What is the correct growing medium for a green roof?

Some of the most frequently asked questions I encounter when running CPD green roofing courses are about growing medium.  So here are the basics on why one should use green roof substrates rather than less expensive soil.

Weight

I’ve heard green roofing referred to as “engineering meets ecology” and I think that’s a pretty accurate description.  

When it comes to growing plants and creating an eco-system – even if it’s a very small one – success depends on three things.  Sunshine, water supply and the growing medium.  How much room is there for roots to grow and for water to be retained? What is the drainage like? Does it hold nutrients? Is it acidic or alkaline?

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 in can support.   For example, if a roof can only take the weight of 20mm depth growing medium – you’ll be restricted to growing sedums (or moss in the shade)

On another subject – deeper substrate layers offer better insulation for the building.

Here’s the thing.  At ground level, where load bearing capacity is irrelevant, plants grow in soil.  Soil is heavy, especially when wet.

Green roof substrate weighs considerably less that topsoil.  That means less strain on the building.  It also means it’s easier to lift onto the roof.

Just to satisfy myself that I’m not fibbing I did a quick experiment.   I part filled one ice-cream tub with green roof substrate and another with topsoil.  There was around 400 cm3 in each tub by volume.  Then I weighed each one on my kitchen scales.

400cm3 green roof substrate weighed 800g

400cm3 garden soil weighed 1020g  - in other words around 20% more

Over a 100m2 green roof with 10cm depth of growing medium, the weight saved by using substrate instead of soil is considerable.  This of course can be reflected in the cost of building materials.

 

Particle size 

Green roof substrate has been researched, developed, blended, and improved over several decades.  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.

close photo of chalky-loam topsoil

Garden soil.  The particle size is mixed but what you can't see all that well is that those bigger particles crumble easily.  This is a chalky loam from my own garden in Norfolk.  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 in but needs regular top ups with compost.

close up of green roof substrate

Green roof substrate.  Like topsoil it has a range of particle sizes but the larger ones, although light weight, cannot be crumbled or crushed.  The pale colours suggest it is more mineral based. 

These photo's don't do justice to either product.  To really appreciate the differences between substrate and soil you need to handle them both.  Feel the texture and the temperature, smell them, assess the weight, pour water on them and see what happens...

 

 

You might be able to see from the photos that 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 – where it belongs.  Silt or clay particles tend to be small, very small, 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 definitely mean somebody will eventually need to top up the levels of growing medium on the roof.  Expensive and inconvenient.

So particle size is about sustainability

Drainage

We’ve already touched on drainage when discussing particle size so I won’t bore you by repeating myself.  Suffice to say water retention vs 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 for weight management.

Green roof substrate is blended to get that balance just right.

Controlled nutrient content

Plant nutrition is a huge subject – too complex for this blog post so I’ll look at just 2 aspects of it.  

Firstly, 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.  That’s not ideal.  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 so plants (provided they have the correct supplementary feeds applied at the right time of year) 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 they can cause havoc.  Plant nutrients are water soluble.  On a green roof, they 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 very much 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.

Compaction

Roots need a firm but friable medium to grow into.  Very few plants can root into solid surfaces.  Farmers and gardeners when they grow plants 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 the thing is, you can’t plough or dig a green roof without endangering the waterproofing.  So you cant relieve that compaction.  Your only option is to use a growing medium that is less prone to compaction.  We’re back to our larger-particled green roof substrate.  Far more manageable than soil!

Oxidisation

Have you ever wondered where the soil in your potplants disappears to?  It hasn’t necessarily been washed out through the holes in the bottom of the pot.  It has 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 mimimal long term maintenance and maximum sustainability

To summarise

It feels as though I’ve rambled on a lot about this subject – but it is important.  Green roofing and the reason we do things the way we do is not about suppliers getting rich, it’s about making sure that when you invest in a living roof it will serve you well for a very long time.   

PS if you are installing a simple sedum roof using Enviromat sedum matting, you have no need to worry about substrate – it’s an incorporated into the product.

 

More on creating a living green roof with Enviromat sedum matting

 

Further learning for professionals – discover more about our green roof CPD courses