A green roof is a living, sustainable rooftop system that incorporates drought tolerant plants with a soil growing medium to receive, process and transpire rainwater, providing protective, insulative and other benefits to the building structure.
Green roofs may be either extensive or intensive. Site characteristics and objectives will determine which green roof is appropriate for your property.
An extensive green roof will have 8 or less inches of soil and vegetation heights lower than 36 inches, typically. The extensive green roof is lower maintenance, more economical, and usually offers the most green coverage hence the greater environmental benefit.
The intensive green roof will have soil depths between 8 inches and 4 feet and requires the normal maintenance functions typical of a traditional garden. Intensive green roofs are usually accessible for maintenance functions and often offer additional public uses such as recreational, social, contemplative, or vegetable production activities.
The necessary components may vary from extensive to intensive roofs and also by roof site configuration. Regardless, most green roofs are generally comprised of four layers beneath the plant material - the waterproofing layer, the drainage layer, the root barrier, and the soil medium.
Waterproofing Layer
A waterproofing membrane is laid directly on the roof decking. The layer is commonly of bitumen, rubberized asphalt or compounded thermoplastic composition. Depending upon the composition, this layer is either loose laid or adhered to the roof. The adhered system is usually bonded to the concrete, steel or wood substrate through hot-welding or permanent fusing. The advantage to adhered systems over loose laid is the permanent seaming.
Thermoplastic compositions tend to hold up better against acid conditions caused by fertilizers and acid rain.
The membrane should be a minimum 80 mil or greater in thickness for best durability and performance. The waterproofing membrane must also be impenetrable by roots and resist the growth of algae and other organisms. Leak detection systems can often be installed to make any leakage easy to detect, locate and correct.
Drainage Layer
The drainage layer must simultaneously capture and retain rainwater or irrigated water and also allow excess rainwater to drain away. Drainage layers are typically of an egg carton style configuration, offering closely spaced reservoirs to capture water. Drainage layers are available with reservoir depths between 1/4 and one inch. Between the reservoirs drain holes are located to release excess water so that it does not rise to the soil level. The drainage layer is commonly made of formed polystyrene.
Root Barrier Layer
The root barrier layer prevents roots and soil from entering the drainage layer. This is important for three reasons. Roots and soil in the drainage layer reduce the rainwater storage capacity of the reservoirs. Second, roots submerged in trapped rainwater will rot, introducing potentially fatal fungal diseases to the plant material. Finally, roots in the drainage layer are one step closer to the waterproofing medium.
The root barrier layer or filter fabric is water retentive and, through capillary action, transmits water from the drainage layer's reservoirs to the growing medium so the roots are able to receive water.
Soil Medium
Specialized soil mixes are usually used with green roofs. The precise mix and amount will depend on whether the roof in intensive or extensive as well as the types of vegetation grown.
Soil mixes for green roofs are often a combination of expanded clay, shale or slate; organic matter such as bark and worm castings; and inorganic coarse matter such as limestone and gravel. The extensive green roof might be 80% expanded material whereas the intensive roof might be 55% expanded material.
The expanded material is kiln-fired to over 2000 degrees Fahrenheit, causing it to "explode", thereby creating pocks and voids in the material. In this expanded state the material offers an environment that has both good aeration and water absorption capacity.
Green roofs benefit from these specialized mixes because the mixes hold water in reserve for the plant roots instead of soaking or flooding them and provide a lightweight, disease retardant, relatively wind-proof medium for good root health.
Since the weight of green roofs - both extensive and intensive - can be considerable, engineers should be consulted to determine roof load capacity and total saturated roof weight before completing the estimating phase for any green roof project.
Vegetative Layer
Plant material will vary between extensive and intensive green roofs. Extensive roofs will require highly drought tolerant species growing 1 - 30" in height. The deeper the soil layer, the higher the plant that can be sustained. Intensive green roofs can sustain small and medium sized trees if designed properly.
Sedums are popular green roof plant choices due to their drought-tolerant habit. Hundreds of varieties exist and extensive green roofs can achieve varied aesthetic effects based on creative combinations. Delospermas are another family frequently present on the green roof. Overall, plant trials are conducted throughout the country and the world to test new finds and cultivars and the palette of suitable plants is constantly expanding.
Modular Systems
Several modular systems are available. Modular systems are self-contained units that require only the addition of the soil medium and vegetative layer for a functioning green roof. These systems have advantages over area systems for steeper-pitched roofs, smaller installations, or where routine maintenance cannot be conducted at installation height. Although they are not less expensive than area systems, installation is much easier.
Cost
Costs vary widely. Extensive green roofs may cost as little as $12 per square foot, consistent with traditional roofing systems, if the soil layer is very minimal. Intensive green roofs may vary between $20 and $40 per square foot[1], depending upon soil depth and design complexity.
The installation costs should be factored against the savings due to the extended life expectancy a green roof provides versus traditional roof life values, as well as the annual energy savings and any qualifying tax credits.
[1]D'Antonio, Peter. "Thermoplastic Waterproofing Membranes in Green Roof System Construction 2004." Interface. Feb. 2004.
