What is a Green Roof?
Green Roofs are conventional roofs that have a waterproof membrane, growing medium and soils that support living plants also known as living roofs, vegetated roofs, or eco roofs. The engineered soil used on a green roof is specially formulated to be lighter in weight than regular garden soil, to reduce the weight load on the building. The green roof must be designed so that the weight of the soil and plants is less than or equal to the weight the roof structure can safely support. Green roofs are designed to help manage rainwater by reducing the rate and amount of runoff into the storm drain system.
Types of Green Roofs
Green roofs range from small, simple, owner-constructed designs on sheds and homes, to extensive proprietary systems installed on large commercial and industrial buildings. Of the latter installations, there are two basic types: intensive and extensive.
Intensive Green Roofs
Intensive roofs typically have deeper soils and irrigation systems, allowing for a much wider range of plant types and sizes. They provide significantly better insulation. Intensive green roofs are usually designed to be accessible (available to be walked on or used as amenity space). This type of eco-roof requires more technical expertise to design and install, and more maintenance once built than extensive types.
Extensive Green Roofs
These roofs are characterized by their low weight, low capital cost and minimal maintenance. Soils (or growing medium) are very thin, and irrigation is rare, allowing only smaller, hardier plants to survive. Extensive green roofs are more suitable for larger areas and for sloped roofs. They are easier to retrofit onto existing roofs but are not usually designed for public accessibility.
While differences exist between the range of plants possible for extensive versus intensive green roofs, those best suited to any rooftop environment are:
- Drought-tolerant, requiring little or no irrigation after establishment
- Self-sustaining, without the need for fertilizers, pesticides, or herbicides
- Able to withstand heat, cold, and high winds
- Very low-maintenance, needing little or no mowing or trimming
- Perennial or self-sowing
Suitable plants also have growth patterns that allow them to thoroughly cover the soil, and include a variety of species to ensure establishment of a self-maintaining community which is appealing to animal and human users alike.
Advantages of Green Roofs
Reduced Runoff into Storm Sewers
Growing medium, and any additional substrate or integrated water storage, can store significant volumes of rainwater, preventing runoff in small storm events, and delaying peak runoff for larger storms. Runoff is extended over several hours, reducing peak flows. The plants themselves can return some of this moisture to the atmosphere via evapotranspiration. Studies in Berlin show that green roofs absorb up to 75% of precipitation that falls on them. A study from Portland confirms similar results, with green roofs mitigating from 65% to 94% of runoff.
As green roofs reduce runoff volumes for most rainfall events, this can improve the performance of other on-site cleaning, infiltration, detention or storage facilities.
Pollutant Removal
The temperature of water runoff leaving a green roof will be cooler than that coming off a standard roof surface, especially during warmer months of the year. This helps to protect fish habitat from heat pollution. In addition, C02 is absorbed, and some airborne and precipitation-borne pollutants are filtered out by the plants and growing medium.
Energy Use
Green roofs are effective summer heat insulators, reducing the energy required for air conditioning buildings. They can also be used for water-based heat exchange systems and industrial cooling. Research indicates that green roofs also increase the efficiency of rooftop solar panels, as they keep the surrounding air temperatures cooler in warm weather.
Research also shows that winter green roofs can reduce the energy used for heating by more than 10 per cent during the cold season. (Test houses used include the National Research Council of Canada's field roofing facility in Ottawa.)
Heat Island Effect
Thermal modelling has demonstrated that adding a percentage of green roofs to a city can reduce the ambient air temperature during hot days, reducing the need for air conditioning and improving livability.
Design & Construction
Sources agree that a flat surface is suitable for installation of a green roof, but they differ on the maximum slopes recommended. Recommendations range from 30% (4:12 pitch) up to 40% (or 5:12 pitch).
Green roofs can be used on most types of commercial, multi-family, and industrial structures, as well as single-family homes and garages. Though green roofs are simpler to create in new construction, it is possible to retrofit an existing roof to become a green roof provided the roof has suitable structural strength.
Sound Insulation
Green roofs can be designed for noise attenuation. The growing medium blocks lower frequencies of sound and the plants block the higher frequencies. Tests show that 12cm of growing medium alone can reduce sound by 40 dB.
Longevity/Maintenance
Green roofs protect roofing membranes against ultraviolet radiation, extreme temperature fluctuations and physical or puncture damage from recreation or maintenance. US sources estimate that green roofs last twice as long as conventional roof surfaces.
Building Incentives
Some cities such as Toronto, Ontario award amenity bonuses for proposed buildings with green roofs, increasing the building space that would otherwise be allowed. Many green building rating systems such as LEEDTM assign points in their recognition schemes to green roofs.
Wildlife Habitat
In high density urban areas, green roofs can provide helpful habitat for birds and insects, especially if the roof area is not accessible to regular human use. These micro habitats can function as stepping stones, linking natural habitat areas otherwise separated by urban development.
Food Production/Amenity
Green roofs hold potential for local food production, and have special applicability for outdoor classrooms and horticultural therapy gardens if used on institutional buildings. They can also provide beautiful and useful green spaces for people living in otherwise hard-surfaced urban landscapes.
Limitations of Green Roofs
Root penetration
To avoid damage to the roof and building, plants with known invasive root systems should not be used on green roofs. Many new installations also use an industry standard chemical or physical root penetration barrier.
Roof Design & Construction
As with all roofs, proper waterproofing, leak detection systems and drainage is required, as is protection from damage during construction. With green roofs, leaks are more difficult to find than on a standard roof. Design and plantings must allow easy access to drainage outlets for inspection and maintenance.
Costs
From a simply financial perspective, eco-roofs cost more to construct than conventional roofs. However, several sources list them as competitive on a life-cycle basis, because of reduced maintenance and replacement costs. In addition, other advantages (energy savings, air cleansing, green space provision, habitat replacement, stormwater detention and cleansing, etc.) should be factored into any cost considerations.
Weight
When calculating structural loads, designers must consider the saturated weight of each material. The structural strength of existing buildings must be assessed to ensure they can accommodate the additional weight of a green roof retrofit.
Fire Safety
Sedum and other succulents are naturally fire resistant, almost eliminating fire concerns. There is evidence from European manufacturers which suggests that green roofs can help slow the spread of fire to and from the building through the roof, particularly where the growing medium is saturated. Other types of vegetation could be of concern and may need to be watered, mowed, and/or maintained to prevent fire hazard before they go dormant during the dry season.
Aesthetics
Plants on roofs are exposed to very harsh conditions and proper maintenance is required to ensure long term appeal. Some observers find that extensive eco-roofs are not appealing during the dormant season, but this should be tempered by a fair comparison to standard roof finishes, particularly flat roofs, and not to standard ground planted gardens.