Green Roof Monitoring

Extreme weather and a changing climate are now normal. The effects these changes can have on communities are numerous and sometimes serious.

Flooding and pollution can cause economic damage to infrastructure and buildings, not to mention the costs to human health. This is why smart solutions for rising temperatures and increasing rainfall are so crucial.

Green roofs are among those smart solutions. These living roofs can act as natural building insulation, reduce stormwater runoff, and offer a more aesthetically pleasing environment—but only when they are well-designed and functional.

NexSens real-time green roof monitoring systems can ensure that green roofs live up to their promise. On this page, you can find discussions of green roofs and their field performance, sustainable architecture, benefits of eco-roofs, the importance of sustainable roofs, and challenges in green roof design, construction, and maintenance.

We will also describe our facility where we monitor a wide range of green roof conditions, including air temperatures, drainage, evapotranspiration, rainfall, retention, runoff, and soil temperatures.

Sustainable Architecture and Green Roofs

Throughout the United States, as the Earth’s climate warms, extreme weather appears. Heavy spring rains in many places produce summers with flooding, especially in cities and towns in the Midwest.

Thanks to flooding, these communities suffer devastating losses. Dams, levees, and roads are damaged, while wells and other water supplies get—and sometimes stay—contaminated. Drinking water supplies can remain out of commission for weeks.

Adding to this problem is the current state of critical water infrastructure in the US. According to a recent report card from the American Society of Civil Engineers’ (ASCE), dams, drinking water systems, inland waterways, and levees are all at a D level, failing or in danger of failing.

All of this means that sustainable architecture and new infrastructure solutions are essential to healthier, happier communities. Achieving this goal will mean thinking about infrastructure in new ways.

As our atmosphere creates bigger, more powerful storms, our infrastructure will need to withstand more moisture. One way to make this happen is to design and implement smart green roofs that can help buildings last through more extreme rainfall events—and the flooding those events inevitably produce.

How can we design our buildings to ensure they remain competent for the future and ease the impact of major weather events?

Part of the answer isn’t just avoiding water damage. It’s also making better use of the water that comes. This is another great benefit of green roofs and sustainable infrastructure.

Although serious storms can bring large quantities of water quickly, they might also be followed by periods of dry weather or even drought. This means repelling excess water only solves part of the problem.

Instead, a more holistic, greener solution is ideal. With the right green roof monitoring system in place, a user can mitigate pollution through soakaways, flow control, and attenuation, and limit surface water discharge rates to greenfield runoff.

For this kind of engineered, smart solution, it pays to have the right team working on your problem. Only an advanced environmental monitoring team can focus on both adaptation and mitigation in the right proportions to generate environmentally-led, evidence-based, engineered solutions.

Eco Roofs and Other Green Infrastructure

Until more recently, most thinking about infrastructure has focused on gray infrastructure. Gray infrastructure includes the things we use to meet basic energy and irrigation needs, such as dams, pipes, pumps, and plants that manage floods and treat water.

However, green infrastructure is just as important and demands more attention than it receives. Natural green infrastructure includes floodplains, forests, and wetlands, while human-made green infrastructure might include anything from green roofs to green spaces in cities.

There is an important relationship between green infrastructure and gray infrastructure because green infrastructure protects gray infrastructure from risks, higher costs, and both natural and human-made hazards. For example, functional, healthy wetlands reduce flood risk in coastal cities and make treating drinking water easier and less costly.

Furthermore, integrating green and gray infrastructure can save operational costs. It can also render overall processes more efficient to rely more on green infrastructure where possible.

Even so, there is no “green infrastructure report card,” for example. Instead, it is the responsibility of individuals and lawmakers to protect green infrastructure and the resilience and multi-functionality it brings to communities. This is why green roof monitoring is so critical.

Research proves that green spaces help keep water clean. However, these green infrastructure elements also promote human health and stronger communities. Urban green spaces also mitigate against increasingly deadly heat waves and lower greenhouse gas emissions by reducing energy consumption in buildings.

In a world of risk to property and human health and life from extreme weather events, green infrastructure is a crucial component of risk management. This will continue to be true, and over time, it will only get more important to create and manage green public spaces with care.

Green roofs are an excellent example of transforming an urban space to be more sustainable. Green roofs may not be a new idea, but they are a proven, reliable, smart solution. Eco roofs that function well can help even individual households counteract flooding and other climate change impacts.

What Is a Green Roof?

Green roofs, also called living roofs, eco-roofs, or brown roofs, are a popular and valuable mitigation technology, particularly in urban settings. These multi-layered engineered systems look like this, bottom up:

Layer 1 is a waterproof membrane that lines the bottom of the green roof, which is root-proof. Layer 2 is a drainage layer that traps excess water. The material in these layers is highly permeable and open, channeling gravitational water quickly off the roof, but preventing penetration below the lining.

Atop the draining layer is Layer 3, a filter fabric. Instead of soil, Layer 4 is a lightweight, engineered growth medium, usually consisting of clay minerals and shales, forms the basis for the plants. This allows the plants to root and grow, but it also allows for excellent drainage. The light weight reduces the demand for increased structural support in new buildings and enables retrofit installation on existing buildings.

Layer 5 consists of the plants, which are typically sedums. These are a cactus-like succulent, well-suited to drought, and tolerant of many weather conditions. Beyond that, which plants to choose for your green roof will depend on your climate and structure—and it pays to have expert advice.

This vegetation forms the top layer of the green roof, and the porosity and depth of the media are also important for growth and the retention of stormwater. The plants themselves intercept rainfall, shade the roof’s surface, and reduce the rate and amount of direct runoff.

Green roof systems may be classified as deeper “intensive” or shallower “extensive” systems. These classifications refer to the depth of the growing medium. There is no exact cutoff for these classifications, but in general, “intensive” systems include those equal to or greater than 15 cm, while “extensive” systems include those less than 15 cm.

This means that “intensive” green roofs may have more variety and choice in terms of plant options. It is also possible that “intensive” green roof systems offer better environmental performance. For example, using a more ideal plant or one better suited to local conditions may be more possible in an “intensive” system, and this could improve water retention capacity or reduce the temperature more.

In fact, the planning that goes into a green roof is critical. Variety in ecosystem services on any given eco-roof is closely linked to diversity in the plant communities living on that roof. Water availability is central to every decision, controlling health, growth, and survival.

Mastery of the relationships between coverage, survival, and species richness as they relate to temperature, moisture, and water supply is critical to optimal green roof management and service provision. To build a better green roof, so to speak, and manage it more effectively, it is essential to know how green roof substrates and various plant species work together to form diverse types of ecosystems under a range of micro-meteorological variables over time.

Benefits of Green Roof Systems

The layers of the green roof do much more than keeping the roof alive. They also enable the structure to address multiple problems, including runoff, heat and other challenges. This versatility is among the strongest arguments for the widespread use of green roofs in urban areas.

Environmental Benefits of Green Roofs

Green roofs manage stormwater and other runoff like water storage tanks that leak slowly and enable water vapor evaporation. When fully saturated, a typical 10-cm green roof medium can hold approximately 1 gallon of water per square foot.

In most urban areas, rooftops comprise a considerable percentage of total land surface area, a massive yet mostly unexploited water retention and storage capacity. Storing water in soil adds the advantages of insulation, shading, and latent heat cooling as evapotranspiration takes place without added ground level pressures on land uses.

Smart green roof systems can help nurture more sustainable urban communities that are closely linked to their natural environments. Eco roofs that are well-designed improve the efficiency and function of building infrastructure for humans, of course. But they simultaneously provide a full range of services to the local ecosystem—including improved air and stormwater quality, flood mitigation, and creation of supplemental habitat.

Green roofs can collect and temporarily store rainwater in a safe, sustainable way. They then release the water more gradually, through evapotranspiration, so it is transferred back into the atmosphere. This means that green roofs are actually improving water quality before it reenters the draining system.

Green roofs are also improving air quality, reducing air pollutants as they work. Furthermore, they add insulation to buildings, all year long, reducing the consumption of power and emissions. This lowers ambient daytime temperatures and reduces the urban heat island effect, which can cause increased air pollution, energy consumption, and heat-related illnesses and deaths.

Economic Benefits of Green Roofs

Green roofs save money over time, as they can last twice as long as conventional roofs. They also add value once they are installed, as well as aesthetic beauty.

A well-designed eco-roof can reduce maintenance and operations costs. This is because the monitoring system required by the green roof provides data on irrigation, plant material replacement, weed management, and other critical issues automatically. In other words, the system itself ensures that you are managing the roof in a cost-effective way.

According to the GSA, commercial and public buildings with green roofs actually earn money. In fact, an eco-roof with a standard net present value of $2.7/square foot will provide an ROI of 224 percent.

Green Infrastructure for Living Roof Systems: Monitoring Green Roofs

Monitoring green roofs has two larger goals in mind: efficient water management and optimal green roof performance. These two end goals, along with the circumstances of the roof and the geography of the region, determine which green roof infrastructure is best suited for monitoring your living roof system.

Sensors that measure soil temperature, soil moisture, surface temperature, and precipitation and flow all contribute to effective water management. The data these sensors collect helps green roof managers estimate how much irrigation water is needed to support the vegetation. Ongoing analysis of data collected from green roof systems can help researchers better understand which specific factors shape water needs in a particular location at a specific time.

As far as optimal green roof performance goes, ongoing monitoring enables evidence-based green roof management decisions. Any living system is dynamic and demanding. Improving such a system’s operation and performance requires constant data.

Systematic monitoring of a huge number of conditions is important in this context. To ensure optimal performance, engage in ongoing, systematic monitoring of:

  • subsurface soil temperature and moisture conditions
  • plant-available moisture
  • surface temperatures
  • photosynthetically active radiation
  • solar radiation
  • essential micro-climatic variables such as air temperature changes, precipitation levels, relative humidity, and wind speed
  • changes in vegetative coverage
  • changes in rates of survival of plant species over time

Use this data to quantify evapotranspiration, confirm that lab test data is applicable, observe performance in erratic weather, eliminate excess irrigation, parameterize characteristics of a performance or even identify detention days after a storm. And each time there’s precipitation in the area, your green roof system generates more data automatically.

Collect runoff in a vessel linked to a pressure transducer that enables continuous measurement of the water level. Connect the pressure transducers to a data logger which can then record pressure at set intervals and either transmit the data or save it for downloading later.

Thermistors can measure temperature in and over green roof systems.

Round a green roof monitoring system out with a weather station, including sensors for humidity, light, temperature, wind direction, and wind speed, and a tipping bucket rain gage. You can program your green roof infrastructure to record and download weather station data in tandem with the pressure transducers.

Although both the tipping bucket rain gage and the runoff bucket will collect water, you can still use a sampling tube to collect discrete runoff samples if you install one in a gutter. You can also grab samples from any bucket or other collection vessels that are part of your system.

Challenges in Eco Roof Design, Construction, and Maintenance

Potential challenges in green roof design, construction, and maintenance can include knowing the right codes, mastery of structural considerations, roof construction and maintenance issues, and problems related to installing a green roof on a historic structure.

And sometimes, green roofs just don’t perform at their designed level. This might be due to any number of possible failures, including erosion of soil, inadequate drainage, leaks, plant loss, and slope instability.

There are some existing best practices out there that can lend some guidance to a green roof project—although each one is unique. Here are some of the more common problems seen in eco-roofs, and some possible solutions.

Problem: Leaks, and leak detection

Just like any other roof, a green roof shouldn’t be leaking. However, staying on top of a green roof and maintaining its functionality is more complex.

Solution: Leaks in a green roof are a wholesale failure of the system to perform at its designed level. Leaks in an eco-roof can be related to inadequate drainage, plant loss, soil erosion, slope instability, and other problems.

A standard roof can be tested with a flood method, but this isn’t as effective with a green roof. Instead, low-voltage electric leak detection is a better bet in most cases, enabling a more exact location of leaks.

Problem: Over-fertilizing and over-watering

These problems are easy to get wrong, particularly when your green roof is losing plants and you’re going without hard data from continuous monitoring. In fact, the signs of over-fertilizing and under-fertilizing are very similar, just as the signs of overwatering and under-watering resemble each other closely.

Solution: Stop over-watering by cutting off excess water. Determine how much is the right amount using the soil moisture, soil temperature, and other sensors.

Balance your over-fertilizing problem by amending the soil based on your testing data. Remember to keep monitoring over time, as nutrient levels stabilize gradually.

Problem: Poor drainage

Improperly installed drainage components or unsuitable drainage medium can result in impaired drainage conditions. These kinds of conditions can cause plant loss, overload the roof, and force water into the building through skylights, door thresholds, hatches, and other vulnerable spots.

Solution: Unsuitable drainage medium is the most frequently seen factor in green roof failures. Plant failures often happen with medium and high silt or clay content. Aim for moderate to high porosity and permeability in your green roof media instead.

Problem: Suboptimal plant selection

The single most important factor in successfully establishing your green roof is optimal plant selection. Sedum and other succulents are the types of plants seen most often on green roofs, for good reason. For extensive green roofs, they are often the only appropriate choice, and this is also true in some arid climates.

Solution: To some extent, techniques such as supplemental irrigation and increased medium thickness can enable more plant variety, but these methods can never entirely change the facts. Green roofs must survive and thrive under extreme conditions, and each one is unique.

Suboptimal plant selection that runs rampant, however, will mean a ruined project. Green roof designers must determine which plants are appropriate, and which will help the eco-roof system meet the set goals.

Problem: Wind scour

Wind scour can cause growth medium loss, even green roof delamination in extreme cases. The green roof perimeter is most vulnerable to wind scour.

Solution: Work wind scour into your green roof design as needed. Wind scour is less concerning in the field of the green roof because the plants’ foliage is naturally scour-resistant. Their rough surfaces disrupt the wind and create low-level turbulence even as the plants themselves stay stable thanks to an integrated root network within the green roof medium.

Not all problems are human-made; some are attributable to biological factors, such as the biodegradation of medium and root penetration.

Problem: Biodegradation of medium

Green roof medium is sometimes filled with organic matter which can biodegrade. This can in turn cause drainage capacity to drop, the soil structure to collapse, and anoxic conditions to occur. Anoxic conditions that hurt or prevent plant growth usual signal the end of a green roof, as an amendment is typically not enough to fix failed medium.

Solution: Follow local recommendations for green roof media in your local climate.

Problem: Root penetration

The waterproofing layer of a green roof can fall victim to aggressive roots from plants over time. Those made from asphalt are at an especially high risk.

Of course, using chemical products to stop root growth is not always the wisest choice on a green roof system. These can lead to polluted water and leaching. On the other hand, damage to the waterproofing layer can be catastrophic, ending the life of the eco-roof.

Solution: Test all waterproofing membranes for green roofs for resistance and vulnerability to roots. Monitor ongoing performance and signs of damage.

Why Monitoring Green Roofs Matters

The biggest limiting factor for green roof performance on a larger scale is the unpredictable ways particular roofs respond hydrologically to individual storm events. Right now, many localities try to solve this problem by modeling to estimate how stormwater runoff will flow.

However, green roofs are complicated. Their overall performance is influenced by a host of factors, including local conditions at the time the storm starts, geometry, the hyetograph of that particular storm, and even soil media depth and type.

Simplistic models do not accurately predict the hydrological response of green roof systems. Instead, accurate, ongoing data from green roof stormwater runoff monitoring is essential to creating more accurate models and developing more appropriate hydrological solutions. This kind of data can also clarify the full range of benefits green roofs provide to local communities.

Despite their amazing advantages, green roofs alone are not enough to prevent all problems stemming from urban stormwater runoff. Only well-designed and carefully maintained green roofs that are part of a larger green infrastructure solution are up to that task.

City planners, water resource engineers, and other decision-makers must water management tools such as green roofs to protect communities and watersheds, but they can only do that with accurate monitoring. That’s why green roof monitoring matters: it’s the key to taking full advantage of every stormwater management solution at hand.

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