GREEN ROOFS + LANDSCAPING THEORY

Click on this video to get acquainted with what is greenroof is all about!

Green Roofs are Cool!

A green roof system is an extension of the existing roof which involves a high quality water proofing and root repellant system, a drainage system, filter cloth, a lightweight growing medium and plants.

Green roof systems may be modular, with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids, or, each component of the system may be installed separately.Green roof development involves the creation of "contained" green space on top of a human-made structure. 

This green space could be below, at or above grade, but in all cases the plants are not planted in the "ground'.  Green roofs can provide a wide range of public and private benefits.

A green roof can certainly make a building look nicer, but can it measurably lower energy requirements and improve water management? At Penn State University, engineer Jelena Srebric and horticulturist Rob Berghage are working together on a project to measure and model how a green roof affects the way buildings use energy and water. In Jelena's lab, undergraduate student Tyler Meek does weather experiments indoors to determine the effects of light, wind and water on the roof. Graduate student Paulo Tabares Velasco works with Tyler to make a mathematical model to explain the experimental data and predict what will happen in the real world. The researchers hope that architects will someday use their model to make building designs even greener.

How do "Green Roof" saves energy?

Here are selected videos to show benefits of green roofs from covering office buildings to gourmet garden plots on your home.

Green Roof® Saves Money. 

Prevegetated modules discourage weeds and save the time and money traditionally required to develop and maintain a green roof grown from plugs, seeds or cuttings. Other systems may require three or more years for establishment and intensive care is needed during this time period. Well-established LiveRoof® plants and roots mean an instant green, successful, and instantly functional green roof. 

Maintenance cost estimates for traditionally planted green roofs vary from $1.00 to $2.00 per square foot annually. LiveRoof® can be maintained for as little as $0.15 to $0.30 per square foot. Being fully grown, LiveRoof® plants act as a living mulch.

Architecture + Landscape Engineering 

Here are list of issues that need to be addressed by your architect and engineers on your green roof:

Annual Rainwater Absorption

BIM Components

Downward Force Against Parapet For Sloping

Effect of Supplemental Irrigation

How Much Slope Is Too Little?

How Much Slope is Too Much?

Fire / Spread of Flame

FM Approval

Irrigation vs. No Irrigation

Job Site Safety

Level or Gently Rolling?

Native Plants

Parapet Design

Plant Basics

Positive Wind Pressure

Rainfall Absorption

Rainfall Intensity

Retrofit Roofs

Recommended Plants

Roof Slope

Runoff Basics

Runoff Coefficient

Seasonal Weather and Rainfall Patterns

Soil Characteristics

Soil Depth

Tall Buildings:  How High is Too High?

Wind and Sun

Wind Pressure

Wind Uplift?

What Will My LiveRoof Look Like In The Future?

Wind Pressure

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

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, Uplift

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.

How Much Uplift?

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 accepted testing standard or code.

• However, in 2010 the American National Standards Institute (ANSI) accepted RP14 Wind Design Standard for Vegetative Roofing Systems as an American National Standard. This document provides design and installation recommendations to help eliminate the risk of wind uplift on green roofs in high wind areas. LiveRoof modules are fully vegetated at the time of installation and have subterranean overlapping lips, which allows them to be sheltered from direct wind exposure. 

• The LiveRoof Lite System is 2.5” deep and has a dry weight of approximately 10 psf, and thus meets the definition of #4 Ballast (3.13.1). The deeper Standard (4.25”), Deep (6.25”), Maxx 8” and Maxx 12” systems meet the definition of both #4 Ballast and #2 Ballast (3.13.2). This standard serves as a guide but should not replace the involvement of a qualified professional.

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.

Parapet Design

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.

Wind Challenged Applications

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).

• A. 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.

• B. 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.

• C. Adhering the LiveRoof modules to a fully adhered rooftop using special two-sided adhesive tape.

Tall Buildings: How High is Too High?

Tall buildings present three substantial challenges for green roofs. The first is wind uplift. This is a physical phenomenon that presents certain design and engineering considerations, discussed in detail under the wind uplift section.

The second is wind scour. This is the physical displacement of soil and/or plants due to the force of the wind. The best defense against wind scour is full-vegetation, which LiveRoof provides. It is also important to remediate any “bare-patches” that might arise in the future. Bare patches can be caused by weed encroachment, nesting, birds, or physical damage.

The third challenge involves the plants and their ability to resist the wind and cold at high building elevations.

The answer to the question “how high up can green roof plant survive?” is not well understood, at least not at this time. Very tall buildings, for example over 20 stories tall, are subject to virtually constant wind. This in itself is not a problem for the plants, as most green roof plants hail form alpine environments where there is also a lot of wind.

The problem seems to be an issue of dehydration and wind chill, particularly in areas of persistently cold winter temperatures. In such cases, the soil will be frozen for prolonged periods of time, and therefore, the plants can not extract water from the soil to replace the moisture lost from wind desiccation. If this occurs for too long of a duration, the plants essentially become “freeze dried”, and their tissue dies.

At the present time, the level of experience and bona fide research with this problem is not sufficient to be able to quantify the effect in such manner that makes it simple to properly plan and design for tall building applications. No one can say “on the 35th floor in New York City, you can effectively grow these five species of plants”. Therefore, at present we must rely upon anecdotal experiences, and upon common sense.

What we suggest is the following:

The issue will likely be more significant in Chicago than Atlanta: The tallest LiveRoof brand green roof, to date, in Chicago is at 28 stories. Installed in 2010, but it has not yet seen a winter. We have, however, installed a LiveRoof brand green roof on a 16th. story exposure during 2008, and it has persisted without a hint of plant damage during wintertime (note: the winter of 2008/9 was one of the coldest in recent history).

Deciduous plants: Plants that drop their leaves are likely better equipped to resist the rigors of constant winter chilling winds. This is because they present less surface area for chilling.

Irrigation is good: In areas where the winters tend to be dry, giving the plants periodic irrigation can help them to resist dehydration during cold spells.

As we learn more about this issue, we will make that information available.

Downward Force Against Parapet For Sloping Applications

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.

How Much Slope is Too Much?

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.

How Much Slope is Too Little?

While this question is seldom asked, it is important to design for adequate drainage. Most authorities state that a roof needs ¼”/12’ slope to provide adequate drainage. Without this, water may accumulate and damage the health of your LiveRoof plants.

Level or Gently-Rolling?

Most LiveRoof installations simply follow the contour of the roof for a lovely, gently-rolling, meadowlike appearance. If a dead-level LiveRoof is required, it can be realized by applying a tapered closed cell foam to the roof above the waterproofing layer. If this is done, the closed cell foam must allow for adequate water drainage.

Job Site Safety

Remember, wind uplift should be managed during the entire installation process. High winds can come at any time and will not wait for the installation process to be completed. Be sure to cover materials with appropriate temporary ballast.

Retrofit Roofs

Retrofit Projects are exciting as they represent a tremendous upgrade to aesthetics and environmental quality. Of course, they bring their own particular challenges that need to be addressed from an architectural and engineering standpoint. Here are some of the main considerations for retrofit green roofs.

Determination of the construction, condition, and load capacity of the pre-existing roof and suitability to accept a LiveRoof.

Determination of the condition of, remaining warranty lifetime, and terms or warranty of the existing waterproofing system as it pertains to being retrofitted with the new LiveRoof.

Compatibility of the existing waterproofing system with the proposed slip sheet membrane.

All the same issues regarding positive and negative wind pressure, slope, and forces against the parapet as they relate to new construction also apply to retrofit roofs.

In 2010, the American National Standards Institute (ANSI) accepted GRHC/SPRI VF-1, Fire Design Standard for Vegetative Roofs as an American National Standard. This document provides design and installation recommendations to help eliminate the risk of fire on green roofs. A code change proposal has been submitted to the International Building Code to include this standard in the 2012 edition of the International Building Code. Depending on the plant selections, LiveRoof systems meet the requirements to qualify as generic fire-resistant “Succulent based systems” (4.1.1) or “Grass based systems" (4.1.2).

To Irrigate or Not to Irrigate?

While irrigation may only be needed during protracted hot dry weather (to sustain the plants), there are other reasons to install an efficient means of irrigating one’s green roof. Irrigation allows the green roof to be fully optimized. With the ability to irrigate during hot dry weather the rooftop can be turned into one big cooling unit and save money on air conditioning. Remember water liberates 8000 BTU of energy during evaporation (latent heat of evaporation), and pumping water is efficient and cheap, but running air conditioners is inefficient and expensive. The cooling effect derived by irrigating allows for the conservation of energy in comparison to the energy wasted on cooling by less efficient methods.

According to some authorities, and dependent upon the particular climate, during the cooling season the temperature in the room below an irrigated green roof may be reduced 16 to 27ºF compared to a reduction of about 11-13ºF for a non irrigated green roof. This difference is substantial and can mean considerable savings on air conditioning costs. Estimates of cost savings for air conditioning range from 25% to 50% for the floor under the green roof. Irrigating during hot dry weather allows for the optimization of the green roof’s cooling ability.

In rough figures, when an extensive irrigated green roof shows an average summertime temperature of 80 degrees, the same roof without irrigation will average about 100 degrees. Similarly, the membrane below the irrigated roof might fluctuate an average of only 7 or 8ºF during a 24 hour period, while the same green roof without irrigation may fluctuate ± 20 degrees. Less fluctuation may mean less wear and tear via micro-tearing on membranes, and therefore potential extension of the lifetime of the waterproofing membranes.

Judicious irrigation also keeps the green roof plants fat,

full and beautiful. This means better coverage, fewer weeds, less labor, and happier owners, occupants, and visitors. It also means lower maintenance costs and safeguards one’s investment in the green roof.

Finally, judicious irrigation should not significantly

impact stormwater management as irrigation typically occurs only during low rain/low runoff periods when the roof will dry out quickly from evapotranspiration.

Benefits of Irrigation vs. No Irrigation

• Net energy savings

• Reduced temperature fluctuation (less wear on membrane)

• Less maintenance cost

• Plants will be optimally beautiful

• Avoid plant loss due to drought

• Greater owner satisfaction

Plant Basics

The LiveRoof Standard and LiveRoof Lite systems are “Extensive” green roof systems. In other words, their soil depth is less than 6 inches. And, while extensive green roof systems optimize evaporative cooling and storm water management (in part because they can dry down between rain events), their shallow substrate depth means that the plants they can support must be extraordinary at resisting drought. Practically speaking, the plants that work best in “extensive” green roofs must be exceptional “water conservers” as opposed to “water sourcers”.

Water conservers are plants that store copious amounts of water in their fleshy stems and leaves. Cacti are the poster children for “water conservers”. They absorb water when available, and conserve it by closing their leaf pores during the day, by having a waxy cuticle over their leaves and stems, and by having relatively little surface area. Water sourcers, on the other hand, are plants that have extensive and deep root systems that go deep into the earth in search of water. Good examples of “water sourcers” are prairie plants such as little bluestem, purple coneflower, and prairie dock.

Recommended Plants

The LiveRoof system is typically vegetated with a palette of deciduous, semievergreen and evergreen “base mix”

and “accent plants” that are exceptional “water conservers”. These are succulent, water-holding plants like Sedums, Alliums, Sempervivums, Euphorbias, Delospermas, and other species. The best LiveRoof plants both store water and have a special type of metabolism called Crassulacean Acid Metabolism, CAM for short. CAM plants are unique in that under drought conditions their stomates (leaf pores) are open at night rather than during the day (as is the case with most plants). CAM plants exchange gasses (oxygen and carbon dioxide) in the dark when it is cooler and less windy and therefore conserve water. And, CAM plants are up to ten times more efficient with water conservation than non-CAM plants.

Can Native Plants Be Used?

While it is popular to say that native plants are better adapted because they evolved here, this notion is not necessarily true. A plant’s toughness or suitability, is dependent upon genetics and ecological and environmental adaptation (evolving with time and exposure). There is nothing magical about latitude and longitude as there may be similar or more demanding environmental conditions on the other side of the globe. In reality some native plants are tough, some aren’t, and a few will grow in an “extensive” green roof without frequent irrigation. The list, however, is quite short as the native ecosystem parallel would be a giant rock covered in 2 to 4 inches of gravelly soil with loads of reflected light from bordering rocks. Such “real world” parallels are few and far between.

Even though there is not a long list of native plants for use in extensive green roofs (unless one plans to frequently irrigate), there are a few to choose from. Such species as Sedum ternatum (white flowered sedum, a shade lover), Opuntia humifusa (prickly pear cactus), and Allium cernuum (nodding onion) are such plants. Of course, with regular and frequent irrigation, many others can be sustained, and plants that fall into this category include purple coneflower (Echinacea pallida) and little bluestem (Schizachyrium scoparium). These plants are very drought resistant in conventional landscape settings, because they are great “water sourcers”. On a rooftop with 4 inches of soil, however, they won’t survive for long unless regularly irrigated. Such plants are better suited to the LiveRoof Deep system.

The bottom line: LiveRoof growers are interested in using plants that will be successful, regardless of regional nativeness. Rest assured, LiveRoof growers only use plants native to this planet.

What Will My LiveRoof Look Like In The Future?

All plants are unique, and are opportunistic in one way or another. LiveRoof plants are no exception, and practically speaking, some species tolerate heat better than others, some cold better than others, some dry conditions, and others moist conditions. By combining species of varying growth characteristics, we strive to design each LiveRoof plant assortment to perform optimally in all seasons.

Over time, depending upon the particular plant assortment, geographic site, climate and microclimate, the plant assortment will adapt and evolve. One species will increase its presence while another decreases its presence, from season to season, and from year to year. It is this evolutionary dance that helps to make each LiveRoof fresh and exciting now and in the future.

Rainfall Absorption

From the perspective of civil engineers and city planners, the capture of rainfall may be the greatest perceived benefit to green roofs. Sewage infrastructure and retention tunnels are expensive, and green roofs can have a significant impact on reducing the need for such infrastructure.

Runoff Basics

It is common to ask how much water the LiveRoof system will absorb. Or, how much of the initial rainfall (e.g. first 1/2 inch, 3/4 inch, etc.) will be absorbed prior to system saturation and run off. The answer must always begin with five words: “It depends upon many variables.”

LiveRoof, like other green roof systems is expected to absorb a relatively predictable amount of water throughout a full season (based upon soil depth and many other variables). And, absorption at the onset of a rainstorm may capture all, some, or no water. This depends upon how moist the soil and plants are at the onset of the storm. For example, if Monday evening is cold and drizzly, and Tuesday brings a tremendous downpour, then most of Tuesday’s rainfall would flow through the system. On the other hand, if the system were quite dry at the onset of Tuesday’s storm, then the absorptive capacity of the soil would be much greater, and any runoff would come some time later than the onset of the storm event, as the soil must first reach a point of saturation before it allows water to pass through. This process is referred to as “delay in peak flow” and is important as the distribution of stormwater discharge over a longer period of time allows for smaller more efficiently used stormwater infrastructure.

Annual Rainwater Absorption

Most research (Liesecke, 1998; Moran et al., 2004; DeNardo et al., 2005; VanWoert et all, 2005) has shown an annual runoff reduction of 60-100%, impacted most significantly by climate. The precise amount for any given locale and any given rooftop is not one size fits all. The factors that come into play are numerous and include the following:

Seasonal Weather and Rainfall Patterns

When comparing the average monthly rainfall and daily average temperatures of various cities, the differences are amazing. In Phoenix, temperatures tend to be hot and what little rain falls is spread evenly over the year. In Portland Oregon, the temperatures tend to be moderate with wet winters and dry summers. In Chicago and New York the winters are cold, spring and fall are cool, and the summers are hot; precipitation is spread pretty evenly throughout the year. Miami is warm in winter, hot spring through fall, and has a defined summer rainy season where rain can fall in torrents. With this in mind, each climate will have different absorptive qualities. A city like Phoenix may experience nearly 100% rainfall retention, because it is typically hot and dry with little rainfall. Cities like Chicago or New York may see annual stormwater retention in the range of 60% to 85%. And a city where the rain comes in torrents will experience less annual rainfall retention. Retention will also vary somewhat from year to year as determined by the year’s weather.

Rainfall Intensity

Soil acts as a sponge in capturing and holding onto water. But, soil is less porous than a sponge, and will take longer to absorb and hold onto water. For this reason, if a rain event is very fast and intense, such as 1 inch over 15 minutes, a certain amount of water may sheet across the soil surface to the roof drains, before becoming absorbed by the soil. On the other hand, if a 1 inch rain comes in a gentle soaking drizzle over the course of a couple of hours, the efficiency of water capture is much greater.

Runoff Coefficient

The approximate "flat-roof" runoff coefficient may be calculated using our Flash Calculator. Runoff will be greater with increasing roof slope.   On roof slopes over 15 degrees, runoff increases by 10 to 15 percent.  In order to use the calculator, one needs three things.

Dry and saturated weight of the medium (soil). 60 pounds per cu. ft. dry and 90 pounds per cu. ft. saturated serves as a ballpark figure. But, verify with local grower as LiveRoof media varies somewhat regionally.

Depth of growing medium. Approximately 4-4¼” for LiveRoof Standard, approximately 2¼-2½” for LiveRoof Lite, and approximately 6” for LiveRoof Deep.

Local Rainfall Data. See MIFAB Rainfall Considerations for Roof Drains website for useful rainfall information: www.mifab.com/pdf/r-sizing-us.pdf.

Note: it is advised that the information rendered from such calculations be considered ballpark, and that an appropriate safety factor of 20% or greater be built into the calculation, and that aggressive rainfall data be used so as to present a conservative view of runoff. LiveRoof, LLC makes no warranty or representation regarding stormwater and runoff calculation. Such information is the domain of engineers skilled in the science of such calculations and matters related to stormwater management.

Wind and Sun

Windy sites, particularly those that are windy during hot dry weather, will dry out more and retain more rainfall on an annual basis. Taller buildings will tend to be windier and those with reflected light or heat will dry out faster and therefore absorb more water as well. Of course, all things being equal, sunny areas are going to have less stormwater runoff then shady areas.

Roof Slope

All roofs must slope in order to drain, and what most people refer to as a flat roof will have a slope of 1/4” per 12’. Such roofs will retain more water than roofs with greater slope. This is because soil holds water by cohesion, but there is a limit to how much cohesive force soil can provide. Practically speaking, soil acts as a sponge. If a moist sponge is angled upward, additional water will run out of it. The same is true of soil. The greater the angle, the less capable the soil is of retaining water. Researchers Bradley Rowe, Kristin Getter, and Jeffrey Anderson have conducted studies regarding slope and water retention with inclines of 2%, 7%, 15%, and 25%, and found that annual water retention in Lansing Michigan ranged from approximately 85% with 2% slope to 76% with 25% slope.

Soil Depth

One might logically presume that the deeper the soil substrate the more storm water a green roof will mitigate. This is not necessarily true as independently discovered by Dr. Bill Retzlaff, et. al., of Penn State and Civil Engineer Drew Gangnes of Magnusson Klemencic Associates of Seattle have published their findings that a 4 inch soil depth performed optimally for storm water retention. This is because at 4 inch soil depth, the combined ability to hold water along with the ability to evaporate water between rain events, created the least amount of runoff. In comparison, a 6 inch system might hold somewhat more water, but won’t dry out as effectively between rain events and a 2 1/2 inch deep system may dry faster but won’t hold as much rainwater.

Soil and Plant Characteristics

Plants and Soil also play a role in stormwater retention. Soil aggregate sizes and composition affect pore space, air space, and absorptive capacity. The volume and type of soil will also influence each system’s ability to absorb water. And, while soil is mostly inanimate, it will gradually change over time due to natural freeze-thaw cycles that can alter particle sizes of some of the mineral components. Neither is the organic component static and can either rise or fall and therefore affect the soil’s absorptive capacity. The process of plant growth and decay (leaves roots and stems) can contribute organic matter, particularly deciduous plants which shed their leaves to nourish the soil. Fully evergreen plants on the other hand may actually reduce organic matter from the soil.

Plants also affect water retention and runoff by their use of water. They extract water from the soil and combine it with CO2 to make sugars, and they liberate water to the atmosphere by transpiration (similar to sweating) when it is sunny and there is sufficient water available. In either case, plants help to keep rainwater out of the stormwater system and the more densely they cover the soil surface, the more absorptive capacity they have.

Effect of Supplemental Irrigation

While irrigation is used to sustain the LiveRoof system during hot dry periods, a particular roof will likely experience little reduction to its absorption of stormwater. Typically irrigation is used, temporarily, during times of sparse rainfall and the added water is mostly evaporated or sequestered by the plants.

Here's time lapse video on the installation of greenroof landscaped roof deck.

Essentials of a sustainable Green Roof: