If your green roof has a parapet then you are lucky. Most testing has shown parapets minimize wind damage to a green roof - though in a large hurricane, all bets are off.
Even if your green roof does not have a parapet it can still be designed to minimize tropical storm wind damage.
The video here is of a small green roof in Jacksonville, Florida on August 25th, 2008 when Tropical Storm Fay pummeled the northeast Florida area with over twenty inches of rain. The video shows how the roof reacted to gusts up to 50 mph. Wind speed was recorded using an ExTech anemometer.
The roof shown in the video does not have a parapet and is sloped. The video illustrates how the irregular surface of he vegetated roof interrupts wind generated uplift that can damage asphalt shingle roofs. The plants range in height from two inches to six inches and can be seen moving back and forth in response to the wind.
Wind racing across a flat surface can create lift - or a vacuum - and literally lift the shingles or roll roofing up off the decking below.
The plant habit acts to break the shear flow of air, creating turbulence and working against damaging uplift. Planting more wind tolerant plants, such as some succulents or grasses may actually create wind breaks in a manner similar to the way a parapet would act.
Another important quality of a well designed green roof is the drainage factor. The roof here is allowing a rapid drain of roughly 18" of rain over a 24 hour period without washing out.
Monolithic hurricane mats used as the basis of the planting system allow for quick drainage of the stormwater and create a mechanism where plant roots can attached and anchor themselves to the roof all the while holding soil in place. A well established root architecture is important for tropical green roofs subject to high winds and heavy downpours.
We will always deal with the 5 H's here in Florida - High Heat, High Humidity, Hard Frosts, Hurricanes and Hard Desiccating Winds, but with good green roof design your Florida Green Roof can hopefully withstand a severe storm, including tropical storms!
In the meantime, there are some precautions the green roof owner can take to prepare for tropical storm season (May 15th for the Pacific area and June 1 for the Atlantic regions).
At a minimum, we recommend;
- No large trees on a roof.
- Small shrubs and small trees may be used successfully depending upon the final design. This may seem like a common-sense guideline but people try to put all types of tall, large trees on patio or garden roofs. During a 130 mph cyclone, the tree may be blown over and may cause damage from the fall against the structure or to the street below. It may also become airborne if the winds are strong enough. Growing up in Hialeah I saw plenty of hurricanes come through our area and witnessed first hand the power of these storm events.
- Anything and everything on a roof should be permanently attached.
- Walkways should be constructed from a permanently attached TPO, EDPM or other mat and permanently affixed to the roof.
- No loose chairs, tables or other items should be present. If you wish to have a chair and table stay on a roof during a cyclone, they must be permanently attached.
- All green roof components must be permanently attached to the structure.
- Any trays, plastics, pots, containers or other green roof components must be permanently attached to the building structure. Florida Building Code does not allow for loose items to be installed on a roof - they must be attached.
- Green Roof Irrigation components must be permanently attached to the roof.
- Make sure all tools and gardening utensils are picked up and put away.
- It is very easy to forget the pair of shears, scissors or pliers on a roof. Remember what you were using and where you liad them.
- Plant selection should be focused on those species that have historically survived cyclone and hurricane incidents. There are several good books available at most bookstores here in Florida on proper cyclone resistent landscaping and many resources on the web, such as the Brevard County Landscaping Guide for Hurricane Areas.
- Check on the NOAA National Hurricane Center website daily. The NHC webpage is a wonderful resource, full of links to climatic data.