics

Preservation of parkland in urban areas has historically been challenging, and housing close to parkland has typically been more expensive, limiting the amount of green space accessible to lower income areas. The installation of green roofs on residential buildings would provide healthier, aesthetically pleasing communal space to more people.


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