Why is AIR-to-WATER Ratio on Green Roofs so important?

The air-to-water ratio is the ratio between air and water in the pores of a media. (Download PDF: click here)

By Green Roof Expert Extraordinaire Jorg Breuning, March 2020

Introduction (Challenges)

On ground disconnected structures like roofs the available rooting space for plants is typically limited because of weight restraints. Additionally the Green Roof components and the plants are exposed to more extreme environmental impact like higher wind speeds, reflecting surfaces, higher temperatures, air pollution, artificial light at night, lower bio-diversity and more. In such locations the Air-to-Water ratio within the Green Roof system can fluctuate much more often beyond the desired range what causes stress on the plants and stress on beneficial microbes. 

If more extremes are expected, selecting the right plants and right Green Roof system is crucial. In some cases an irrigation system (temporary or permanent) or enhanced drainage system should be considered to reduce stress conditions on the plants and to improve soil biology. However, operating an inappropriate irrigation system (like sprinklers) or drainage systems that were never engineered specifically for Green Roof applications can be counterproductive because the Air-to-Water ratio is not considered. Constant adverse Air-to-Water ratio is the problem on over 60% of the Green Roofs in North America and with changing weather patterns this problem is getting worse.

What is AIR-to-WATER RATIO (Introduction)

The vast majority of plants used on extensive or intensive Green Roofs thrive best within a specific Air-to-Water ratio range in the root zone of a Green Roof system. Over the course of a year and depending on weather conditions, a Green Roof system’s desirable the Air-to-Water ratio shall vary between 35/65 to 65/35 (65% water/35% air to vice versa). An Air-to-Water ratio in such a range is important for the resilience of plants, for nutrient exchange capacity, pH buffering capability, and beneficial (aerobe) microbes. Peaks in one or the other direction (adverse Air-to-Water ratio) can be tolerated by the plants if this is temporary and very limited in time.

To successfully reduce adverse Air-to-Water ratio it takes, extensive experience, profound horticultural knowledge, understanding the historic developments, seeing environmental/natural facts, and the ability to evaluate natural indicators on existing Green Roofs. 

Understanding Green Roof Systems 

As long as humans have cultivated plants they have learned that natural parameters in biology, physics and chemistry are given and everything has to collaborate within these nature’s laws. Although we are able to Gen-manipulate plants for better yield, more pest resistant, and for more stress resistance, but the nature’s laws won’t change.

An increasingly number of stormwater professionals recognize that Green Roofs are an excellent, the least expensive and most natural stormwater management tool with many added values for the environment we live in.  However, as often we talk about Green Roofs as often these specialized professionals forget about the Quarterbacks or key players of such systems: the plants!

It is a simple fact that the more we do for healthy plants with a natural diversity, the better a Green Roof system can retain and detain stormwater.   

Within the development of modern Green Roof technology over the last 50 years most Green Roof systems are built with different continues layers.

The basic layers are (from roofing up):

1. Protection layer to separate between roofing and green roofing trade and to protect the underlying components.
2. Soil/Media extension layer (often described as drainage layer) to mimic natural soil profiles and avoid anaerobe conditions in rooting zone.
3. Filter layer that avoids fine particle migration within the system.
4. Engineered Growth Media or soil that meets parameters of FLL*.
5. Vegetation.  

INFO! Trays, pre-planted containers or modular systems try to mimic the same layers within a small defined area like a plastic box, metal container or soil filled bags. These systems are solely made for easy shipping which is blink of eye in the life span of a Green Roof. These systems are not continuously installed and so these systems are considered as a temporary solution for a limited time (5-8 years).  In most cases these planters or trays are getting re-planted or removed over time, many already ended up on landfills.

Understanding Green Roof Plants

Because of the highly fluctuating Air-to-Water ratio, experienced Green Roof designers prefer utilizing hardy plants. Ideally plants that can cope with no irrigation, that are more resistant to drought and temporary extensive water, that are more heat and wind tolerant, and vegetation that can fast regenerate after stress.

In most cases and on shallow extensive Green Roofs succulent type of plants (like Sedums) are often the preferred and common choices.  Even so the Sedum family is very large, colorful and low growing, for enhanced resilience they also rely on competition with other plant species like mosses, perennials or grasses of comparable hardy properties. Horticultural experts call this “natural plant diversity”. Such natural plant diversity stands in a full contrast to Sedum monocultures that often come with tray (modular) systems or systems with pre-vegetated mats. Pre-cultivated systems are very popular in North America where instant gratification and more revenue play a pivot role.

The variety of different plant families or allowing a natural diversity increases root activity within the Green Roof system, creates healthy rooting space, soil structure and texture, with desired micro and macro pores for enhanced soil biology, all resulting into a more ideal Air-to-Water ratio, better water retention and increasing stormwater detention.

Understanding the Current Green Roof Industry

With the focus and current trend on stormwater management, it is “natural” that many people try to improve the stormwater managing ability of Green Roofs. In this regard the Green Roof industry is not getting tired in developing a deluge of products and components that are supposed to help with stormwater management, stormwater retention, stormwater detention, stormwater neutralization, and stormwater clarification. Unfortunately most of them are forgetting about the champions; the Plants!

Interestingly many of these innovators are often highly decorated with desirable titles, have done a lot of commercial reductionist research, micro- and selective-research over a short time with floods of data, have written a lot papers that are piling up in the cloud, and they have introduced new words or terms that sound fancy, makes them look trendy or educated.

Learn From Common Sense

Our ancestors preferred Green Roofs as an extension of living green and private recreational space, for growing supplemental food or for building protective purposes. Today’s trend moves away from insulating purposes (because that was wrong in the first place) to stormwater management. Environmental aspects go under in the craving for a Return-of-Investment (ROI). With an entire Armada of specialty Green Roof products, with an army of engineers, and a deluge of data people are trying to narrow-down a living machine (a.k.a. Green Roof) into a static, man-made tool by altering, avoiding or eliminating natural processes which are elementary for healthy and resilient survival of plants/vegetation. It is like feeding less to livestock to reduce Methane gas emissions.    

Some of the problematic and counterproductive specialty Green Roof products are:

• Very thick layers of synthetic fabrics with increased retention and/or detention capacity based on internal capillarity properties or hydrophilic properties. These specialty fabrics often reverse natural capillarity of engineered soils what dries out the soils faster or when infiltrated with organics particles there is waterlogging and rotting.

• Some Retention and Detention/Drainage Boards (mainly made from plastic) have no time-release overflow or no overflow that allows air between the water reservoirs. Not talking about polystyrene plastics that are bad for the environment in all regards.

• Water Crystals, Hydrogels are small chunks (or crystals) of man-made, water-absorbing polymers. The chunks are like sponges – they hold a tremendous amount of water by increasing in size. The liquid is then supposed to release gradually into the soil.  These products have a limited lifespan that is generally less than 1/10 the lifetime of a Green Roof and so not worth the investment.

• Mineral Wool is a manmade material from glass fibers either spun together or adhered together. Beside that mineral fibers are a possible human carcinogen these specialty products support an Air-to-Water ratio from 95/5 to 5/95 (95% water/5% air to vice versa) whereas the strong water holding capacity /waterlogging is much longer than in natural soil profiles and creates temporary anaerobe conditions = water stress on succulent type plants and root rot.

• High organic content (over 50%) in growth media (soil). Decomposing of organic matter increases biology activity which is related to heat, causes unusual nutrient increase or decrease, and an increase of carbon dioxide in the soil profile. This and the additional risk of waterlogging increase the stress on plants.

• Slow release or fast release, solid or liquid fertilizers of synthetic/chemical origin behave on extreme locations like roofs very different to on-grade applications. Especially in combination with the above specialty products there could be a serious and snowballing risk for the vegetation and the environment through run-off. It can increase salt concentrations or chemical process that cause deterioration of other components (e.g. roofing) of the Green Roof or the structure.
• Chemical weeding Chemical weeding is a common practice mostly used in North America where environmental laws and standards are very low. Chemical on extremes locations behave very different to on-grade locations and succulent type of plants that have a slower metabolism what means they might die very slowly. What looks like chemical “resident” may be affect such plants years later, weakens them and exposes them to other diseases easier.

Conclusion

Vegetation placed on ground remote or ground disconnected location is a century-old tradition for very different and combined purposes. Optimizing only the aspect “stormwater” will ultimately affect all other natural cycles within a Green Roof system. Especially in North America these considerations for natural cycles on a Green Roof are often not known, disregarded or purposeful ignored for a desperate craving of profit and ROI.

This might looks like a society accepted approach but it can result that Green Roofs in North America might not get as popular and as accepted as in European countries where commons sense, decades of monitoring, fundamental education, and solid experience is the driver for their success. Most countries in Europe understand that healthy driven plants, plant diversities and plant communities on roofs increase the benefits as they progress, develop, and generate a natural ecosystem.

End

State of the Art Health Care and Green Infrastructure

BG Hospital Tübingen: The BG Hospital Tübingen is one of the largest trauma centers in Germany. 

Construction Going Up – Ecology Going Down?

From 2010 to 2015 warehouse new construction in the US tippled from around $5 Billion to $15 Billion with all the economic benefits. 15 Billion in new construction equals around 1000 new warehouses every year or approximately 250 million square feet rooftops plus the same amount of infrastructure provision. Annually, 500 Million square feet or 8800 football fields of lost nature by building new warehouses in the United States alone.

With some of the existing warehouses, this translates to a potential of at least 1 Billion of square feet of new extensive Green Roofs every year that could retain the volume of Lake Constance that is surrounded by three countries (Germany, Switzerland and Austria) or over the double volume of Lake Powell (UT/AZ)!

Assuming that these Green Roofs are done right, maintained by professionals and without artificial irrigation, the natural and controlled succession of these Green Roofs would allow a unique habitat for many endangered species over the upcoming years.

These green roofs can also reduce the re-roofing costs by multiple billion Dollars with a related relieve for landfills and a huge reduction in energy use.

Most important however is the climate effect.

Warehouse belts surround any large city in North America. These warehouse belts with endless impervious areas actually preheat the air that is conveyed into our cities by the wind. In the worst-case scenario, these large warehouses have reflective roofing membranes that might cool the building but also increase – due to reflection – the heat in the atmosphere substantially.

Warehouse belts around cities are like a forced air heating system in a building only on a much larger scale – Warehouse belts are a wasteful forced air heating system for entire cities

Fundamental sustainable thinking and large scale long-term research hardly exists in the US, so it is no wonder that experts are likely to miss the entire picture.

Currently in the US there are no significant requirements or ecological considerations in master plans that address this counterproductive climate habits towards our cities. Actually, it seems that warehouse belts grow faster than Green Roofs!

North American ecological organizations, LEED, or other industry associations are also missing the bigger picture. They often work locally – from one block to the next, they are focused on one or two rare plants, on a distinctive bee species, butterflies or other human pleasing critters as long as there is an instant payback in popularity. It seems worth more having a highly unsustainable fully irrigated Green Wall in the cafeteria, a nice green plaque on the wall, or growing some carrots on the roof to demonstrate social and sustainable responsibility. Most marketing efforts for green washing are beyond the costs of a simple highly efficient extensive Green Roof that helps the future generations.    

The United States is a great country and North America is a large continent where thinking big was always wanted and welcome. Now it seems people are getting lost in their little plastic boxes, made in China with some nice flowers and a watering can next to it.

It is very romantic but also very embarrassing.

Do Green Roof Failures increase with Roofing Manufacturers?

In the last two days I visited four of my projects that are between 14 years and 1 year old.

Each of these Green Roof has an own interstesting history and with the exception of the “cozy” Green Roof, each of the extensive green roof host over 25 and 40 plant species in excellent health.

None of them was ever irrigated and the oldest was fertilized the last time 10 years ago. All of them require hardly any maintenance (2 hours per 5,000sf / year).  One Green Roof is maintained only once every 3 years.

For these amazing, inexpensive and resilient green roofs a special thank to all who participated!

In contrast:

I was on one roof that was installed and maintained by a roofing company with materials from a famous roofing manufacturer.   This roof is basically going down the drain because of incompetence, sloppy work, and inappropriate materials.

This roof came with a costly single source warranty that –as always – doesn’t cover anything of the green roof. The cost for this worthless application was around 25% higher than a fully functioning Green Roof from well trained gardeners.

This is one of many projects around the Nation where I am questioning myself whether roofing manufactures and roofing companies should work in the Green Roof field. As a matter of fact in last decade 90% of roofing manufacturers offered Green Roof Systems that consistently underperform or that are not maintained properly. Actually most of these manufactures changed their systems and approaches to Green Roofs more often than they changed the management of these multi-million Dollar corporations. 

While many of these manager focuses on how increase revenue and some of them how to fix the problems with Green Roofs their quality of the waterproofing and craftsmanship of their installations went down. Striving for more revenue is not a wrong thing in general but when this is done on the backbone of the Green Roof industry and our all prospect clients it is more than questionable.

As client you have to figure out whether it is more important to make unknown stockholder happy or yourself.

I don’t fall for roofing warranties and sales forces with nice brochure

First Extensive Green Roof in the Dominican Republic.

More than 12 years ago my friends Miriam and Buschi decided to settle in Las Terrenas, Dominican Republic to educate the local people and tourists about animal welfare and a better interaction between humans and animals.

An awesome story that has grown and created support from all around the world. Universities, veterinarians and engaged individuals from North America and Europe spend their free time by helping them with hard work and donations, making a better life for animals and humans.

Beside their endless efforts for animal welfare their home has two large extensive green roofs that flourish since 12 years. It is quite an effort finding the right plants that can survive on less than 3 inch of soil mix. A thick, flexible, non-bituminous and root resistant waterproofing is the only German product of the green roof system. All other components were sourced locally and evolved over the years.

These green roofs are the first in the Dominican Republic and represent the basics of modern green roof technology – simple and effective without plastic elements or pre-planted containers.    

http://www.amigos-de-lucky.org/

https://www.facebook.com/Amigos.de.Lucky

Fireworks, Green Roofs and Happy New Year!

Practically all dry organic materials are combustible at relative low temperatures. Roof shingles or many waterproofing membranes on roofs are typically petroleum-based products (including Asphalt) and there is hardly a difference to dry organic materials. Some products are equipped with chemical fire retardants that could decrease the risk of spreading fire, not the risk of leaks cause by impact.

The living vegetation of functional and well-engineered green roofs contain moisture and most plants (especially succulent plants on large extensive green roofs) have a fraction of the energetic potential comparing to all other components used in a building. The green roof growing media (soil for the green roof plants) is a blend of different mineral components with an organic content of typically less that 15%. It is practically impossible to set this material on fire – not even considering its natural moisture content.

Potential fires or leaks on roofs caused by consumer or display fireworks can be prevented if the building has a fully functional green roof.

A fire on any roof is the worst-case scenario. I many cases smoldering firework parts (and cigarettes) are causing leaks on unprotected roofs that are discovered much later. These problems are unknown on fully functional green roofs. When a green roof prevents a leak or even a fire it is typically not recognized but will save expensive repair or even loss of a building.

Nature is (literally) so cool.

Happy and successful green 2016!

Zero Run-Off for 20 Years!

From 2010 to 2015 warehouse new construction in the US tippled from around $5 billion to 15 Billion this equals around 1000 new warehouses every year and translates to a potential of at least 1 Billion of square feet of extensive Green Roofs that could retain the volume of Lake Constance that is surrounded by three countries (Germany, Switzerland and Austria). On top of that and if these Green Roofs are done right and without irrigation, these Green Roofs would allow a controlled succession that could develop over the years to a unique habitat for many endangered species.

As the nationwide building codes require expensive engineered fire suppression systems the same building codes can require simple and highly efficient Green Roofs. Unfortunately most Green Roof or other environmental organizations don’t focus the low cost, high efficient simplicity – they focus on adding carrots or a rare native plant species to an environment that is far away from being connected to the ground.

Common sense exists as term but it isn’t in the head of most people because it doesn’t generate the attention and “individualism”.

The main purpose of an extensive Green Roof is stormwater retention and delaying stormwater runoff amongst many other added values.

Above: An Irrigated thin layered Green Roof System – without irrigation it would be dead.

In the last decade in North America, we've seen many Green Roofs where the intended plants never really flourished. I estimate that in the US at least 50% of the green roofs are not performing to their fullest potential. This can be observed by simply looking at the most obvious of indicators, the plants themselves, regardless if they were planted on purpose or somehow found the space to take root. 

Most of these less healthy extensive Green Roofs are thin-layered systems, pre-planted boxes (often called modular systems or hybrid systems), Green Roof Systems from roofing manufacturers with ineffective components, single source warranty-systems, or design intents without consideration for the needs of plants. Typically these systems much more costly (whether thin or thicker) and the mid to long term results are often far below systems that are assembled in place with common sense.

Thin-Layered system with irrigation - doesn't work and doesn't retain much water

We all know that the transition from being a common nursery-grown plant (including pre-planted boxes) to the extreme environment of a rooftop poses severe challenges. Green roof plant nurseries typically have "great" advice and recommend the installation of temporary or permanent irrigation systems. In some areas this might be helpful when operated correctly but in all cases this advice is defeating the purpose of an extensive Green Roof (water retention). In almost all cases any type of irrigation caused a spiral of failures and it is wrong to irrigate extensive Green Roofs in areas with limited water for the cause of just having a green roof (Green Washing). Most company sponsored experts don’t see the entire picture or they only see the short term profit and commission (“Mow-Blow-Go”).

If a problem occurs and since many Green Roof system developer lack the proper and specific green roof horticultural knowledge, they may not be able to identify problems by simply looking at the indicators (the plants). This could cause the problem to gain momentum. In addition, the false conclusion that technology (Google search, Apps, synthetic growth media or sophisticated soil moisture control devices) can fix the problem supports my theory of hardly experienced or misinformed green roof professionals. They rely heavily on technology to fix any issues and miss the big picture.

Nature has the ability to take care of itself, as long as the appropriate design, materials and plants are used in addition to being familiar with the immediate location and climate.

Thin layered, super lightweight systems or systems with “innovative” components are just by physics not meant to retain “more” stormwater for the plants or they retain too much water for specific plants. As a matter of fact the costs for retaining stormwater in thin layered systems are extremely high comparing to a standardized extensive Green Roof with a thickness of 4-6 inch (depending on location). Besides the initial costs the lack of water retention (don’t forget all studies are done by the companies!) won’t support any significant bio diversity plus the operational costs (because of the required irrigation) are tremendous over time. Typically these Green Roofs never pay back with they are not utilized for “Green Washing” purposes in advertising.  

Irrigated extensive Green Roofs and/or thin layered, super lightweight systems (below 25 lbs/sf) will increase the nutrient pollution in runoff, require higher fertilization application, have hardly any tendency for self-healing, and may increase the pressure of unwanted plants and have a high risk for failure. Irrigated extensive Green Roofs are not environmentally friendly in most loactions!

Green roof designers and green roof professionals must understand that certain advertising, studies, White Papers or project references are all assembled by specific manufacturers and their goal is to make money – unfortunately on the back of a very advanced technology and on the back of their clients.

When we advise our clients they get the full picture because we have seem all solutions in the last 35 years, we have tested them and we are currently working millions of square feet where these systems require upgrades, reinstallation or massive repair (around 20 million in damage so far).

We also help our clients doing it as efficient as possible and to get their money back from prior failures through warranty claims and sophisticated lawsuits. This might help to standardize the function of Green Roofs and helps to get the black sheep out of the market. The building owner has a right that things are done right from the beginning.

Never irrigated in the last 10 years.

Picture: Stuttgarter Nachrichten