Growing media nourishes green roof plants and helps detain stormwater, but what form does that media take – and which is best for your roof?
Which media to use while building or maintaining a green roof is a question with many possible answers today. While choice is a good thing, it also leads to confusion and analysis paralysis.
Don’t worry; we’re not judging. “Analysis paralysis” is sometimes our middle name.
Yet in the case of green roof media, we can’t afford that. Choosing the best media type during the initial stages of green roof design is critical for the roof to function at its best, detain and retain stormwater with maximum efficiency, and enable future stewardship.
So here’s your brief primer on the best green roof media type, so whip out that notepad (or the tablet that’s already wired to your brainstem) and let’s get started.
Are We Ditching Soil as a Green Roof Media?
Formerly, green roofers made use of the same material other gardeners and landscapers used: soil.
... Ever heard of it?
Kidding. But with such a wide range of soil alternatives or soilless growing media available today, and with each offering a different set of benefits, soil is not always the best choice. That’s why we’ve always got our ear to the ground for the latest and greatest type.
Does that mean we’re ditching soil entirely?
The short answer: No. That would be throwing out the baby with the bathwater, and we like our babies.
However, soil isn’t appropriate for all circumstances, such as the thinner profiles used in extensive systems. (You know, the stereotypical ones with low-growing sedums). Soil has a high number of organics, and so it compacts. Also, those organics tend to flash off in thin profiles, offering a quick flush of growth, then departing the system, leaving malnourished plants behind.
In many cases, therefore, we need a different type of media.
What Types of Green Roof Media Exist?
In deeper profiles and more intensive systems, we do make use of topsoil (or at least a version of it).
But even then, that soil is engineered to prevent compaction and address the tendency of nutrients to funnel off. It’s not a perfect system, though it does work.
That said, most people in green roofing are moving away from soil as a green roof media component. Today, some green roof media types incorporate a limited percentage of organics, while others use none at all. It all depends on the specific type of engineered growing media, which can take many forms.
The main point is that you need retentive components to hold onto water and nutrients, and provide structure, for the plants.
For that reason, growing media usually includes a component such as a lightweight aggregate, helping minimize compaction, maximize aeration and aid in water retention. It also needs to be able to stand up to many freeze-thaw cycles. Products that do this well include:
- Expanded shale
- Expanded clay
- Pumice
There are also some systems that avoid organics altogether. XeroFlor, for instance, makes systems that use all fabrics, including a drainage layer and a water retention layer. We’re intrigued by this, though we’re not convinced it’s right for every roof, especially more intensive ones.
Green Roof Media Types to Avoid
Not every green roof media type is good. In other words, just because it exists doesn’t mean you should use it – no matter how fabulous its marketing image.
Take peat, a perfect example. While many industry associations claim it’s a renewable resource, it’s actually not. In fact, peat mining leads to disastrous consequences for our environment, releasing massive amounts of carbon into the atmosphere. It’s hard to stop a juggernaut of an industry, but what we can do is stop using it.
You should also avoid any green roof media type that is either too porous (which won’t retain water and nutrients) or too dense (which can clog drains and lead to standing water). Either will cause serious problems with the roof, making long-term stewardship difficult.
Which Green Roof Media Type Is Right for You?
When looking for your perfect green roof media, there are a few key elements to consider:
- Drainage
- Weight
- Water-holding capacity
- Organics
The perfect mix depends on the plants you’re growing. Your range of species is the most important dictator of the type of soil or media you need. The bottom line is, there is no one-size-fits-all. That’s why it’s important to stay apprised of all the options.
We always advise working with a good regional soil blender, who have both the expertise and the materials to cater to your green roof. Some of our favorites include:
What Does the Future Hold?
Right now, we’re watching many of the green roof guidelines set by organizations such as the American Society for Testing and Materials and FLL carefully. These dictate the performance criteria of green roof soils or green roof media.
As the industry receives increasing pressure to reduce expenses and hold onto more water, we’re seeing new substrates all the time. Novel mineral wools and high-performance fabrics lead the field, many of them being trialed as we speak. Some have proven quite effective, but the jury’s still out on their longevity.
For now, we’re sitting tight, excited for what the future might bring.
If you’d like to learn more about our latest green roof project or check out our portfolio, we invite you to get in touch today. We’re happy to chat about our work, answer questions and steer you toward the right media type – and the right green roof – for you.
Call us today!
