Designing for Wind
The task of designing a vegetative greenroof can be intimidating, yet can be the most rewarding task given the blank canvas a rooftop presents. When designing a vegetative green roof, consideration to accessibility, loading issues, maintenance expectations, and selecting the appropriate vegetation to meet the design vision is crucial. But careful consideration with regard to protecting the vegetative green roof from wind also needs to be factored into the design. Wind can damage a vegetative green roof to a point of no return. A properly designed vegetativegreen roof shall be designed and engineered so that it retains its integrity during a high wind event. Consideration of wind pressures and associated variables, such as the building’s geographic location, surrounding terrain (hills, valleys, escarpments, etc.), height, openings, parapet height/design, and other features shall also factor into the equation. To assist with any technical questions, WESTON’s dedicated vegetative green roof professionals, engineers, and scientists are available.
This Wind Design Standard for the GreenGrid Vegetative Green Roof System was developed from lessons learned on projects and guidance from the following sources. For more specific wind design recommendations the following guides/standards shall be reviewed by the designer/engineer of record:
Click below for links to:
Understanding Wind Pressures
It is extremely important to recognize that wind blowing over a roof exerts varying uplift forces on different areas of the roof. For simplicity, the roof can be divided into three areas: (1) corners, (2) perimeter, and (3) the field. The roof perimeter and corners are exposed to higher uplift forces than the field of the roof. The corners and perimeter areas are where the greatest effects of the disrupted airflow over the building will occur and shall be avoided when designing a vegetative green roof. The worst-case scenario is the wind coming onto a corner at a 45-degree angle. These situations generate wind vortices along the roof edges, causing low-pressure areas over the roof system as well as wind turbulence that can scour a vegetative green roof system and ultimately destroy the system. The maximum uplift force along the windward perimeter occurs when the wind blows at 90 degrees to the perimeter. Actual pressure coefficients for the corners and perimeter vary depending on the building height, parapet height, roof slope, geographic location, and surrounding terrain (hills, valleys, escarpments, etc.).
Designing a GreenGrid Vegetative Green Roof for Wind
When designing a vegetative green roof the factors that need to be studied include basic wind speeds, building height, parapet height, and exposure. Further evaluation of the type of vegetative system (extensive or intensive) and type of assembly (pre-planted or pre-grown) shall determine the design criterion on the vegetative green roof design. After concluding the intent of design, begin to lay out the modules to fill in the “Field Zone” of the project roof. Determination of pre-grown or pre-planted assemblies can be made once the basis of design is determined. As previously stated, the roof can be divided into three areas: (1) corners, (2) perimeter, and (3) the field. Below are the design definitions of the three areas:
- Corner Areas - The space between intersecting walls forming an angle greater than 45 degrees but less than 135 degrees. The corner area is defined as the roof section with sides equal to 40% of the building height. The minimum length of a side is 8.5 ft (2.6 m).
- Perimeter - The perimeter area is defined as the rectangular roof section parallel to the roof edge and connecting the corner areas with a width measurement equal to 40% of the building height, but no less than 8.5 ft (2.6 m).
- Field - The field of the roof is defined as that portion of the roof surface that is not included in the corner or the perimeter areas as defined above.
Through the maintenance program associated with the vegetative green roof, monitor areas that may be more prone to wind scouring. If conditions persist relocate those modules affected to an interior portion of the roof.
Designing with Slopes
The GreenGrid Vegetative Green Roof System is suitable for low-slope roof, concrete deck/plaza applications, and other comparable scenarios. A low-slope roof is a roof surface with a maximum slope of 2 inches of "rise" for 12 inches of "run" (16 degrees) as defined in American Society for Testing and Materials Standard (ASTM) E 1918-97. Minimum required slope shall be 0.25 inch per 12 inches. If roof/plaza is level, water may accumulate and damage the health of your GreenGrid plants. Other safety measures that shall be addressed during the design include:
- GreenGrid applications between 10 degrees and 16 degrees - shear forces will increase with roof slope and the growing media must be protected against erosion. Plant selection and planting methods shall be adjusted to the relevant slope and exposure - Consult your GreenGrid Representative for solutions.
- Engineers shall evaluate sheer forces on GreenGrid applications between 10 degrees and 16 degrees in frost zones and recommend mitigation proceedures. This shall also be performed on applications in frost-free zones because waterproofing membranes and their relative slickness vary across the nation and seasonally.
- For long-sloped vegetative green roofs ranging between 10 degrees and 16 degrees, an abutment or structural support should be incorporated into the design to transfer the overall load throughout the slope and lessen the load on the lower GreenGrid units. Consult a structural engineer and roofing professional on placement and design of abutments.
GreenGrid modules can be cut to size and shaped as necessary to fit odd dimensions, angles, or arching spaces. When a design indicates customization of modules, WESTON will include a few additional empty modules as part of the package. We’ll pre-grow the overall square footage needed, but will use the empty modules to make the odd dimensions, angles, or arching spaces. The vegetation and growth media from the pre-vegetative modules will then be transplanted into the customized modules to complete the installation. This is a much easier and less destructive method than cutting pre-vegetated modules, which other companies advertise.
The cut side of the module will face the outward side of the modular array and be enclosed with our bendable edge treatment to contain the growing media.
Integrating hardscape design features into a GreenGrid vegetative green roof is simple, effective, and attractive. Since the system was designed on a 2’x2’ grid pattern integrating pavers (concrete or rubber) is a snap as most of these accessory items are also based on a 2x2 grid. Decorative edging, planter boxes, and other garden accessories can be easily integrated as well. The true flexibility of the GreenGrid System allows designers to create amazing projects, yet still upholds the ease of repair without damage to the overall vegetative green roof system.|
Irrigation requirements will be dependent upon project location and plant selection. For Extensive GreenGrid systems planted with a mix of highly drought-resistant ground covers, an irrigation system is generally not needed (exceptions do apply to some arid climates). However, GreenGrid strongly recommends a backup system to irrigate the vegetative green roof during prolonged droughts or during hot dry windy weather patterns. Simple overhead spray system with spray heads, or spigot/hose/sprinkler systems are inexpensive and effective methods. These also provide the means to optimize the evaporative cooling effect of the GreenGrid Green Vegetative Roof System during such weather events.
Using GreenGrid as a Stormwater BMP
GreenGrid is the perfect solution for municipal requirements to incorporate Best Management Practice (BMP) and Low Impact Development (LID) into your development. The use of GreenGrid as a BMP allows the developer to reduce the impact the building will have on the total impervious surface of the site. This reduction leads to smaller collection pipes, channels, and detention facilities.
A reduction in impervious surface is not the only advantage that GreenGrid offers when considering its stormwater management advantages over conventional roofing. The modular system, with the presence of vegetation for evapo-transpiration and porous media with available storage, offers a volumetric reduction in total stormwater leaving the roof drains, resulting in lower volumes and flows entering the collection system.
GreenGrid provides for a “rougher” runoff surface than conventional roofing. This “rougher” surface increases the total time it takes for peak flows to develop from the roof, thereby attenuating the peak flow through the total development. This reduction in the peak flow allows the developer to install smaller pipes and conveyance channels.
Designing to Meet Fire Standards
Clear and concise green roof details and roofing details shall follow the local building codes associated with designed green roofs. Through our ongoing Research and Development of the system we have performed fire tests on the system. The Fire tests were conducted in accordance with ASTM E108 (2007) “Standard Test Methods for Fire Tests of Roof Coverings” and UL 790 (2004). Only Burning Brand and Spread of Flame tests were conducted. Upon completion of the tests the system passed a Class “A” rating in each of the Burning Brand and Spread of Flame tests. Further testing is planned and we will update this information when it becomes available.
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