What is the Ideal Green Roof Substrate Composition - The 7 Essential Factors
For flora and fauna to thrive on your green roof, you need to use a substrate that:
- Supplies essential nutrients
- Is porous enough that roots can ‘bed-in’
- Lightweight so there’s no undue stress placed on the roof’s structure
- Has optimal water retention
- Allows for adequate drainage
- And stormwater runoff
- Alongside – ideally – supporting biodiversity
Supplies Essential Nutrients
Green roof plants typically don’t require a lot of nutrients. A substrate with too high a nutrient level will cause plants to grow too large too quickly, too quickly.
This means your maintenance requirements will shoot up (excusing the pun). As will the weight on the roof. So, rather counter-intuitively, you want a substrate with low nutrient density.
And we’re lazy gardeners. Especially when the gardening in question requires climbing a ladder.
Is Porous Enough to Allow Roots to Bed-In
Porousity is essential in aerating the growing medium, aiding water movement through the substrate and providing a light soil structure.
A denser, non-porous soil (i.e. your typical garden soil) will retain too much water, increase the load on your roof, over-saturate the plants and eventually, force you to re-do your green roof.
Not part of the lazy-gardeners manifesto.
Is Lightweight enough to not Place Undue Stress on the Roof
Typical garden soil can weigh almost 1000kg / per cubic yard and 50% more when wet.
Now I don’t know you or your shed, but I am 100% sure your roof cannot take that type of strain. So a lighter, porous substrate is essential.
Maintains Optimal Water Retention for the Vegetation you want to Grow on your Roof
Rather obviously, plants require varying degrees of moisture to thrive. Green roof plants like sedums and wildflowers tend to prefer dry – medium soil.
Which is a key part of their inclusion into the green roof sphere. Extensive roofs need plants that can survive in lightweight, low nutrient soil to reduce the weight on the roof.
Allows for Adequate Drainage and Enables Effective Stormwater Runoff
An over-saturated rooftop soil will cause the plants you lovingly picked to wither and die, whilst potentially causing your roof to cave in.
And when vegetation is saturated, it behaves like an impermeable surface. Water runs off it like it would on concrete. And in urban areas where we’re trying to mitigate flooding, this would be disastrous.
Supports Biodiversity and Promotes Indigenous Wildlife
Using organic materials and soil native to an area will help support indigenous wildlife, encouraging their colonisation. Part of the reason why biodiverse living roofs are so brilliant.
Many green roof experts encourage the use of mycorrhizae – fungal associations between plant roots and beneficial fungi – and lightweight drainage material like pumice to native soils. This allows for sufficient drainage – the drawback to native soil – and improved root growth because of the symbiotic fungal relationship.
Green Roof Soil vs Substrate
When it comes to creating an ecosystem on a green roof, there are three key factors that define your success or failure.
- Water Supply
- Growing Medium
Of course this isn’t unique to a green roof, but the lack of available soil depth, lightweight necessity and inability to store much water do make a living roof ecosystem unique.
We know that the amount of growing material you can use to support life is limited – particularly if you’re building on a shed or garage.
And it’s probably limited by the pitch of your roof. Generally you’re not advised to undertake a DIY green roof project if the slope on your roof exceeds 10 degrees.
Whereas at ground level these limits are non-existent. You can use the most nutrient-dense, moisture hogging mulch you can find.
But add 70m² of garden topsoil to your roof and we recommend calling a roofer as the last grain leaves the bag. And, as substrate is significantly easier to lift onto the roof, it fits nicely into the lazy gardeners manifesto.
Soil vs Substrate: Particle Size and Nutrients
Like topsoil, green roof substrate has a range of particle sizes to encourage proper drainage. But the larger ones, although lightweight, can’t be crumbled or crushed. This makes it perfect for a lightweight, structurally sound base.
And topsoil is very nutrient rich. It’s designed to create the largest, healthiest possible plants. And that’s not what we need. We need a slow, steady rate of growth to mitigate disease, weight and elemental damage.
If you’re ready, our summation of the best green roof substrates will help you start your green roof.
What makes Green Roof Substrate special?
Green Roof Substrate Particle Size
Decades of research and development. Quality living roof substrate blends should vary by location, climate, plant type, rainfall alongside a host of other factors.
Even something as seemingly minute – like particle size – has been taken into consideration.
The larger particles common with substrate create pockets of air within the soil. Not only do these air pockets allow for optimal drainage, but they help insulate the building. A key benefit of green roofs.
This helps the growing medium hold just enough water for the plants without increasing the weight on the roof.
Smaller particles, like silt or clay, tend to be moved around by rainwater. If enough of these particles are washed away:
- The amount of maintenance required has just increased. Directly against our lazy gardener manifesto.
- Drains could become blocked – increasing the risk of flooding
Green Roof Substrate is Designed for Rooftop Conditions
Rooftop conditions are harsh as plants are exposed to the elements.
If the plants are over-fed, they’ll be prone to frost and wind damage. Hence the need for a low nutrient growing medium. It doesn’t pay to have the plants get too large. From a maintenance (ugh) or weight perspective.
Soil microbes play the foremost role in soil ecology and formation and regulate the amount of nutrients in plants. And There are more microbes in a teaspoon of soil than there are people on earth.
But not in green roof substrate. Green roof substrate purposely has fewer soil microbes. Restricting their rate of growth allows rooftop plants to live healthier, longer lives as they’re protected from elemental damage and disease.
Green Roof Substrate and Runoff
Green roof substrate is specifically designed to minimise the risk of contaminating rivers and streams. Plants absorb macronutrients (nitrogen, phosphorus and potassium) from the soil.
And when these nutrients stay in the soil, there’s no danger. But as runoff causes these nutrients to seep into the ecosystem around it, it can upset the balance as plant nutrients are water soluble.
The organic matter used in green roof substrate – created with recycled secondary aggregates and organic components – is created to minimise the rate of growth.
The fact that this substrate reduces less nutrients into the environment – and therefore minimises it’s potential impact – is a key secondary benefit.
Green Roof Substrate and Roots
Roots need a firm, but porous soil to grow into and bed-in. And as soil compacts this gets harder and harder.
And natural compaction occurs via:
But because you can’t dig-up your green roof in case you damage the waterproof membrane (top tip, don’t do that), you must choose a soil that minimises compaction.
Exactly what the larger particles in green roof substrate are designed to do.
Green Roof Growing Medium FAQs
You shouldn’t need to use any fertiliser as most of the plants recommended for use on green roofs prefer a lean soil, only requiring a small amount of organic material initially.
After a few seasons the system should become self sustaining with the dropping of foliage and accumulation of dust particles from the air.
If plant growth is really lacking, you could use worm castings which are easy to spread and organic. One annual application should be adequate.
Don’t use checmnical fertilisers – it would detract from the whole purpose of attempting to enhance the environment, only to pollute local waterways with chemical run off from your green roof.
Recommended installation depth varies for green roof soil, but typically somewhere between 50 – 100mm for extensive living roofs should suffice.
If you plan on planting some trees or shrubs with more complex root systems than sedum plants or wildflowers on an extensive roof, then you could go up to around 200mm in depth.
More than that and we’d be concerned about the additional weight on the roof. In the short and long-term.
However intensive roof soil depth can be upwards of 400mm due to their structural stability.
Intensive green roofs use growing mediums that are deeper and slightly more nutrient rich than extensive green roofs. Intensive roofs soil can be upwards of 400mm in depth.
It can be significantly more than twice the depth of extensive roof soil.
The deeper soil allows intensive roofs to accommodate larger plants – their root systems and nutrient requirements. Intensive green roofs also require more maintenance than extensive green roofs because of the more complex plant varieties they tend to support.