BEST TECHNICAL PRACTICES
Rooftop Garden Designs
Using our provided best technical practices will ensure the long term performance of your southeast green roof project. James Greenroofs provides in-depth project involvement; from design consultation to on-site installation training and oversight. Utilizing the LiveRoof vegetated tray system will guarantee you are starting with a product designed to withstand the unpredictable (and often predictably hot and humid) climate in the Southeast United States.
Our green roof experts have developed a set of tools and understanding gained only from years of experience growing and researching plant species. James Greenroofs utilizes numerous different plant genus and species for each individual project, because each project has unique site requirements. Once delivered, the LiveRoof product is fully vegetated to help out compete invasive weed pressure.
We strongly recommend that you download and reference our Designer’s Checklist for every green roof project you design.
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.
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.
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 and Maxx systems.
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.
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.
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.
- 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
In his paper, “How Green Roofs Partition Water, Energy and Costs in Urban Energy-Air Conditioning Budgets,” Paul S. Mankiewicz PhD of the Gaia Institute illustrated the hidden relationship of water and energy when he asked the question, “Is it more cost-effective to utilize potable water for cooling than electrical air conditioning?”
He found that the cost of electricity for a ton of air conditioning in New York costs $13.50, while the evaporation of 33 gallons of water produces an equivalent ton of air conditioning for only 26 cents.
Central to his comparison is the fact that virtually all electricity has substantial water cost for its production. Each ton of air conditioning requires 84 kilowatt-hours to produce. In four major cities, this quantity of electricity takes between 24 and 89 gallons of water to generate. On the average, each gallon of water used to cool a structure will save 1.7 gallons of water used to produce the equivalent cooling using air-conditioning.
Overhead, matched-precipitation rotor systems are the best for irrigating green roofs, and they do a better job with less water than drip or sub irrigation methods. In 2011, an irrigation study was conducted in the MSU Plant Science Greenhouses by Bradley Rowe PhD, in LiveRoof® green roof modules. Substrates that were watered with overhead irrigation retained more of the water dispersed with less waste than sub-irrigation and drip systems. This is because green roof substrates are designed with coarse-textures for drainage, and therefore do not wick water or move it horizontally very well.
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. The National Fire Protection Association defines “high-rise building” as a building greater than 75 feet (25 m) in height where the building height is measured from the lowest level of fire department vehicle access to the floor of the highest occupiable story. Low rise buildings (75 feet and lower) are less affected than high rise buildings (greater than 75 feet) 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 RoofEdge 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.
During 2012, LiveRoof® LLC worked with a team of code officials and engineers from the US and Canada to become the first green roof system in North America to undergo full scale dynamic wind uplift testing, giving us empirical data related to how the LiveRoof® system performs against uplift, with and without LiveRoof®’s patent-pending WindDisc™ technology.
Suffice it to say, during laboratory testing, the LiveRoof® system with subterranean overlapping edges and full vegetation performed admirably and its performance was further enhanced with WindDisc™ technology.
The WindDisc™ is a simple way to secure LiveRoof® modules together to improve wind uplift resistance. At uplift pressures exceeding 200 psf, the modules remained connnected across the green roof surface with the WindDisc™ module connectors in place.
The WindDisc™ technology allows for any size LiveRoof® modules or RoofStone pavers to connect together across a green roof installation. Please contact us for more information.
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 mitigate 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 lbs per sq ft, and thus meets the definition of #4 Ballast (3.13.1). The deeper Standard (4.25”), Deep (6.25”), Maxx 8” and systems meet the definition of both#4 Ballast and #2 Ballast (3.13.2).
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.
- 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.
- Overlaying the LiveRoof® with a mechanically fastened stainless steel netting such as CarlStahl’s Decorcable, flexible stainless cable mesh, firstname.lastname@example.org, 800-444-6271 or G-Sky Netting.
- Adhering the LiveRoof® modules to a fully adhered rooftop using special two-sided adhesive tape.
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 as it can easily be offset with irrigation, and we would consider a built-in irrigation system mandatory on any tall building regardless of climate.
At the present time, the level of experience and bona fide research with green roofs on tall buildings is limited. 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, common sense, and horticultural know-how.
The tallest LiveRoof® brand green roof, to date, in Chicago is at 28 stories. Installed in 2010, it has persisted without a hint of plant damage. LiveRoof® has also been installed on a test plot at 50 stories in 2010 and it has performed well.
We suggest the following:
Going forward, we suggest that green roofs be applied to tall buildings with a cautious and thoughtful approach. Use the toughest, most cold-tolerant species, install them early in the season to allow for acclimatization to winter, and always incorporate a built-in irrigation system into the design. Work with the local LiveRoof® representative to determine whether WindDisc™ or other uplift mitigation measures are required and to ensure that proper plants are selected for your high-rise application.
In January 2008, LiveRoof® was tested to see how it performs when its surface is exposed to flame via a test method typically applied to other roof coverings. In this case, a Sedum-populated LiveRoof® Standard modular system was installed on top of a plywood deck and subjected to a direct flame for 10 minutes. Following 10 minutes, there was no ignition of the plywood deck and no spread of the flame via the plant material. While, the plants in the path of the flame were scorched and reduced to ash, they did not ignite and spread the flame. Neither did the LiveRoof® soil, and the module itself remained intact. Note: if the LiveRoof® modules were populated with plants other than Sedums, which are succulent, the result may vary. For example, if dry grasses were planted in the system, one might expect them to burn and propagate the flame.
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).
In July 2011, the LiveRoof® Hybrid Green Roof System became the first to be FM Approved according the FM Standard 4477. Developed by FM Approvals, LLC, FM Standard 4477 is the approval standard for vegetative roof systems. It evaluates green roof performance related to fire, foot traffic resistance and water leakage. In addition to testing the LiveRoof® system, FM Approvals examined LiveRoof®’s manufacturing facilities and audited its quality control procedures to verify that the company produces a consistently uniform and reliable product.
Recognized and respected worldwide, the FM Approvals certification process assures that products and services have been objectively tested and proven to conform to the highest property loss prevention engineering standards. Building owners and facility managers who rely on FM Approved products now have LiveRoof® as a certified green roof solution.
The approval is granted to the LiveRoof® Standard and Deep Systems populated with succulent groundcovers. For specific information on Approval requirements and compatible membrane assemblies, view our FM Approval Report.
- 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.
LiveRoof® is compatible with nearly any roofing system on the market. Over the years, we have seen our system successfully installed atop membranes manufactured by nearly every supplier in North America.
In recent years, more of these manufacturers have decided to introduce their own green roof systems. The quality and the integrity of these systems is variable, thus certain manufacturers have used warranty provisions to limit options for their customers to only system which they sell or market. The most common warranty used to control the green roof options are overburden removal warranties.
Overburden removal warranties cover the costs of removing and replacing the green roof system in the event of a roof leak. To make sourcing a single source easy, LiveRoof®, LLC has developed relationships with the following manufacturers who offer single source overburden removal and leak repair warranties:
- CertainTeed (US Only)
- Flex Membranes (US, Canada)
- Sika Sarnafil* (US, Canada)
- Versico (continental US Only)
Other membrane manufacturers may consider adding overburden removal coverage on a case by case basis.
When overburden removal coverage is not available from the membrane manufacturer, LiveRoof® Overburden Removal Warranties may be purchased for terms of 10, 15 or 20 years. This warranty provides coverage for the costs to remove the green roof and to replace it after the leak has been serviced. Only Diamond Level LiveRoof® Certified Installers are authorized to provide LiveRoof® overburden removal coverage. Contact your local LiveRoof® representative for referrals to qualified contractors in your region.
The slip sheet is the protective layer used between the modules and the membrane. The material is typically selected and provided by the membrane manufacturer.
For Conventional Membrane Roof Assemblies, we recommend a minimum 1-1.5 mm (40-60 mil) thickness with overlapped and effectively bonded seams to ward against root penetration and to keep waterproofing layer safe and clean from soil during installation.
Slip sheet/root barrier typified as follows: Welded Seam Types – 1 mm (40 mil) or greater thickness
- TPO, with seams heat welded
- PVC, with seams heat welded
- Polypropylene, with seams heat welded
- HDPE, with seams heat welded
Glued Seam Types – 1 mm (40 mil) or greater thickness
- EPDM, with seams overlapped a minimum of 75 mm and glued with roll out adhesive or double sided tape adhesive of the type that is impervious to and not affected by moisture, and recommended by the manufacturer.
- Low profile drain board of appx. 0.5 mm (17 mil) thickness, with edges overlapped 75 mm and glued with manufacturer approved adhesive.
For Protected Membrane Roof Assemblies, we recommend a minimum .25mm (10 mil) thick slip sheet of woven polyethylene or other non-moisture holding material to be installed above the membrane and below the insulation, as specified by membrane manufacturer.
A minimum .25mm (10 mil) thick slip sheet of woven polypropylene or other non-moisture holding material to be installed above the insulation and below the green roof modules, as specified by membrane manufacturer.
Do not use duct tape or adhesive for seaming that is not approved by the membrane manufacturer. Never use moisture holding fabric, such as needle-punched polyethylene or felt, under the green roof system. Such materials are trap aggregate and are impossible to sweep during installation and stay wet and encourage root growth and root penetration, which is especially detrimental if woody plants become established as such plants have woody root systems and may potentially cause roof leaks. This could lead to impeded drainage and compromise plant health.
In cases where electronic leak detection may be desired, a fiber-backed drainboard may be used. Fiber-backed drainboards are only recommended when electronic leak detection is desired, and only when vegetated with Sedums or Sempervivums, or other succulents, as these plants are sparsely-rooted and not prone to rooting into the fiber of the drainboard.
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.
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.
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.
Rainfall and Stormwater Management Considerations
Rainfall and Stormwater Management Considerations
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.
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.”
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:
For an in depth review of precipitation for your region, visit the National Weather Service website.
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.
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.
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.
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.
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.
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.
Confirm with you local LiveRoof® Licensed Grower.
|(dry weight basis)||lbs/ft3||60.37|
|Moisture (as received basis)||mass %||21.0|
|Total Pore Volume||Vol. %||58.8|
|Maximum Water-Holding Capacity||Vol. %||48.3|
|Air-Filled Porosity||Vol. %||10.5|
|(at max water-holding capacity)|
|(saturated hydraulic conductivity)||in/min||0.434|
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.
The map below contains data and information links for rainfall absorption rates from studies performed across North America.
View Green Roof Stormwater Retention Studies in a larger map.
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.
Additional Design Resources
LiveRoof® BIM Components for the LiveRoof® Standard, Lite, Deep and Maxx Systems are now available on Autodesk® Seek and our details page.
LiveRoof® Global is pleased to provide the online continuing education course, Green Roof Design Considerations. The course satisfies AIA, HSW, GCBI and many more continuing education requirements.
With a focus on hybrid green roof systems, this course provides an overview of green roofs, including system options and design and specification considerations, such as plant selection, irrigation, mitigation of wind pressure and fire risk, sloped applications, and warranty options.
AIA/CES Info: Course # AEC698
AIA/CES Learning units: 1.00
AIA approved course. This course qualifies for 1.0 LU/HSW hour.
GBCI Info: Course # AEC698
GBCI CE Hours: 1.00
USGBC/GBCI approved for GBCI CE Hours. Course approval #910000152.
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