The Chicago City Hall green roof, completed in 2001, is helping to control both stormwater runoff and the urban heat island effect. RoofScapes, Inc. Installing a green roof involves more than just growing plants on your roof; you will need to consider structural integrity, accessibility, and maintenance issues. Green roofs offer a host of benefits, particularly in urban environments where they can provide a visual connection to nature, reduce the urban heat island effect, and help mitigate stormwater runoff.
Green roofs may also be called living, vegetated, or eco-roofs, and consist of a of a layered system: waterproofing membrane, root barrier, drainage layer, lightweight soil or engineered growing medium, and vegetation. This layered system provides extra insulation for the building, which can reduce heating and cooling demands. Green roofs can offer habitat for native plants and animals, and may also be designed to be accessible to building occupants as an amenity. Although they usually cost more than a typical roof, green roofs protect your underlying roof system from damaging ultraviolet rays and temperature fluctuations, enhancing its durability and extending its life.
Consult with an experienced professional when considering a green roof, and involve the architect, engineer, and landscape architect in an integrated approach to design, code, and maintenance issues.
There are generally two types of green roofs, which are distinguished by soil depth and maintenance needs.
Extensive green roofs have 2–6 inches of growing media, are lighter-weight, are able to support drought-tolerant plants, and require less maintenance.
Intensive green roofs have 6–24 inches or more of growing media, accommodate a wide variety of plant species, are heavier, and require regular maintenance.
When considering a green roof, also factor in accessibility needs for maintenance and occupant use. Extensive green roofs are usually only accessible for maintenance purposes, whereas intensive green roofs are often made accessible for occupant use.
Depending on the soil depth of the green roof, the saturated weight can range from 10 lbs/ft2 to 120 lbs/ft2 or more. On both retrofits and new construction projects, consult with a structural engineer to evaluate your roof load capacity and any necessary structural enhancements. Roof slope will also affect the design of the building. Most green roofs are installed on low-slope roofs. Green roofs with a slope greater than 30% will require special engineering.
Not all plants will stay green on a green roof. Roofs are exposed to intense sun, high winds, fluctuating temperatures, and rooftop plants don’t have the benefit of groundwater or adequate shading. Consult with a landscape architect to select appropriate plants for your roof type, climate, and maintenance requirements. Low-maintenance, drought-tolerant plants such as sedums and succulents are common choices.
Workers above are planting a green roof on a Chiropractic Center under construction in Pennsylvania. The curved roof surface has varying pitch. Roofscapes, Inc. At least one year of irrigation may be needed for plants to become established. This should be factored into the design and maintenance costs. Weeding, monitoring leaks, ensuring proper drainage, as well as clearing drainage pipes and cleaning sediment filters (where applicable) should all be part of a regular maintenance plan. Provide access to the green roof for regular maintenance.
Installation costs can range from $9/ft2 to $25/ft2 for extensive roofs and $25/ft2 to $40/ft2 or more for intensive roofs. Variables include type of system, depth of growing media, and vegetation used. Modular or tray systems can be more expensive than rollout or integrated systems.
Code requirements vary, so check with local authorities for specific requirements in your area. For example, some local codes don’t allow access to green roofs unless high railings are installed. Make sure to obtain the appropriate permits before building a green roof.
Due to environmental and economic benefits, several cities offer incentives for green roofs. These include Chicago, Seattle, Boston, New York, and Portland, Oregon. Check for green roofs incentives in your area.
It will take a few years for the newly-installed low-profile green roof on this Pennsylvania Chiropractic Center to fill in entirely. Roofscapes, Inc.Green roofs are most often chosen based on their soil depth and structural requirements, intended purpose, and maintenance needs. Early on, you will need to decide whether the green roof is going to provide occupants with an outdoor space and a park-like feeling, or if the green roof is intended for visual and environmental purposes.
An extensive green roof is great for projects that want many of the environmental benefits of a green roof but without the added structural concerns and additional maintenance. This type of green roof has a thin soil layer, making it much lighter and only capable of supporting smaller, drought-resistant plants such as some grass varietals, sedums, and succulents. These plants can usually survive without supplemental irrigation once they have been established, and will require less maintenance than an intensive roof. Except for paths, patios and maintenance access, extensive green roofs are not meant to be walked on. They vary in weight from 10–50 lbs/ft2, and can be grown on flat or moderately sloped roofs.
An intensive green roof is best for projects that want to provide building occupants with a connection to the outdoors and a wider array of plants, including shrubs and small trees. However, these increased benefits require additional structural support, irrigation and typical landscape maintenance. Intensive green roofs can weigh anywhere from 80–120 lbs/ft2 or more when saturated, and usually require flat roofs that can support the extra load.
You can use a variety of methods to install green roofs, including modular, roll-out, and built-in-place systems. Expect to pay more for convenience, and for custom design and installation.
Green roof designs and components may vary depending on the type of system and product manufacturer. The list below includes the main components of a green roof. Extensive green roof systems may combine (or exclude) some of the optional layers.
Offers extensive resources, techniques and technologies on extensive green roofs, including design considerations and analysis tools.
The City of Chicago’s comprehensive guide to green roofs including common green roof plants, obtaining a permit, and a map of Chicago green roof projects.
The not-for-profit industry association site consists of an overview of green roofs and their benefits, design considerations, products and manufacturers, and educational, training and workshop opportunities related to green roofs.
An informative site focusing on determining appropriate green roof design for your project, installation techniques and considerations, as well as a helpful breakdown of cost factors for green roofs.
Site lists ASTM industry standards and Green Roof Testing Methodologies laboratories.
RoofNav is a free Web-based tool developed by FM Approvals that provides fast access to the most up-to-date FM Approved roofing products and assemblies.
Site includes a number of useful documents particular to Green Paks and Green Roof Blocks, and a calculating spreadsheet including a stormwater retention calculator.
I'm working on calculating the green roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1. and I was wondering if the 3' gravel border around the building edge and gravel paths/stepping stones to access the mechanical units are included in the area or should they be subtracted from the green roof area? The whole roof including those areas are being used for the stormwater management points but I wasn't sure if they should be subtracted from the vegetated roof for SSc7.2 (and factored into the Option 3: Combination path of high-albedoAlbedo is synonymous with solar reflectance./vegetated). Thanks!
Can someone tell me that does a green roof have to be entirely covered in greens in order to be considered a functional green roof? (Ex:absorbs storm water. reduce energy costs, last longer than conventional roofs..)
Yushan, any green roof will have some soil that is exposed, or walkways for maintenance. This is just a practical reality. The benefits of the green roof will certainly vary to some extent as a result. How much they vary will depend on the specific situation.
Green roofs have a positive impact on energy consumption, especially cooling during the summer in hot climates. How do you account for this in an energy simulation however? One way is to lower the U-valueU-value describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions. The greater the U-value, the less efficient the building element is as an insulator. The inverse of (1 divided by) the U-value is the R-value. of the roof due to a green roof. But by how much and how do you justify that? Next, I wonder if the SRI value could be adjusted. ASHRAE allows an SRI factor of 0.45 in case the roof has a SRI o more then 70%. What is the SRI of the roof? Is is above 70%?
Green roofs are not included in U-ValueU-value describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions. The greater the U-value, the less efficient the building element is as an insulator. The inverse of (1 divided by) the U-value is the R-value. calculations as they are considered to be continually saturated. The U-Value will be taken to be the same as the roof without the extra green roof layer. The roof will however benefit from increased protection from heat in the summer and some protection from cold in the winter.
Green roofs are not the only roof treatment that cannot be described by an R-value and whose energy performance is highly dependent on location. To a certain extent, "cool" roofs suffer a similar lack of thermal simplicity.
But having said that, both design builder and energy plus allow you to model green roofs quite effectively by accounting for:
- Long wave and short wave radiative exchange within the plant canopy,
- Plant canopy effects on convective heat transfer,
- Evapotranspiration from the soil and plants, and
- Heat conduction (and storage) in the soil layer
In our experience a well irrigated green roof also provides some level of radiant cooling because the cool water cools the concrete slab. For this reason we avoid drought tolerant species on our roofs instead we use treated grey water to keep the roof well irrigated. This aspect of green roofs would be very hard to model.
Thank you very much for your comments. Very interesting and insightful.
We have a green roof system that have been designed by the Designer. The system comprises a 7.5" layer of growing medium + gravel of top (light or Coated White). The vegetation is varying between shrubs and ground covers (like; Salvia Farinacea, Lavaudula Multiflda and Halyoxylon Sallcornicau, ..etc) with a density of 11 No./sq. mt.
It is expected to have considerable area between the plantings exposed with gravel.
Is this considered as Green Roof; or we should consider it as reflective materials (i.e. white-coated gravel)
Mohamed, this is a bit of a gray area in the LEED requirements: what is the definition of a green roof, and the difference between a green roof and a regular roof with some plantings on it?
From your description I would say it's a tossup. Maybe you should consider it half green roof, and half reflective surface?
We are working on an 32.980 m2 (354.862 sqf) core an shell office project in Frankfurt, Germany. The area quoted includes all floors, including basement areas, with the subtraction of the area of the 2 underground parking levels and ramps.
Because of the downtown location, we choose to cover more than 20% of the site project area with green roof with local plants.
We currently check compliance with WE Credit 1: We plan to install a 10 m3 rain water collection tank and plan to switch from an intensive green roof to an extensive green roof with local plants.
Could you educate us, if we can consider the portable water saving from changing from an intensive green roof to an extensive green roof, to contribute to the required 50% reduction by WE Credit 1, Option 1?
In my opinion, no. You would be giving yourself credit for sometihng you could have done but chose not to, and would perversely be an incentive to choose a landscaped area of arguably lower ecological and social value. It would be like paving an area that could have been planted and taking credit for that. You could instead choose plants and irrigation methods that lower the water use of the extensive roof, with an extensive roof as the baseline case.
I agree with Tristan. It very much sounds like cheating. You can account for water savings based on your choice of plants, but your baseline is a standard landscaping (green roof), middle of the road choice of plants. But be very careful, all this is object to the reviewers interpretation. So rather underestimate your savings based on that if anything.
Our Landscape architect considered both scenarios, including an intensive and optional extensive green roof. The intensive roof would require ca. 84.000 Liter per months in the July design case scenario, the demand of the extensive roof can be covered by the supply of rain water. As a compromise our Architect plans to create mainly an extensive green roof in combination with an intensive green area around the roof terrraces at 7th floor an 12th floor level. We also intend to increase biodiversity by using a mix of local plants (no monoculture) for the roof.
With this secrario we can substitute more than 50% of the demand for irrigation of the outside green areas including roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1. by collected rainwater. Due to the downtown location the availabe space for installing a rainwainwater tank is limited to 10.000 liters.
Do you think this will be an acceptable solution, given the local circumstances in the inner City of Frankfurt.
I also have project in Frankfurt. The baseline for a green roof's irrigation demand would be zero, since a standard green roof wouldn't need water. So in order to achieve points you will have to design without any irrigation or reduce your demand by 50% and use only non-potable waterPotable water meets or exceeds EPA's drinking water quality standards and is approved for human consumption by the state or local authorities having jurisdiction; it may be supplied from wells or municipal water systems.. the 50% reduction is not possible if your baseline is already zero. I understand your dilemma.
However if you can argue that a standard intensive green roof requires irrigation, you may be able to get there. I would challenge the landscape architect to find plants, which need little or no irrigation.
Also native and adaptive plants might gain you points under protect and restore habitat.
Can some explain why Green Roof are not built in South Florida? What is the reason...
If there are some Green Roof where are they located. Thanks!
I would check Green Roofs for Healthy Cities for resources on this. I think there are green roofs, maybe it's just not an area where they've taken off in a big way. It really varies in part based simply on whether there's someone who cares enough to make it happen, like Chicago's mayor.
I wonder if hurricane codes affect requirements, though?
We've looked at Live Roof system for a Florida project. Live Roof has been tested to 120mph - but our project is in a zone of 140mph.
Thanks for the links on green roofs, however, these do not give me the delta cost difference between different roofing systems or how much cost premium is involved for green roofs over other systems.
You will find cost premium for green roofing and other roofing systems in the Cost of LEED report.
Several resources are available on Green Roofs.
"Green Roof Systems" by Susan Weiler and Katrin Scholz-Barth.
Also, take a look at the Green Roof Systems section listings on BuildingGreen.com
Please educate me on costs for the following:
4" green roof:
6 to 8" green roof:
Regular coal-tar built-up roofA roof covering consisting of several successive layers (each of which is called a "ply"), usually of roofing felt, with mopping of hot asphalt between layers and topped by a mineral-surfaced layer or by gravel embedded in a heavy coat of asphalt.:
White coating on built-up roof:
Fully adheared Epdm roof with white reflective surface:
Built-up roof with Gravel:
We charge $4/Sq Ft here in India for a basic green roof (i.e no design element) this includes waterproofing/root barrier, drainage layer, filter medium, light weight growing media, plants and installation. Our brand name is BuildScape.
For our BuildScape - Cool Roof (triple coat reflective paint) we charge $0.70/SqFt. This includes 8 year warranty on waterproofing and paint chipping.
The general thumb rule is you triple the rate and you will get the average cost in the US.
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