Green roofs, garden roofs, vegetative roofs, landscaped roofs - no matter what you call them, they are here to stay. The movement toward energy savings and environmental stewardship is legitimate, and green roofs are a big part of it. Green roofs have a number of benefits; however, there are considerations that need to be dealt with whether you are a designer, contractor or building owner.
Green roofs can also provide usable outdoor space. Changing portions of a building’s rooftop to accessible space is almost like finding new land! Even changing the visual appearance of a more traditional roof to a green roof can be beneficial.
ASTM International (ASTM) has a task group devoted to sustainability: ASTM E06.71. The group has been working for a number of years developing standards regarding sustainability. Many standards are currently available and more are in process.
Some state and local governments are requiring buildings be constructed in ways and with materials that are sustainable. Green roofs clearly are part of the sustainability movement.
However, there are some concerns the industry must deal with. At this point, the building code implications are as yet unresolved. In general, roof systems are required to meet certain fire- and wind-resistance requirements. Green roofs are subject to the same requirements, or at least should be. There is considerable work under way within the International Code Council (ICC) to put in place specific requirements for roof gardens and landscaped roofs. (“Roof gardens” and “Landscaped roofs” are the terms used in the International Building Code [IBC].) Specifically, fire- and wind-resistance requirements are being discussed.
Three Types of Green RoofsA green roof combines roofing, waterproofing and landscaping concepts.
NRCA defines three types of green roofs: extensive, semi-intensive and intensive. The principal difference between these three types is the depth of the growth medium. Growth medium for extensive systems is 2 to 6 inches in depth; for semi-intensive systems, 6 to 10 inches in depth; and for intensive systems, 10 inches or more in depth. Extensive systems generally are not irrigated and the reservoir layer is optional. This is primarily based on the common plant varietals used with extensive systems. Semi-intensive systems may include aggregate in the reservoir layer. Intensive systems often require irrigation and need large reservoir capacity.
The basic components of the three types of green roof systems are similar. Figure 1 shows a cross-section of an extensive green roof system. NRCA recommends all green roof systems include a waterproofing membrane adhered to a stable substrate, protection course, root barrier, drainage layer, insulation, aeration layer, moisture-retention layer, reservoir layer, filter fabric and some type of growth medium or engineered soil.
It’s important to point out there is an aeration layer above the insulation and a drainage layer beneath the insulation. When insulation gets wet or saturated, its R-value is significantly reduced. Having an aeration layer above and a drainage layer beneath the insulation helps maintain the R-value by giving water a place to evaporate and drain. If these components are not included, additional insulation should be included in the design knowing the in-place R-value will be reduced when the insulation contains some amount of water.
NRCA recommends a waterproofing membrane be used for green roofs, and that it be adhered to the substrate. NRCA believes this is the most appropriate method to ensure a long service life for a green roof system waterproofing membrane. Even though a green roof uses a waterproofing membrane, there should be positive drainage. Positive drainage should be provided by sloping the deck. Anticipating localized deflection will help ensure positive drainage. Perhaps additional structural support is required to prevent localized deflection. Insulation is installed above the membrane. Insulation should have high compressive strength - adequate to support the weight of all components and the expected live load - and be resistant to deterioration from moisture.
Many of the traditional waterproofing membranes are used with green roofs. NRCA recommends five membrane types for green roof systems:
- Hot-fluid-applied polymer-modified asphalt membrane.
- APP- and SBS-polymer-modified bitumen sheet membrane.
- EPDM membrane.
- PVC membrane.
- One- and two-component, fluid-applied elastomeric membrane.
Installation guidelines and construction details for each membrane type are provided in The NRCA Green Roof Systems Manual–2007 Edition.
Design ConsiderationsThere are many other design considerations to bear in mind when designing a green roof system. Membrane flashing details should terminate at or above the top surface of the growth medium. Membrane flashings and counterflashings below the top of the growth medium are more susceptible to leakage and are more difficult to maintain. With that in mind, NRCA’s green roof construction details include access to the flashings at perimeters and interior penetrations. Installing pavers or gravel along the flashings is a method to provide access. Uplift issues should be considered where gravel/aggregate is used, especially at perimeters and corners. The IBC, 2006 Edition, has specific requirements which limit the use of gravel/aggregate on roof systems.
UV and mechanical protection is needed for membrane flashings. The activity on a green roof - from landscaping maintenance to public access - raises the possibility of flashing problems. Figure 2 shows a low parapet flashing detail for a thermoplastic waterproofing membrane for an extensive green roof system. Also, expansion joints should be located above the membrane surface, and ideally they should be above the growth medium. Drainage should be provided within any area on a green roof that is bounded by expansion joints, area dividers or perimeters.
Safety and access need to be considered during the design of a green roof system. Many green roofs allow public access; many publicly accessed green roofs are many stories off the ground. The 2006 IBC includes provisions requiring roofs used for roof gardens or assembly purposes be capable of supporting 100 pounds-per-square-foot live loads. Walls, parapet walls or railings likely are needed to ensure safety of the users of a green roof. Where access to a green roof is allowed, there needs to be appropriate passage from the interior to the exterior - similar to the access required for a public plaza. Recognize there are additional building code requirements for green roofs that are readily accessible.
Safety of maintenance personnel is also a concern. Many landscape contractors are not well versed in rooftop safety. If parapet walls or railings are not provided, should anchorage points or safety rails be provided? Or are temporary safety devices all that is needed for rooftop workers? Do green roofs need to include walkways and footpaths for rooftop maintenance workers? These are not necessarily new questions regarding rooftop access, but with more and more publicly accessible green roofs, safety in being thought of more often. Regarding plant maintenance, is there a water source required on the roof? Some green roof plants may require periodic watering.
Because green roofs hold water intentionally, the design loads are larger than more traditional roof systems. Extensive roofs start at 12 pounds per square foot; semi-intensive roofs start at 40 pounds per square foot; and intensive roofs start at 60 pounds per square foot. The structure needs to be able to handle the anticipated design loads.
Most green roof systems are installed on low-slope roofs; however, it is quite possible to install green roofs on steep-slope roofs. Steep-slope green roofs are generally limited to extensive systems. Installations over 2:12 or 3:12 will likely need special detailing methods. Loose-laid components will need ballast or some type of restraint to keep them from sliding down slope, and the growth medium also will need to be held in place.
One very important item for all parties involved in a green roof project is a water test. NRCA recommends a water test be performed to verify the integrity of the waterproofing membrane before proceeding with subsequent construction activities. This provides an opportunity to correct any defects in the installation or alter details that aren’t working properly. Establishing the green roof waterproofing membrane is functioning properly prior to additional work being performed also provides a benchmark that can be used if disputes regarding functionality arise in the future.
The NRCA Green Roof Systems Manual-2007 Edition is an excellent source for information regarding the waterproofing of green roof systems. I encourage roofing professionals involved with green roof system projects to use this manual as a reference tool. CR