Norfolk Botanical Garden installed a “Green Roof” on our public programs hut in the early spring of 2008.
A green roof is one that is at least partially covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane. There are two distinct types of green roofs: intensive and extensive. The roof installed on our public programs hut is extensive.
Thick soil depths (8″ – 4′)
Plantings may be elaborate and include shrubs and trees
Installed primarily over concrete roof decks
These park-like green roofs require considerable maintenance
Thin soil depths (3″ – 7″)
Plantings are typically sun and drought tolerant ground covers
Can be installed over various roof decks on slopes as steep as 45°
Require periodic maintenance
History of Green Roofs
The history of green roofs dates back thousands of years. The most famous were the Hanging Gardens of Babylon, constructed around 500 B.C. and considered one of the Seven Wonders of the World. These terraced structures were built over arched stone beams and waterproofed with layers of reeds and thick tar. Soil, plants and trees were then planted on top.
Modern green roofs, which are made of a system of manufactured layers deliberately placed over roofs to support growing medium and vegetation, are a relatively new phenomenon that were developed in Germany in the 1960s. Since then, green roofs have spread to other European countries and are starting to appear in some US cities such as Chicago, Atlanta and Washington DC.
Benefits of Green Roofs
Storm water management
As urban sprawl continues, many areas can no longer accommodate the volume of storm water discarded off of roofs, roads and other impervious surfaces. Green roofs decrease the total amount of runoff by retaining up to 75% of rainwater gradually releasing it back into the atmosphere via condensation and transpiration.
Improved water quality
Green roofs, like wetlands, hold rain like a sponge. This helps reduce sedimentation loads that can choke streams while allowing the soil to filter out any pollutants present in the rain water.
Urban heat islands
Traditional building materials soak up the sun’s radiation and reflect it back as heat, making cities at least 7°F hotter than surrounding areas. This phenomenon is referred to as “urban heat islands”. Green roofs help reduce this effect by lowering roof top temperatures by 25°-80°F.
Reduced energy requirements
By lowering the ambient temperature of the roof’s surface, green roofs can reduce summer cooling needs by as much as 26%. Conversely, green roofs provide great insulation and can reduce heat loss during winter months by that same 26%.
Extend roof’s lifespan According to a Penn State University study, greening a roof can lengthen a roof’s lifespan by two or three times.
Improved air quality
One square meter of leaf surface supplies enough oxygen, through photosynthesis, to supply one person’s requirements for an entire year. Since the foliage in plants binds dust, green roofs further improve air quality by reducing dust.
Reduced noise pollution
Green roofs superior insulating qualities help reduce noise pollution caused by highway traffic and airplanes.
Green Roofs can provide attractive and useable outdoor patio spaces not only for people to enjoy, but also provide habitat for plants, insects and animals that otherwise have limited natural space in cities. Even in high-rise urban settings as tall as 19 stories high, it has been found that green roofs can attract beneficial insects, birds, bees and butterflies. Rooftop greenery complements wild areas by providing “stepping stones” for songbirds, migratory birds and other wildlife facing shortages of natural habitat.
Plants On Our Green Roof
- Delosperma ashtonii
- Delosperma cooperi
- Delosperma dyeri
- Delosperma floribunda “Starburst”
- Delosperma herbeum (usually misspelled as D. herbeau)
- Delosperma “John Proffitt”
- Delosperma “Kelaidis” (aka “Mesa Verde”)
- Delosperma nubigenum
- Echeveria runyonii “Topsy Turvy”
- Sedum acre
- Sedum acre “Aureum”
- Sedum album “Jellybean”
- Sedum album “Murale”
- Sedum cauticola “Mini Ewersii”
- Sedum dasyphyllum
- Sedum dasyphyllum “Major”
- Sedum divergens
- Sedum ellacombianum
- Sedum “Gold Carpet”
- Sedum hispanicum
- Sedum kamtschaticum
- Sedum kamtschaticum var. floriferum “Weihenstephaner Gold”
- Sedum kamtschaticum fa. variegatum
- Sedum makinoi “Limelight’
- Sedum makinoi “Ogon”
- Sedum makinoi “Variegated”
- Sedum platycladus
- Sedum reflexum “Blue Spruce”
- Sedum rupestre “Angelina”
- Sedum selskianum “Variegatum”
- Sedum spurium “Bronze Carpet”
- Sedum spurium “Dragon”s Blood”
- Sedum spurium “Dragon’s Blood Tricolor”
- Sedum spurium “Fuldaglut”
- Sedum spurium “John Creech”
- Sedum spurium “Voodoo”
- Sedum tetractinum
- Sempervivum braunii
- Sempervivum “Commander Hay”
- Sempervivum x giuseppii
- Sempervivum pilioseum
- Sempervivum “Purple Beauty”
- Sempervivum “Red Cloud”
- Sempervivum “Saturn”
- Sempervivum tectorum
- Sempervivum tectorum “Silverine”