Green Roofs at Harvard
Harvard Green Roof Case Studies
Harvard Green Roof Case Studies
There is no easy answer to the question as to the definite number of green roofs at Harvard. The total number depends on how we define a green roof. We have opted for a somehow inclusive definition and are going to call “green roofs” all the roofs that are located below-, at- or above-grade and that are covered with plants that are not planted directly in the “ground”.
However, we are not going to consider the numerous roofs located in the Harvard Business School area atop underground tunnels, green roofs.
If we held to common opinions at Harvard, we would conclude that there are only two green roofs: 60 Oxford Street and 29 Garden Street. Their managers are aware of the singularity of their roofs and appreciative of their environmental and social benefits. Moreover only in these two cases, sustainable consciousness played an important part in the design and creation of the green roof. These are also the only green roofs that are located above grade.
In other cases, the reasons for green roofs were driven by either economics or aesthetics and their managers refer to the roof as “the patio” or “courtyard”. There is clearly no perception of those roofs as “green” and their environmental benefits pass unnoticed.
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GUND HALL TEST (April 2006) The GSD is experimenting with soils and sedum plants on the roof of Gund Hall, to test the feasibility of retrofitting Harvard's existing gravel roofs with green roofs. The system is 3,000 square feet (20% of roof area) and could potentially absorb 500,000 gallons of water per year. The cost was only $1 per sq. ft., and is designed to double the roof life span. The project is being led by GSD Professor Katrin Scholz-Barth, an expert in green roof technology. More information can be found in a recent Harvard Gazette article, "Gund Hall Sports a Thick Head of...Sedum."
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CGIS (2005) This roof, at ground level and above underground space, is 15,000 square feet. It includes grass and maple trees, two to three and a half feet deep, with a multi-layered waterproofing membrane integrated with a soil support system. It is irrigated daily in the summer and some fertilizers are used.
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29 GARDEN STREET (2003) This EXTENSIVE 10,000 square foot green roof cost approximately $50/square foot. A primary impetus for its installation was the views from the adjacent housing. It covers the previously asphalt roof of a parking garage. The system uses a MM6125-EV waterproofing membrane, and was installed by American Hydrotech, who provided a 2 year warranty for plants, 20 years for the roof membrane, and a 2 year maintenance contract.
The system American Hydrotech provided at 29 Garden St. consists of the following:
· Monolithic Membrane 6125 EV (Environmental Grade) FR (Fabric Reinforced) 215 mils
· Hydroflex BR (Root Barrier) 160 mils
· Hydrodrain 300 drainage course 3/8”
· Floradrain 40 Water Retention/Drainage/Aeration layer 11/2”
· System Filter fabric
· Hydrotech Lite Top growing Medium 4” in the field, 12” in shallow planters, 36” in
deeper planters
· Hydrotech concrete pavers and precast curbs & walls
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60 OXFORD STREET (2003) This roof garden is approximately 900 square feet (stone, sand and gravel: 70%; plants 30%), which is 16% of roof area. It consists of soils, crushed granite, pygmy bamboos, amur maples surrounded by birch mulch, which protects their roots from cold and prevents moisture from evaporating. The roof is a combination of intensive and extensive roofing. Water is filtered through gravel. The entire roof garden consists of the same plant species. The trees are Redbuds.
60 Oxford Street management is maintaining the roof garden. There are facility management people coming on a weekly basis to clean the gravel, prune the trees, trim, water the plants and perform other maintenance tasks. In the spring they also do mulching. They come from April through December, for 1 day a week for 3 hours. The roof garden is irrigated.
What is a green roof?
A green roof, also called an “eco-roof” or “living roof” is an extension of the existing roof, covered with vegetation. It can be located below-, at- or above-grade – but in all cases the plants are never planted directly in the “ground”.
No green roof systems are identical but they all include:
- a synthetic, high quality waterproof membrane
- drainage layer
- soil layer and
- light weight medium plants
Ideally, green roofs are low maintenance and their self-sustaining plants and soil do not require neither irrigation, fertilizers nor pesticides.
There are two major types of green roofs:
EXTENSIVE: minimal or no-required maintenance with a restricted variety of plants; resistant to frost, wind and drought; sedum, herbs and grasses; located on almost any flat or low slop roof deck that maximizes water retention
INTENSIVE: regular maintenance required (irrigation, fertilizing, pruning, mowing); greater variety of plants (sod grass lawns, perennial, annual flowers, shrubs, small trees); deeper, heavier and richer soil
In a nutshell, they differ from each other by weight, types of plants, accessibility, costs, maintenance and water requirements.
Benefits
ECONOMIC CONSIDERATIONS
In Europe, where the green roof market established and thriving, prices range from $4-$13 per square foot. In the U.S., prices are much higher, ranging from $10-$25 per square foot. Initial costs are often cited as a deterrent and sometimes discourage consideration of green roofs. Of course, the total cost varies depending on the type of the roof, its depth and size, plants and maintenance.
In spite of higher initial costs, life cycle costing analysis reveals that green roofs provide a payback over the lifetime of a building through:
- a longer material lifespan (the green roof can be expected to last two to three times as long as conventional roofs), which leads to reduced maintenance and replacement costs
- savings on heating and cooling, due to reduced peak energy demand
- potential reduction in stormwater/wastewater charges from the municipality
- potential grants
- potential greenhouse gas emissions trading credits, stemming from energy savings
ENVIRONMENTAL BENEFITS: Air
- Carbon Dioxide/Oxygen Exchange: (1.5 m2 (16.15 ft2 ) of uncut grass, produces enough oxygen per year to supply 1 human with their yearly oxygen intake requirement)
- cleaner air (1 m2 (10.76 ft2) of grass roof can remove between 0.2 kg of airbourne particulates from the air every year)
- reduction of the ambient air temperature (prevention of the heat-island effect as green roofs do not absorb as much heat as conventional roofs)
ENVIRONMENTAL BENEFITS: Water
- cleaner storm-water (removal of approximately 95% of the cadmium, copper and lead)
- storm water management: reduction of the storm water volume and a slower water flow (runoffs from impervious surfaces can cause overflow in sewage systems) “green roofs absorb between 50-95% of the rainfall on the site”
- water-proofing capacity (green roofs act as sponges)
- soil erosion reduction (through mitigation of storm-water volume)
ENVIRONMENTAL BENEFITS:
Resource Conservation
- replacement of an impervious roof surface with a living, breathing and permeable green space
- reduction of the building’s footprint
- increased life-span of a roof (2-3 times)
- more habitat for wildlife
- improved views for neighboring building occupants, as they look at a greenscape instead of asphalt or mechanical equipment
Plants
Ideally, green roofs include low maintenance and native, hardy plants that require no irrigation, fertilizers nor pesticides. The choice of plants depends on geographic and climatic conditions (rainfall, temperature, wind, sun and wind exposure) and on the green roof type (extensive versus intensive). Hardy plants are the best bet as they can survive in atypical conditions and a roof is not a typical place for plants to thrive. Sedum is the most popular choice. There are approximately 1000 varieties to choose from, and many are hardy and drought resistant.
A number of planting methods can be used, ranging from vegetated mats (which provide drainage and moisture retention), sowing seeds to using containerized plants.
Maintenance
Irrigation systems are optional. However, green roofs need additional water during the first few seasons. Sprinklers are the most popular irrigation method but more ecological options are available, such as passive irrigation (a membrane located underneath that stores rainwater) or cisterns used for rainwater catchment.
Cost
In Europe, where the green roofs market is already established and thriving, prices range from $4-$13 per sq.f.. But in the US prices are much higher - $10-$25. Huge upfront costs are an important deterrent, discouraging people from investing in green roofs.
Of course, the total cost varies depending on the type of the roof, its depth and size, plants and maintenance.
Applicable LEED Credits
Depending on design and their integration with other building systems, green roofs can contribute to up to 15 credits under the LEED certification system and can therefore strongly improve LEED’s rating of a given building.
They can lead to direct LEED points in the following areas:
- SS credit 5.1: Protect or Restore Habitat
- SS credit 6.1: Quantity Control
- SSc credit 6.2: Quality Control
- SS credit 7.2: Heat Island Effect: Roof
- WE credit 1.1: Water Efficient Landscaping
- Innovation in Design
However, when integrated with other sustainable building components, green roofs can earn a given building even more LEED points.
Resources
- Berlin: Half of the city's new government buildings have green roofs.
- Tokyo: All new construction uses green roofs, and existing buildings must convert 20 percent of their rooftops.
- Chicago: The city introduced its green roof program in 2001 and now has more than 200 green roofs in some stage of development.
- Portland: In 1994, it became the first American city to pass legislation promoting the use of green roofs.
- Seattle: The city hall and its justice center have rooftop gardens.
- Des Moines: The green roof on the city's newly opened $32 million downtown library can retain as much as 187,500 gallons of rainwater.
Websites
Green Roofs for Healthy Cities
City of Chicago (then search for green roofs)
Books
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Planting Green Roofs and Living Walls, Nigel Dunnett, Noel Kingsbury
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Roof Gardens, by Theodore H. Osmundson
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Sky Gardens, by Signe Nielsen
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Sustainable Landscape Construction, by J. William Thompson, et al