Green Roofs
Green Roofs are native plant installations on the roof of a building. They can be placed on new or existing construction although they are typically installed in concert with new construction as the design considerations can be taken into account when the building is planned. Roofs are one of the primary sources of runoff as they are large impervious areas. By placing hardy native vegetation on the roof, the building can greatly reduce the amount of stormwater it produces by using the plants to evapotranspire much of the water received during a storm. Additionally, there are energy benefits inherent to green roofs as they act as an insulating layer both in the summer and during the winter.
More Detailed Information
- Design Details
- Plant List -Blue Thumb Program plant selector
- Vendor List
Design Considerations
Siting of Facility
Each rooftop should be considered unique. Retrofitting existing buildings or new construction should take the following into consideration: local climate conditions, roof slope, function of the roof (e.g. stormwater management, public access and/ or habitat creation), size of the project, budget, degree of accessibility, structural loading and infrastructure located on top of the building.
Structural Loading
The load bearing capacity of a green roof’s underlying structural support is a major factor influencing the design. The load bearing capacity of a roof must consider: dead load - the total weight of materials including soil, plants, snow, ballast and any other roof materials and live load - people, including maintenance workers and any other activities that the roof will need to support.
Type of Green Roof
There are two main divisions of green roofs - extensive and intensive. Generally, extensive roofs are lighter, have less than 4 inches of planting medium, use drought tolerant vegetation and are able to handle a limited number of people for maintenance concerns. Intensive systems are heavier with a much deeper planting medium, allow for greater live loads and require a much higher structural capacity load.
Vegitation
Successful rooftop vegetation must be able to: rapidly stabilize soil, quickly repair itself from damage, absorb and transpire water despite extreme conditions of heat and cold, wind and drought. In general, as the planting medium’s depth increases, so does the list of viable plant species. Sedums and mosses have been successfully used in shallow depth areas, while native grasses and forbs may be used in deeper soils.
Growing Medium
The growing medium must meet the selected planting’s nutrient, water, oxygen and pH needs. However, the structural load capacity of the building often determines the depth and material of the medium, which ultimately determines the vegetation that can be supported. Most growing mediums are made up of approximately 3/4 mineral material and 1/4 organic material. Mineral materials can be natural - sand or gravel, artificial - perlite or vermiculite, or created from recycled building materials.
Filter Fabric
Filter fabric below the growing medium prevents soil particles and other debris from migrating to and clogging the layers below. The drainage layer is protected while aeration is provided for the growing medium. Non-woven, non-biodegradable fabrics typically come in rolls and must be overlapped and secured to one another.
Water Drainage and Storage
The drainage layer helps prevent leaks by moving excess water away from the waterproof layer. Roof drainage design must consider: stormwater management goals, roof slope and the depth and nature of the drainage material. There are 3 main types of drainage material including granular materials (coarse gravel, stone, expanded clay, etc.) that have a large proportion of open space when packed together, sponge-like porous mats that can absorb and hold water, and several different types of synthetic drainage modules. Most of the synthetic modules create a rigid platform to keep growing medium and vegetation from contacting the waterproofing. Several of the synthetic modules have small depressions to store excess water.
Root Barrier
A physical or chemical barrier is needed to protect organic based waterproofing (i.e. asphalt) that can be penetrated by roots and broken down by microorganisms. The most common root barrier is a high-density polyethylene (HDPE) membrane. Metal and plastic base plates and PVC rolls are also used. PVC can also serve as waterproofing itself and is available in recycled form. Root barriers and waterproofing must be raised above the planting medium at roof perimeters and at any projections that penetrate the roof vertically, (chimneys, vents, walls, etc.) in order to completely enclose the soil and vegetation.
High Quality Waterproofing
One of the most important aspects of an effective green roof is maintaining a waterproof seal. While there are several types of membranes that can make up the waterproof layer, built-up systems that use bituminous materials are the most common. Another system seals overlapping rolls of synthetic materials, such as poly(vinyl chloride) PVC, rubber (EPDM), hypolan (CSPE) or thermoplastic polyolifins, together in a single-ply membrane. Hot or cold liquid systems can be sprayed or painted onto the roofing deck to create a joint free seal. Overlying soils and vegetation can extend the life of these waterproofing systems by protecting them from damaging ultraviolet light and extreme temperature fluctuations.
Irrigation
Proper green roof design and plant selection should alleviate or limit irrigation needs to new vegetation establishment and/ or prolonged periods of drought. Typical irrigation methods include: surface spray - hoses and sprinkler heads, drip and tube - subgrade tubes deliver water directly to the root zone, capillary - mats hold water under the root zone for plants to take up, and standing water - water is captured from large storms and held for future use.
Leak Detection System
Roof leaks are a hazard to any roofing system; however, locating damaged waterproofing under several layers of a green roof can prove to be very difficult and costly. Electric field vector mapping (EFVM) is a relatively inexpensive leak detection system that charges the planting medium with electricity and looks for grounds, where moisture contacts the metal or concrete roof deck. Modular systems that use trays can easily be removed to fix damaged waterproofing, but locating the source of the leak is still difficult without a leak detection system.