Green roof – soil vs substrate – 07.09.22 – When growing a green roof, the most toiled-over conundrum is which growing medium to choose between substrate and soil. At TurfOnline, we favour the former more than the latter for a list of reasons. Here are the basics of why one should use green roof substrates rather than less expensive soil.
What is the Best Growing Medium For a Green Roof?
Green roofing is where engineering meets ecology. When it comes to growing plants and creating an eco-system, even if it’s a very small one, success depends on three factors: sunshine, water supply and the growing medium. The growing medium has to pass a few criteria to be suitable for growing, like ample room for roots to grow and water retention. The best growing medium needs good water drainage and nutrient retention.
What Can Your Roof Support?
The amount of growing material that can be used to support life on a green roof is limited by the load-bearing capacity of the building, the roof pitch and the depth of any edgings or upstands.
The deeper the growing medium, the wider the range of plants it can support. For example, if a roof can only take the weight of a 20mm depth growing medium, you’ll be restricted to growing sedums (or moss in the shade).
Green roof substrate weighs considerably less than topsoil. 400cm³ of green roof substrate weighs 800g compared to 400cm³ of garden soil. Soil weighs about 20% more at 1080g. That means less strain on the building, and also an easier time lifting it onto the roof.
Over a 100m² green roof with a 10cm depth of growing medium, the weight saved by using substrate instead of soil is considerable. This can be reflected in the cost of building materials.
Green roof substrate has been researched, developed, blended, and improved over several decades. Though it’s not a naturally occurring product, it’s a blend of several different natural ingredients.
Scientists have looked for a compromise between what the plants need and what works best for the building.
Garden soil. The particle size is mixed but what you can’t see here is that those bigger particles crumble easily. This is a chalky loam, and in drought conditions, it turns to dust. In torrential rain, it washes over the path and onto the road. It’s great for growing plants but needs regular top-ups with compost.
Green roof substrate. Like topsoil, it has a range of particle sizes but the larger ones, although lightweight, cannot be crumbled or crushed. The pale colours suggest it is more mineral-based.
However, these photos don’t do justice to either product. To appreciate the differences between substrate and soil handle them both. Feel the texture and the temperature, smell them, assess the weight, pour water on them and see what happens.
Air & Water
You might be able to see from the photos that the substrate has larger particles than topsoil. Larger particles mean bigger air spaces between them. Those are air pockets to help insulate the building and to allow optimum drainage.
The growing medium should hold just enough water for the plants, but not become waterlogged and weighty. Particle size is important too when it comes to keeping the growing medium on the roof. Silt or clay particles tend to be small enough to be moved around by rainwater, and therefore small enough to be washed off the roof into the drainage system and away. That could lead to blocked drains. It could mean somebody will eventually need to top up the levels of growing medium on the roof. Expensive and inconvenient.
We’ve already touched on drainage when discussing particle size. Suffice to say the balance between water retention and drainage is vital for the success and sustainability of a living roof. Not only for creating the correct living environment for the plants but for insulation and weight management.
Green roof substrate is blended to get that balance just right.
Controlled Nutrient Content
Plant nutrition is a huge subject, but let’s summarise it here.
Firstly, it should be stressed that conditions on a roof are harsh. If plants are over-fed and allowed to get too lush, they will be prone to frost damage, wind scorch and possible disease. They need to be treated like athletes, given enough food but not allowed to get fat. As topsoil has a built-in ecosystem that produces and stores plant food, it’s difficult to control growth. Green roof substrate plays host to fewer of the soil microbes that fuel ecosystems and thus, plants are hardier and live longer.
The other matter to consider is runoff. When plant nutrients stay in the soil, they’re fine. If they get into watercourses, however, they can cause havoc. Plant nutrients are water soluble and on a green roof, could potentially be dissolved by rainwater, washed off the roof and into watercourses. By using green roof substrate and controlled fertiliser applications, the risk of contaminating rivers and streams is greatly reduced. Of course, capturing and recycling the rainwater that comes off the roof would be the ideal solution, but it’s not an option for every building.
Roots need a firm but friable medium to grow into. Very few plants can root into solid surfaces. When farmers and gardeners grow plants, they will take every possible precaution to avoid compacting the soil in the first place and to relieve compaction when it happens. You cannot avoid natural compaction. Pressures exerted by heavy rain, by maintenance visits to your green roof are part and parcel of it all. But you can’t plough or dig a green roof without endangering the waterproofing, so you can’t relieve that compaction. Your only option is to use a growing medium that is less prone to compaction. Enter our green roof substrate, which is much more manageable than soil!
Have you ever wondered where the soil in your pot plants disappears to? It hasn’t necessarily been washed out through the holes in the bottom of the pot, but has been oxidised. Oxidisation is a chemical reaction between organic matter in the soil and the air. Some of the soil turns to gas and disappears into the atmosphere.
Imagine if that happened on a green roof. You’d be buying new soil and lifting it onto the roof every 5 years or so. Either that or the plants would perish from lack of growing medium.
That’s why green roof substrate has considerably less organic matter than ordinary soil. There’s enough organic matter in there to support the plants, and of course, with dead leaves, flower stems and roots, any matter that does oxidise is replaced by nature. But you won’t find the levels of substrate dropping in the same way that the levels of soil would.
Once again, we’re talking minimal long-term maintenance and maximum sustainability.
Green roofing and the reason we do things the way we do is about making sure that when you invest in a living roof it will serve you well for a very long time.
Be sure to use green substrate for this, and if you are installing a simple sedum roof using Enviromat sedum matting, you have no need to worry about the substrate, it’s incorporated into the product.