Fixing The Problems Caused by Not Specifying Properly Designed Growing Media
Introduction - Why Growing Media is Not Soil
Getting performance value from your green roof investment requires that you work with knowledgeable design, installation and maintenance professionals and use the right materials for the job. While the waterproofing membrane, protection layers and drainage components can be considered the foundation of any green roof assembly, the real core of any green roof assembly is the growing media. Without a properly engineered growing media, the balance of drainage, nutrients, particle size and airflow will not be met. An imbalance between these elements can lead to disaster. Common issues are that the plants will struggle to thrive, the planting areas can shrink, the drainage components can clog causing the green roof to fail, creating a headache for the building owner down the road, as well as a high price tag for repairs or replacement.
Why Engineered Growing Media
Rooftops, podium and amenity decks are not easy places for plants to grow. There are many different environmental conditions that need to be considered when planning for rooftop green spaces. A critical element is the growing media. Extensive green roof growing media blends are designed to provide a healthy environment for proper plant growth while being shallow depth and in exposed conditions. Intensive green roof media has higher nutrient content than extensive media because it is designed to support woody plants like shrubs and trees, often in planters. These engineered media mixes balance the ability to store nutrients within the blend, meet high water holding capacity requirements, with providing good drainage and air content (pore space). These performance characteristics must work together to support the plants, stormwater management and drainage needs.
Engineered growing media also provides an important ballasting function for protected membrane roofs. where the insulation is above the membrane. In these systems, the growing media holds the insulated panels in place. The saturated and dry weight of the growing media must be consistent and measurable to meet ballasting weight needs, while not exceeding the building’s structural capacity.
Growing media has the following general qualities:
It is an engineered mix of consistent lightweight, well-drained materials blended in proportions intended to address specific qualities. All components when combined together will have good water and nutrient holding capacity.
Commercial growing media blends are now easily accessible from green roof manufacturers and have been in use for years and can be replicated next week or next year using these consistent tested proportions and ingredients.
Media mixes should be tested using well established national ASTM standards that ensure consistency. (ASTM E2399 for weight, porosity, permeability and water holding and ASTM D422-63 for particle size, for example)
A good quality growing media:
Can retain 40-50 per cent or more stormwater by volume
Contains large amounts of pore space for air and roots.
Provides just enough organic matter to retain nutrients and support plant health.
Provides structure by including lightweight aggregate that retains water and creates air and drainage space and won’t break down and shrink over time.
Meets the media guidelines of ASTM E2400 and ASTM E2777 for Vegetated Roof Systems
“But we have access to some great looking topsoil, what’s the big deal?”
The Issues with Using Topsoil on Any Structure: Rooftop, Podium or Amenity Deck
Even though the green roof industry has had time to mature, we still see many instances where topsoil has been used on rooftops or amenity decks instead of growing medium. The use of topsoil often leads to undesirable results both in the short and long term.
Topsoil is highly variable in its qualities. For example, topsoil in the Midwest is arguably some of the best on the planet; the deep prairies created magnificent ancient grazing lands for buffalo and beautiful contemporary corn and soybean fields that feed the planet today. Even so, that ‘high-quality‘ topsoil is still highly variable in its composition even within the same source field. Slight variations in silt and clay compositions can create a wide range of drainage and porosity values which can detrimentally affect green roof drainage performance.
Topsoil tends to be high in silt and clay fines which are deliberately limited in growing media blends because they can clog drains and drainage layers. The fine particles also settle over time and fill internal air spaces. The typical topsoil composition cannot approach the water holding capacity of engineered growing media.
Topsoil and planting soil cannot be readily tested using the ASTM standards for green roof media. Its structure does not lend itself to the testing regimens in those standards. There is no way to ensure that the last load of soil placed on the roof is of the same composition and consistency as the first.
Imagine bringing an intensive green roof media to the roof for a planter installation. Even when moist, intensive green roof media, due to its design, will easily flow into the tight spaces and corners of a planter. Growing media is installed in lifts and compacted as the planter is filled.
Now imagine filling that same planter with recycled topsoil or planting soil from the nearby stockpile. Depending on how it has been excavated and stored, topsoil used in an at-grade landscape can be difficult to spread. It creates clods and clumps consistently. The clumpy material when placed in planters will leave significant voids in the planter that require a lot of compaction. It can often be contaminated with weed seeds and other things when left on the project site. Even imported topsoil can arrive at the jobsite in a clumpy condition. Pulverized topsoil may be used, however, it too creates inconsistencies in the planter fill. While it might look like it is full, over time, even pulverized topsoil will shrink as the voids within the soil become filled with weathered silt and clay fines.
Shrinking lowers the soil level of your green roof or rooftop planter, clogging essential drainage and airflow for plants, exposing roots and irrigation lines. This may lead to soggy, stagnant soil and eventually dead plants. Even worse, maintenance crews may ‘solve’ this problem overtime by replacing plants and adding back more soil to top off planters. This throws off the soil blend and increases compaction at best and exceeds the structural capacity of the roof at worst.
Now that some of these soil-based roofs have been installed for several years, the industry is sadly starting to see structural roof failures including sagging roofs and split membranes from the added soil weight causing roof leaks. Without fail, there will be fingers pointing back to the integrity of the components selected for the original rooftop planting design.
Designers of green roofs must keep this in mind when working on green roof projects, especially where deeper, intensive assemblies are desired. Contractors must keep this in mind when weighing cost implications. While a repurposed topsoil from an on-site stockpile might be enticing to use on the roof top to save a few dollars, the damage that can occur later can be very costly.
Conclusion: Topsoil - Don’t be Tempted!
While the science used in developing green roofs continues to advance, there are certain elements that have remained consistent for decades. An engineered growing media is at the core of these elements. When properly configured and blended, each growing media load has the same great properties from beginning to end specifically engineered for use on rooftops and structural deck conditions. Topsoil and planting soil on the other hand cannot be reliably depended on.
A message for designers from the authors who are formerly practicing Landscape Architects: when specifications are written for green roofs, be sure to strip out the word “soil” from any green roof related document. Not just your written specs, but take callouts about ’Topsoil’ or ‘Planting soil’ out of your drawing details and material lists too! Don’t confuse your installers or contractors- Green roof media is not topsoil. It is not soil either. It is an intentionally developed, configured and blended material that is intended to enhance the growing environment for rooftops, podium and amenity deck plantings.
Green roof technology has been around since the 1960’s and millions of square feet are installed worldwide every year. Make sure you hire people who have the right experience with proven products to ensure you get the performance benefits you deserve for your investment.
Andrea Saven and Vanessa Keitges, Columbia Green and Richard Hayden, GRP, Green Roof and Blue Roof Advisor for Barrett Roofing
