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"Be the change
you wish to see
in the world..."
            -M. Gandhi

t a k i n g   g r e e n   t o   t h e   e x t r e m e

LIVING ROOF

overview
the layers
benefits
challenges
common questions
additional resources
related articles

OVERVIEW

Green roof  at Montgomery Park Business Center, Baltimore

Design by KSB Consulting plants by Emory Knoll Farms

A living roof, or green roof, describes a system that allows plants to thrive on the surface of rooftop.  The plants provide a net cooling effect, while the soil provides some insulation and thermal mass, all of which makes the building more energy efficient.  Even one inch of planted soil lowers overall average roof temperatures and reduces day-to-night temperature swings on roof surface.  The plants absorb and filter rainwater, reducing the negative impacts of excess storm runoff in watersheds. The mini eco-system created by the plants provides habitat for butterflies, bees, and birds.  And the plants protect a roof’s waterproofing membrane from damaging UV sunlight, resulting in increased durability.

Plants have provided benefits on buildings for centuries, with the Hanging Gardens of Babylon as perhaps the best known early example.  Early sod roofs have been found in Scandinavia , Iceland , and Newfoundland .  Today’s living roof systems bear little resemblance to early planted roofs, though the benefits remain.

Two basic types of living roofs exist: intensive (deep) systems and extensive (shallow) systems.  Intensive living roofs, as the name implies, require substantial infrastructure and maintenance.  Extensive systems provide lightweight alternatives to conventional roofing materials.  This article addresses shallow extensive systems, which provide all of the benefits described above with wider applicability and lower maintenance.

THE LAYERS

The basic layers of a living roof include:

1. waterproofing membrane

2. optional root barrier

3. drainage layer

4. filter layer (often combined with drainage layer)

5. growing medium (soil)

6. plants

BENEFITS

BENEFITS OF LIVING ROOFS

• reduced rate and volume of stormwater runoff (averaging 60% to 90% absorption in most climates)

• reduced non-point pollution delivered to waterways

• reduced downstream erosion in watershed

• lower roof temperatures, eliminating an average of 60% heat gain thus reducing cooling loads

• reduces urban heat island effect

• net absorption of CO2 and net release of oxygen

• filters particulates from air pollution

• absorbs airborne phosphates

Stormwater Management
Plants drink water to grow and succulent plants used on green roofs store that water in their leaves.  This means that rainwater falling on a roof gets absorbed by plants instead of washed off down gutters into storm drains.  The percentage of rainwater absorbed by roof plants varies depending on the slope of the roof, plant species, and type of growing medium, but averages between 60-90% of all rain water hitting the roof surface.  Less rain water in storm drain systems, means reduced rate and volume of stormwater in local waters, reduced non-point pollution delivered to waterways, and reduced downstream erosion in the watershed.

Building Energy Efficiency
Plants stay cool in sunlight by evaporating moisture from their leaves, a process called transpiration.  This is similar to how we stay cool by sweating.  While a conventional roof surface heats up in the sun (up to twice the air temperature), a living roof actually cools the roof surface (by up to 15 degrees below the air temperature).  This translates to reduced cooling loads for HVAC equipment in the building below.  The cooler roof surface also helps to reduce the effects of urban heat island effect when living roofs are installed on a city-wide basis.

Reduced Air Pollution
Healthy plants provide net CO2 absorption and net release of oxygen.  Some plants also reduce NOx and SO2, two other major air pollutants.  Plants filter particulate matter (PM) pollutants from the air and absorb airborne phosphates (smog contributor).

CHALLENGES

CHALLENGES OF LIVING ROOF

•  retrofit requires determination of structural capacity & possible augmented structure

•  recommended to hire a structural engineer & an experienced design professional for criteria & details

•  usually requires roofing structure upgrade (2x10’s become 2x12’s or similar)

•  some living roof systems have higher material costs

•  less water for rainwater harvesting

•  requires some maintenance in first year, until plants become established

COMMON QUESTIONS

Suitable Plant Selection
The general rule for living roof plants is to select species that are suitable for rock gardens, with sedum varieties being the most common and generally foolproof.  Rock garden plants have adapted to arid conditions without access to the groundwater table by up-taking rainwater very quickly for storage in their leaves, stems, or root bulbs.  Variables to consider when selecting suitable plants include: climate, shade or full sun exposure of the roof surface, soil type, thickness of growing medium, and aggressiveness of plant roots.

Costs
Types and costs of living roof systems vary greatly, from approximately $1.50 per SF to over $15 per SF of roof area.  In general, complete top-to-bottom systems cost more, averaging $12 to $25 per square foot (about 4x to 6x the cost of metal panel roofing).  Do-it-yourself systems generally cost significantly less (starting at $1.50 per square foot), but require the assistance of an experienced design professional for specifications.

Maintenance
A well-design living roof should create a self-sustaining eco-system that requires little or no maintenance in the long term.  During the first year, freshly installed plants thrive best when given a small amount of nurturing.  This means watering regularly in the first week, watering periodically during the three months (if leaves look thin and translucent), and periodic weeding in the first year, until plants establish full coverage of the roof.  After one year, the plants become well-established and require no additional watering.  An annual check for weeds is always recommended.

When To Plant
Sedums are quite hardy and can be planted nearly anytime except freezing months.  However, springtime planting generally results in higher survival rates and faster plant propagation to cover the roof surface.  Any non-sedums deemed suitable to your living roof application will thrive best if planted in spring.

Design Loads
Typical extensive living roofs range from one to five inches deep.  Depending on the type of growing medium, this translates into an added saturated weight of 10 to 50 pounds per square foot.  An average four inch thick system adds 15 to 25 pounds of live load per square foot.  A structural engineer is recommended to determine exact loading.

ADDITIONAL RESOURCES

www.greenroofs.com

www.greenroofs.org

www.greenroofplants.com

en.wikipedia.org/wiki/Green_roof

horticulture.psu.edu/cms/greenroofcenter/

www.epa.gov/heatisland/index.htm

www.hrt.msu.edu/greenroof

Planting Green Roofs and Living Walls by Nigel Dunnett and Noël Kingsbury

Green Roof Plants: A Resource and Planting Guide by Edmund C. Snodgrass and Lucie L. Snodgrass

Green Roofs: Ecological Design And Construction by Earth Pledge Foundation, Leslie Hoffman, & William McDonough

Green Roof Construction and Maintenance by Kelly Luckett

Green Roof Systems: A Guide to the Planning, Design and Construction of Building Over Structure by Susan Weiler and Katrin Scholz-Barth

Award Winning Green Roof Designs by Steven W. Peck

RELATED ARTICLES

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