What is a Green Roof Substrate? (And what makes a good one)
Essentially a good green roof substrate must contain a blend of inorganic and organic material. Preferably local materials that encourages the growth of indigenous species.
- Coarser grained inorganic materials such as recycled brick, expanded clay or other construction type waste
- Organic material like composted green waste that provides nutrients, moisture retention and a medium to support growth
And the first thing to point out is that if you’re building a green roof, you want to use substrate, not soil. Soil is too nutrient rich, too heavy when wet and provides inadequate growing conditions for green roof flora and fauna.
Substrate is light, porous and low in vital nutrients. This lightweight medium is ideal for green roofs as it doesn’t add too much weight, allows for adequate drainage and slow, steady plant growth minimises elemental damage and the amount of rooftop maintenance you have to do.
You can read more about it on our ideal green roof growing medium article.
Green Roof Substrate Composition
A typical green roof substrate contains 70 – 80% inorganic matter and 20 – 30% organic matter. This regulates plant growth, minimises moisture retention, weight on the roof and the larger particles promote drainage
Best 8 Green Roof Substrates
Planting Depth (inches)
Westland’s Cactii & Succulent Potting Mix
Lightweight Green Roof Substrate
Garden Topsoil Direct
1.6 – 6
1.97 – 3.15
Fine Blend Growing Medium
Coir Coco Soil Substrate
Intensive Lightweight Green Roof Soil
Garden Topsoil Direct
Green-tree Roof Garden Substrate
Best Extensive Green Roof Substrate
Westland’s Cactii & Succulent Potting Mix is a specialist, relatively low nutrient substrate ideal for steady, sustainable growth.
The added grit and sand create optimal conditions for drainage, so root development, runoff and roof stability can be prioritised. If you are building a small, extensive living roof, adding some additional stones or perlite is sensible.
Garden Topsoil Direct’s Extensive Green Roof Substrate is designed to be used on rooftops where a lightweight and free-draining growing medium is required.
The low-nutrient compost is perfect for stable sedum and wildflower growth – particularly extensive green roof DIY projects like sheds and garages.
Boughton’s EX1 is their premier extensive green roof substrate.
It’s relatively low bulk density makes it perfect for rooftop installation, such is it’s lightweight nature. And the fact it’s made from almost entirely recycled material gives it extra environmental points.
Boughton’s EX2 has a higher amount of organic matter than it’s EX 1 companion. Over 3 times higher in fact!
It’s higher organic matter % and 12% increase in porousity compared to the EX 1 make it ideally suited to semi-intensive roofs.
Sky Garden’s sedum substrate is a blended mix of free-draining organic material complete with clean, recycled granular aggregates recycled from building industry waste.
Made from 70% recycled matter in fact.
This provides a low nutrient, water-retentive growing medium. Perfect for steady, stable growth on an extensive roof.
Sedum Supply’s Green Roof Substrate is an excellent blend of lightweight water retaining aggregates and composted organic material.
They blend composted organic material (from sustainable sources) and recycled, clean crushed brick – containing no demolition waste.
They then add pumice and lightweight aggregate to ensure quality drainage and appropriate water retention. Creating a lightweight, robust substrate perfect for extensive roofs.
Best Intensive Green Roof Substrate
Coir Coco’s Soil Substrate is a lightweight, eco-friendly option that is brilliant for green roofs.
The blend of coconut husk chip and fibres & pith provides the perfect balance of aeration, water retention and rooting. Perfect for sturdy, sustainable roof growth without compromising the structure.
There’s no need to use pebbles or perlite as the coconut husk does the job. And as a 100% organic, biodegradable space-saving solution, it’s perfect for encouraging biodiversity.
Intensive Lightweight Green Roof SoilGarden Topsoil Direct
Medium - high maintenance
Absorbent (with excellent aeration capacity)
Better suited to intensive roofs
Contains small stones that provide some drainage
Best suited to deep rooted plants
It’s designed for projects that require a lightweight but nutritious and free draining growing medium.
Much denser in nutrients and can sustain much larger plants and root systems, this soil retains more water, but still has excellent drainage capabilities.
Green-tech’s Roof Garden Substrate is an award winning blend of lightweight aggregate and Green-tree topsoil.
How many soils do you know of that have won awards?
Particularly suited to larger plants, root systems and intensive roofs – even containerised planting – with excellent drainage and appropriate water retention.
If you’re looking for a medium that supports larger plants, this is perfect.
And you need to know what type of green roof you’re looking to build and what you want it to achieve.
There are a host of benefits associated with green roofs – you can promote biodiversity and reduce your energy bills at the same time. But ultimately every roof is different.
We promote sustainable, simple gardening. That’s what green roofs should be and your substrate should be no different.
If you’re ready to choose your rooftop flora, our best plants for a green roof article is ripe and ready.
Green Roof Substrate FAQs
Obviously we haven’t tried every soil on the list, that’s either a) impossible or b) insanely expensive.
But we understand exactly what green roofs need substrate wise. So we breakdown:
- The substrate composition: roughly a 4:1 blend of inorganic / organic materials.
- Is it made from recycled materials? Is it sustainably sourced?
- Does it contain large enough particles to allow sufficient drainage?
- Is it a lightweight medium? Is it suitable for DIY projects.
- Is it low enough in nutrients and organic matter to only stimulate slow plant growth?
- Will it require significant watering or maintenance – if a substrate required a lot of attention it won’t make the list
This varies greatly, largely depending on whether the green roof in question is intensive or extensive.
Intensive roofs typically require a substrate depth of 6 – 9 inches to support larger plants with greater nutrient requirements. Of course this makes the roof heavier.
Extensive living roofs have a much smaller substrate depth – typically anywhere from 1 – 5/6 inches – because the lighter plants don’t need as much sustenance.