As with any roof, high winds can pose a threat to the security of green roofs, and care must be taken to properly design and engineer the green roof so that it retains its integrity during high winds. To do this, consideration of wind pressure and associated variables, such as the building's geographic location, surrounding terrain, shape, slope, height, building openings, parapet design, and other features is essential.

At the tip of the iceberg, of wind pressure, one must consider the typical high wind speeds for that region. Consulting ASCE 7.95 Figure 6-1 Basic Wind Speed, or Factory Mutual Global Property Loss Prevention Data Sheet 1-28 is a good first step. In addition, the engineer must consider the surrounding terrain; for example, is the building situated along water, mountains, open field, surrounded by tall trees or taller buildings?

Of course the building design itself is very important. Low rise buildings (generally regarded as 60 feet and lower) are less affected than high rise buildings (60 feet and taller) which in addition to direct (positive) wind pressure are more greatly affected by negative wind pressure, often referred to as uplift or suction.

Positive Wind Pressure is the force exerted by the wind as it strikes an object, or building. Positive Wind Pressure is evident when a tree (or other object) moves or bends over in a strong wind.

LiveRoof modules, when populated with a base mixture of flexible-stemmed hardy sedums (the backbone of the LiveRoof product line) were wind tested on 1/25/08 with wind speeds exceeding 110 MPH. In this test, the LiveRoof planting (4' x 5') was surrounded with Edging and first exposed to 10 minutes of wind at 95 MPH, followed by 1 hour and 50 minutes at 110+ MPH. The wind was impinged directly upon the surface of the LiveRoof planting as would be the case when testing other roof coverings. Remarkably, at the end of the test period, there was no loss of growing medium and all plants remained well rooted and intact. Throughout the test, the plants simply arched over, held in place by their root systems. This test demonstrated the value of full vegetative cover as a means of stabilizing the green roof system.

Negative Wind Pressure is what causes airplanes to fly, and it's what causes roofs to want to fly. Negative wind pressure occurs when wind passes over an object that causes the wind to redirect and accelerate. This in turn creates a pressure differential and the pressure differential can be substantial.

In the case of roofs, wind accelerates as it passes over the roof edge or parapet, causing a pressure differential and lifting force, uplift, that is exerted upon the rooftop. Redirected winds of this nature tend to whirl and swirl, often in cone shaped vortices which can aggressively scour roof surfaces and components. Such forces are typically greatest in the corners of the roof, secondarily along the parapet walls, and to a lesser degree in the "field" or center part of the roof. Uplift forces vary with the building shape and height, parapet shape and height, overall exposure, size of openings, etc.

In answer to the question, how much uplift force can a green roof tolerate, there is no simple answer, at least today there isn't. Available information is mostly anecdotal and research is slow coming. And, because the weight, vegetation, and porosity of green roof systems is variable, and the particular components in which they interface (edging, pavers, parapets, etc.) are diverse, there has been little testing and there is no generally accepted standard or code.

Given the absence of empirical data, many engineers treat green roofs as if they were pavers of similar mass, and pay particular consideration to negative wind pressure, at minimum reviewing the items discussed below. LiveRoof mentions these considerations as an impetus to diligent design and engineering, but does not purport to have specific knowledge of engineering principles. Such expertise and accompanying liability is the domain of qualified engineers. Now and in the future, LiveRoof will pursue research in hopes of providing more precise information as a support service to engineering professionals. For now we offer the following list of considerations to stimulate a diligent review of design and engineering considerations as they pertain to green roofs.

Low rise buildings in areas of moderate exposure may present fewer challenges in regard to Positive or Negative wind forces. But, taller buildings may cause one to have to be more creative. Design strategies that moderate wind uplift forces and disrupt the formation of surface-scouring wind vortices may be employed in the overall green roof design.

Regarding low rise buildings, a lower parapet design may avoid potential air turbulence and help to minimize uplift forces. And, for buildings containing only a single parapet, as is commonly used as a facade for aesthetic purpose, one should keep in mind that the parapet may dramatically increase the uplift pressures in the corner regions. Conversely, on high rise buildings (over 60 feet), higher parapet height can be an effective tool in moderating uplift forces. Studies on parapet height typically indicate that parapets over 3 feet tall can moderate uplift pressure in the corners of the roof on high rise buildings. Likewise, the use of a partial parapet with attached porous screen may be used to reduce uplift pressures and expand design options for taller buildings. And, parapets of different shapes, e.g. saw-tooth configuration, rounded vs. sharp edges, or the application of spoilers are sometimes used.

Keep in mind, that the taller the parapet, the more Positive Wind Pressure against the parapet itself, both windward and leeward sides.

In very challenging applications an engineer may have to direct the architect to forego using the LiveRoof Lite system (about 9 to 10 lbs per sf when bone dry) in favor of the LiveRoof standard system (about 18 to 20 lbs per sf when bone dry). And, in the most wind challenged applications, an added means of securing the LiveRoof (either LiveRoof Lite or Standard) may be needed to safeguard the LiveRoof system. Accessory products for extreme uplift designs may include any or all of the following. (A-C)

1. Limiting the LiveRoof to the center "field" of the roof top, and using heavier ballast in the corners and along the parapet edges. Such ballasted perimeter design is referred to as a "vegetation free" zone. Vegetation free zones will vary with the parapet height and geometry.
2. Overlaying the LiveRoof with a mechanically fastened stainless steel netting such as CarlStahl's Decorcable, flexible stainless cable mesh, sales@decorcable.com, 800-444-6271 or G-Sky Netting.
3. Adhering the LiveRoof modules to a fully adhered rooftop using special two-sided adhesive tape.

The combination of a green roof (unaffixed object), slope, and gravity imply the need to address physical containment and resistance to downward pressures exerted by the green roof against the parapet and mechanical fixtures of the roof especially in cold climate areas where ice crystals may form on the slip sheet/root barrier surface during winter. For this reason, LiveRoof recommends that the slope and size of the roof be assessed in regard to force that will be exerted against the parapet (or other mechanical features of the roof).

For the convenience of engineers, LiveRoof provides force tables for use in designing each particular LiveRoof project. These tables assume "zero" friction and present a conservative model based upon the assumption of ice between the slip sheet membrane and the LiveRoof modules during the winter months. Obviously, this may not be appropriate for frost free zones, but one must realize that certain roofing membranes are coated in talc or other lubricants to prevent sticking. Others membranes may be slippery when wet. Therefore, even in frost free zones, one should assume a degree of downward force on sloping applications.

For long roofs and roofs with great slope, it may be appropriate to incorporate “stops” or buttresses in the design to prevent all of the load from being exerted against the parapet on the low side of the roof. In all cases, it is important to realize that the low side parapet must be built in such manner as to have the structural integrity to resist whatever forces exist given the design of the particular roof.

Both of the main international green roof organizations, the German FLL and North America's Green Roofs for Healthy Cities agree that green roofs should not be applied to roofs with slope of greater than 40 degrees. This stems both from containment challenges but also from the extreme difficulty in managing soil moisture on a roof of such pitch.

You may be familiar with the properties of a wet sponge, where it will hold so much water when laying on its side. But, after you prop it up on its end even more water runs out. Soil acts the same way and as the pitch of the roof increases, there is a greater tendency for the water to want to run out of the system. Green roofs above 2'/12' pitch are commonly dry at the top and moist at the bottom. And, while the segmental or baffled characteristic of LiveRoof may help to mitigate this phenomenon, pitched roofs will certainly require more irrigation than low sloped green roofs.

LiveRoof Garden Brochure - View