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DIY Green Roof Substrate

Hi all,

I'm looking for some advice on making my own substrate for a green roof please.

I'm putting a green roof on a small bike shed.  It's 2x1m, with a 10cm depth and it's sloped, so drainage won't be an issue. There's a mix of sun and shaded areas, so I'm thinking of using a mix of alpines and small drought-tolerant ferns.

I've been looking online for substrate mixes but as we don't need a lot, I'm not sure this is going to be cost effective. On some websites, the delivery cost is more than the actual material!  The shed is very sturdy but I'd rather not risk the weight of standard soil just in case. 

Has anyone made their own substrate before?  Any tips on composition greatly appreciated!


  • DovefromaboveDovefromabove Central Norfolk UKPosts: 82,245
    Never done it myself, but this

    suggests a mix of potting compost and a high proportion of perlite
    “I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh

  • wild edgeswild edges The north west of south east WalesPosts: 8,876
    What plants are you planning to grow? The pic below was on top of our rabbit hutch and the substrate was 1200 gauge DPM with 15-20mm of gravel on top, terram root barrier, 2 bags of John innes No2 with a bag of sharp grit mixed in and a layer of gravel on top to cover the soil. The plants are all easy going sedums and some drought tolerant saxifage. Given the choice again I'd replace the lower layer of gravel with plastic egg crate (the suspended ceiling light diffuser stuff) or expanded clay balls if I could find any cheap enough. Even broken up polystyrene would work as a base as it's just a drainage layer to stop the roots sitting in water.

    Perlite is also a good choice because it's much lighter than grit. If you go for really drought tolerant plants like sempervivums then you could mix it 50/50 with john innes.

    If you use some bigger rocks and logs on top it adds wildlife value and you can make deeper areas of soil for alpine bulbs or herbs.

    Tradition is just peer pressure from dead people
  • lovegardening77lovegardening77 Berkshire Posts: 332
    Wow that's impressive!
  • Lovely hutch Wild Edges!

    Thanks for the advice all. Looks like John Innes, perlite and clay balls will do the trick for the substrate layer. 

    I haven't decided on the exact plants yet. I've bought a copy of Nigel Dunnett's Planting Green Roofs and Living Walls and there's a good list of plants in it that I'm reading through. The part shade area is a bit of a challenge but Maidenhair Spleenwort and Irish Moss might work. 
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