A vegetated roof design (green roof) is defined as a roof membrane system with a top layer of living plants in an engineered soil blend (growth medium). This type of roof system can be installed as a protected membrane roof assembly or over a conventional roof assembly.
The typical components of a vegetative roof system are:
- Waterproofing membrane
- Membrane protection layer
- Root barrier
- Drainage layer
- Moisture retention layer
- Filter fabric
- Growing medium and vegetation
Vegetative roof systems are available in either a built-in place, modular or pre-grown vegetated systems. The distinction between these types is based on how the components are assembled. A built-in place system is when the individual components are assembled directly on-site including the vegetation. A modular system consists of pre-grown plants in mats, portable containers or trays that also contains the remaining components of the vegetative roof assembly and are generally installed directly over the waterproofing or the protection layer. Pre-grown vegetated systems consist of thin mats or rolls of vegetation grown offsite and placed over a combination of components required for containment, drainage, and protection from root penetration. Additional thickness of growing medium may also be installed in accordance with project requirements.
The waterproofing membrane* may consist of the following
- Single ply membrane
- EPDM,PVC, TPO
- Liquid applied
- Built-up roofing assembly
- Modified bitumen roofing assembly
* Refer to local jurisdiction and/code requirements for acceptable membranes
In a protected membrane vegetative system, the waterproofing membrane will be adhered directly to the structural deck. In a conventional system, the waterproofing membrane is installed above the insulation layer.
Depending on the membrane type to be installed, there are enhancements that may be required by the manufacturer that will provide acceptable performance due to membrane being subjected to hydrostatic pressure. These enhancements may include:
- Increase in the number of plies in a built-up roof assembly.
- Increase in the thickness of the waterproofing membrane.
- Upgrade in the seaming of single ply membrane.
- Upgrade in the flashing details.
- Installation of a leak detection system (i.e. vector mapping).
During the installation of a vegetative roof assembly, the installed waterproofing membrane may be subjected to rooftop traffic. While access to the roof should be restricted during vegetative roof assembly installation, a protection layer may be installed directly on top of the membrane to minimize damage.
Suitable materials for the protection layer include:
- Asphaltic boards or sheets
- Extruded polystyrene boards
- PVC sheets
The type and thickness of the protection material would depend on the type of waterproofing membrane and thickness of the growth medium.
Root barrier layer
The root barrier prevents the migration of plant roots from damaging the waterproofing membrane. As plants grow their roots can penetrate the waterproofing membrane resulting in damage to the waterproofing membrane and potential water leaks.
The root barrier is typically installed on top of the protection layer. The materials that are commonly used are:
- Polyethylene sheets
- High density polyethylene boards
- Modified bitumen membranes
Depending on the type of materials installed, the seams of sheet materials must be overlapped or properly heat welded.
When a root barrier is exposed to environmental elements, such as at borders or upstand, the materials must be UV resistant.
In some products, the root barrier material is incorporated in the membrane material, protection or drainage layers.
The installation of a root barrier should not be limited to those areas covered with vegetation. Any joints, borders, transitions and roof projections must be adequately treated to prevent root penetration.
All vegetative roof systems must be properly drained to maintain the optimum growing conditions and to manage rain, melting ice and snow. A drainage layer allows for water or moisture to travel laterally through the vegetative system. It is recommended that vegetative systems be installed over a minimum slope of 1:50 with positive drainage to roof drains.
A wide selection of materials and components with different performance attributes may comprise the drainage layer including:
- Lightweight, plastic forms
- Cellular plastic foam
- Synthetic fiber mats
- Granular aggregate
- Rigid mineral board
- Profiled drainage boards
In some systems, the drainage layer material may be integrated with the filter layer.
The insulation layer provides thermal protection and can be installed either below or above the waterproofing membrane depending on the type of roof assembly, (conventional or protected membrane) that is specified. The insulation material should be high density with a high compressive strength and be moisture resistant.
Moisture retention layer
A moisture retention layer is installed to retain or store water for plant growth typically above the drainage layer. The material that is used will depend on the type of overburden but is typically an absorptive mat consisting of polypropylene fibers.
Filter fabric layer
A filter layer is installed below the growing medium to prevent small soil and medium particles from entering the drainage layer below.
The filter fabric layer is typically a geotextile material and may be integrated with the drainage board. The filter fabric is installed directly over the drainage layer and prior to placement of growth medium.
Proper installation of the filter fabric included proper overlaps and the filter fabric must be carried up along vertical edges and secured.
A growing medium is necessary for plant growth with a sufficient depth for the root zone to ensure an adequate nutrient supply and a well-balanced water-air relation for the type of plantings. The vegetative roof system is categorized by the depth of the growth medium.
- Extensive (shallow): The growth medium depth is 150 mm (6 in) or less.
- Semi-intensive (moderate depth): The growth medium depth is approximately 150 to 250 mm (6 to 10 in).
- Intensive (deep): The growth medium depth is greater than 250 mm (10 in).
The growing medium is typically formulated by manufactured sources and determines how well the plants will grow and how much maintenance, weeding, and fertilization will be necessary. The growth media type and thickness will also determine the weight of the assembly as well as its water-absorption capacity.
Together with the drainage layer, the growing medium facilitates adequate drainage of excess rainwater. Light-weight mineral materials, with a high water retention capacity and good water permeability, such as lava rock, pumice, expanded clay, expanded shale, and clay tiles, have proven to be reliable for many years. Untreated organic material and top soil have disadvantages in terms of weight and drainage function; they are only used as additions to mineral substrates.
The plants that form the vegetative cover, particularly those in shallow systems, must be able to withstand significant exposure to sun, wind, drought, and heavy rains at times. Proper maintenance is required to ensure the presence of weeds is kept to a minimum.
Vegetation Free Zones
Vegetation free zones are installed at roof perimeters, penetrations, walls and curbs to provide access for maintenance on rooftop equipment, penetrations, and flashings.
Vegetation free zones are typically constructed with concrete pavers and/or mineral aggregate.
At perimeter of roof areas, the vegetation free zone is typically installed with a minimum width of 450 mm (18 in).
At penetrations, curbs, and drains, the vegetation free zone is installed with a minimum width of 300 mm (12 in).
A vegetative roof system should be maintained with regular inspections and repairs as required. The following should be included in a maintenance program:
- Ensure adequate irrigation and watering of plants.
- Remove any dirt or debris from roof system including weeds.
- Inspect working condition of roof drains and remove any debris accumulation.
- Replace or replant any missing portions of the planting/landscaping.
- Repair any damaged or displaced growth medium.
- Materials to be stored in a dry location on platforms/pallets to prevent moisture damage.
- Materials susceptible to UV damage should be covered and/or protected from sunlight.
- Roll materials are to be stored on their end to prevent deformation or damage.
- Water-based adhesives, primers or other materials should be stored as per manufacturer’s requirements.
- Thermal insulation should be protected from environmental conditions and moisture absorption as per manufacturer’s requirements.
Prior to installation of vegetative materials, the substrate should be clean, smooth, and free of voids.
Surface should have a positive slope (1:50) to allow for proper positive drainage.
All openings, penetrations through the roof deck and equipment curbs should be installed before membrane installation.