By Elizabeth Morris, CDT, GRP – Director of Green Roofing for Henry Company
Concerns about wind uplift on green roofing have been a limiting factor for designers and building owners across North America. This is especially true in sensitive coastal watersheds that could most benefit from on-structure stormwater management, but these are also the areas at the highest risk for recurrent hurricanes. Even outside of hurricane zones and green roofing - all roofing projects can be affected by wind, and uplift must be accounted for in every roof design.
In any climate zone, high winds can damage and destroy roofing with wind-born debris during storms. But the biggest concern is displacement of the roofing from pressure pushing it up from underneath. This is where the whole roof can end up in the parking lot, or on top of the building next door. Add on thousands of pounds of plants and soil, and it is clear why this is a primary green roof design considerations.
University of Miami Green Roof Breakthrough
The University of Miami, in a great advancement of sustainable development, pushed through the wind resistance and adopted a hurricane-strength green roof system for their new Lakeside Village student community housing development – a 25 building community of new dormitories. The university is located in a particularly sensitive wetland, calling for extra environmental considerations with each new campus development. Green roofing was selected to top each building, as a solution to manage on-site stormwater, cool the heat island effect on the campus, provide pollinator habitat, and protect the roof from wind and ultraviolet damage. Students often include campus sustainability in the criteria for selecting their colleges, and the sloped design allows them to view their flowery roofs from the ground below.
This project was designed by the architectural firm Arquitectonica and has the rare distinction of being given a notice of acceptance (NOA) by the county of Miami-Dade for hurricane zone green roofing. The green roof and waterproofing components were provided by Green Roof Outfitters (GRO) and Henry Company, and installed by Paragon Painting and Waterproofing, and GreenRise Technologies.
The design intent presented formidable challenges to the green roof status quo. For example, not only are all of the green roofs located in the middle of a coastal hurricane superhighway, they all have slopes of varying degrees, on a cantilever (one corner raised instead of two). The project incorporated many features to account for the sloped decks, and especially for the hurricanes the green roofs will endure.
Wind concerns had virtually eliminated the possibility of widespread green roofing in Miami, until it was fully demonstrated that this system would be capable of withstanding a Category V hurricane. After an extensive review by Cronin Engineering, the firm working with Miami-Dade, and they agreed that the system was safe for use in High Velocity Hurricane Zones (HVHZ) and issued a notice of acceptance for the project.
Special Features of the Green Roofs on University of Miami
- Double parapet enclosure around each roof. The outer parapet wall allows high winds to roll within the more turbulent edges of each roof, while the inner enclosure reduces uplift pressure on the vegetation and wind scour at the corners.
- The waterproofing membrane is fully adhered to the concrete deck for slip resistance.
- A capillary action drainage mat was used in place of a standard molded plastic drainboard, to provide a balance of water retention and positive drainage beneath the soil. It resembles a sponge, with a lot of surfaced area to grip the roof in sloped conditions. This layer manages the hydrostatic pressure of the stormwater, to retain adequate moisture at the higher end of the roof slope and avoid too much water at the base. Soil that is not uniformly moist is more likely to slump, and is less supportive of the vegetation. Thriving vegetation helps anchor the soil during windstorms.
- A slope stabilization system was used, consisting of steel cables and a geotextile mesh within the soil, anchored down into the deck, and also bolted to the parapets. Two layers of mesh were placed within the soil, with curved cleats attached to the mesh, keeping the soil tightly confined, and holding it in place in spite of the slopes and winds.
- A biodegradable jute mat was installed over the top of the soil, for further erosion control during the plant establishment period. The plants quickly grew in to cover the jute and soil, and now with their roots entangled in the mesh, they anchor the soil in a way that mimics the nearby mangrove forests. The mangrove trees serve a critical ecological role during hurricanes. They grow on the coastal waters edge, and their roots hold the sand and soil in place even throughout the highest winds.
Hurricane Strength Green Roof Research and Testing
After the success of this project, additional testing and research led by Henry Company, was conducted in a laboratory setting to see what else could be accomplished in hurricane zones. Results demonstrate that several configurations of Henry 790-11 Hot Rubberized Asphalt Waterproofing with Green Roof Outfitters overburden, and Owens Corning Extruded Polystyrene, can withstand Category IV hurricane strength lateral winds without damage to or displacement of any of the components, and achieving a wind resistance rating of 350 pounds per square foot.
Over a dozen green roof system models were built to reflect common green roof and overburden scenarios across many wind zones. Test models included different planting conditions, parapet heights, and various ways to anchor components.
Despite cranking up the wind tunnel to its highest capacity at Category IV hurricane strength, there was no displacement or damage to any part of the systems, waterproofing through plants. Engineering judgements based on a multitude of lab tests independently confirmed six types of overburden roofing systems that can withstand high velocity hurricane zone winds, in the categories below.
- Extensive green roof systems with anchored components and soil
- Concrete, porcelain and wood pavers, anchored with fastening kits and pedestals
- Synthetic turf anchored to fastened pedestals
Hurricane-Strength Green Infrastructure
Concerns about wind uplift have held back the development of green roofing in many urban hurricane zones, especially throughout Florida, the Carolinas and Texas. It has slowed development of on-structure vegetation for high-rise buildings and those exposed to lake effect winds. Green roofing is the solution to so many of the environmental concerns caused by urban development, including stormwater management, heat island effect, and air pollution. With this recent research overseen by Henry Company, designers and urban planners can now confidently incorporate on-structure vegetation into their green infrastructure strategies, across all wind zones.
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