Wind Design Process for Vegetated Roof Assemblies Using New Standards
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Wind Effects On Vegetated (green) Roofing Assemblies
A vegetated roof assembly (VRA) consists of a roofing system (RS) and a vegetated system (VS). They must work in harmony to resist wind forces and failure in the weakest link can lead to failure of the entire assembly.
When wind travels across the roof, it creates suction or negative pressure that affects both the RS and the VS, but in different ways. A roof membrane is impermeable and attached to the roof structure, so the wind suction lifts it like a sail. In comparison, a VS is unattached to the roof structure and is permeable, akin to a sail with tiny holes. Wind can move through the VS, which creates pressure equalization, thus reducing the net overall uplift.
Wind drag can cause sliding and overturning of the VS or its components. Repeated wind motions can cause fatigue. Scouring or wind erosion of growing medium can occur, especially when the root network has not yet fully established. On the other hand, high vegetation density can increase surface roughness and reduce wind forces experienced by the VS.
While wind damage on vegetated roofs is not common based on experiences in Germany, it is prudent for designers to exercise caution, particularly around corners and perimeters where wind forces are the highest. Without knowing the wind resistance of vegetated roofing assemblies, engineers often over-design the reinforcing/securing mechanisms, thus increasing the project costs.
Modular vs Built-in-Place Vegetated Systems
Vegetated systems can be classified as built-in-place or modular based on their establishment method.
Built-in-place vegetated systems (BVS) consist of vegetation such as plugs, cuttings or seeds planted on or in, growing medium. During the establishment period, when the vegetation has not yet rooted firmly in the growing media, the plant materials and the exposed growing media are susceptible to scouring and displacement, particularly in the corner and edge regions of the roof where localized wind forces are stronger. As typical green roof growing media tend to be mineral-based and granular in nature, they are more likely to be displaced than cohesive growing media with a higher proportion of clay and organic contents. Therefore, erosion control nettings, tackifiers, and hydromulches are often applied to protect the growing media from wind erosion during the establishment period.
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Modular vegetated systems (MVS) contain mature vegetation that are pre-grown on mats or in trays and delivered at near complete vegetation coverage, typically 80 per cent surface coverage. Compared to BVS, MVS have the advantages of significantly shorter establishment time and lower initial maintenance needs. The mature roots bind the growing medium together and the higher degree of vegetation coverage protects the medium from scouring. The modules in an MVS can also be interlocked to allow the uplift force to be shared among them, thus enabling the MVS to collectively resist higher wind forces.
Wind tunnel studies showed the weight and design of the vegetated modules govern their wind uplift behaviour. Wind suction creates uplift force on the vegetated modules. When the uplift force exceeds the module weight, it can become airborne and overturned. The modules are permeable and have gaps at the joints, which allow air to enter and equalize the underside pressure and the surface pressure. This pressure equalization reduces the net uplift force on the modules. The higher the pressure equalization, the higher wind force is needed to lift the module, therefore the higher its wind resistance.
VS come in a wide range of weight, material, and design, so the degree of pressure equalization varies from one system to another. Therefore, it is necessary to test and determine their wind resistance.
CSA A123.24-21 Standard Test Method for Wind Resistance of Vegetated Roof Assembly
Canadian Standards Association CSA A123.24 was the result of a multi-year research program led by the National Research Council Canada (NRC), in collaboration with the members of the North American green roof industry, to study the wind performance of green roofs. Each VRA must undergo both wind uplift and wind flow testing to determine its wind resistance.
This video shows wind resistance testing of vegetated roof assemblies using CSA A123.24 conducted at the National Research Council and UL. Next Level Stormwater Management was a key participant in the development of CSA A123.24 Standard Test Method for Wind Resistance of Vegetated Roof Assembly - the first such test in the world. The results showed that many lightweight extensive vegetated systems on the market could resist high wind without having to secure with geotechnical reinforcing mesh/grid, which meant significant cost saving. This test method also enables green roof companies to innovate and improve system designs to resist higher wind forces.
Wind uplift resistance
The VRA is installed on a test table measuring 6.1 m X 2.2 m (20 ft X 7.2 ft), instrumented with pressure and deflection sensors, and subjected to dynamic wind load cycles to simulate wind suction on the roof in accordance with CSA A123.21 Standard test method for the dynamic wind uplift resistance of membrane-roofing systems. The test is terminated when the specimen fails as defined in CSA A123.21 or when the net uplift deformation of the above-deck components exceeds a critical value.
Wind flow testing of a vegetated roof assembly. The specimen (1.8m X 1.8m) is angled at 45° to the wind flow to receive maximum impact. Photo: Dr. Karen Liu
Wind flow resistance
The VRA measuring 1.98m X 1.98 m (6.5 ft X 6.5 ft) is installed on top of four load cells and placed in front of an airflow machine and yawed at 45 degrees to the airflow to simulate corner wind effects. The test starts at a wind speed of 48 km/h (30 mph) and continuously moves up at 15 km/h (9.3 mph) increments. Failure occurs when any component displaces or overturns more than 25 mm (1 in.) or when the VS loses more than 15% of its initial weight.
Wind Design Process of Vegetated Roof Assemblies Using CSA A123.24
CSA A123.24 determines the wind resistance of an VRA and enables the designers to compare it to the design values and determine if the assembly is secured against the wind forces for a specific project by following a few simple steps:
Step 1: Determine the project’s design wind load (PD) and the design wind speed (VD) using the National Building Code (NBC) or the Wind-MVRA online calculator.
Step 2: Request the wind uplift resistance (PR) and the wind flow resistance (VR) of the vegetated roof assembly as determined per CSA A123.24 from the green roof supplier.
Step 3: The VRA is secured against wind forces for the project if the following conditions are met:
wind uplift resistance (PR) is higher than the design wind load (PD)
wind flow resistance (VR) is higher than the design wind speed (VD)
Conclusion
Like any building envelope components, vegetated roofing assemblies must be designed properly to resist wind forces to ensure durability and public safety. CSA A123.24 provides a means to determine the wind resistance of vegetative roofing assemblies. Designers should ask green roof suppliers for wind resistances of their systems and follow the wind design process to select the appropriate assembly for their projects. Alternatively, some green roof suppliers retain professional engineers to perform the wind design process on behalf of their clients.
Dr. Karen Liu is the Green Roof Specialist at Next Level Stormwater Management. Karen has been working in the green roof industry for 20 years as an academic researcher in the public sector and a product manager in the private sector. She is a member of the CSA A123 Technical Committee that develops roofing and green roof standards in Canada. Karen can be reached at karen@nlsm.ca