Teaching guidance:
Substrate workshop 1
Purpose: The substrate workshop is an opportunity for students to gain
understanding about the different characteristics of substrate components.
Focus is on what kinds of material could, or maybe should not, be part of a
green roof substrate, and the major difference between extensive and intensive
green roof substrate composition.
Time: 2 hours
Materials: Suggested materials divided into four categories
1. Organics Compost / green waste compost, peat, bio-char
2. Recycled: Crushed bricks 0-2 mm (fines), crushed bricks 2- 10mm (or
similar sizes), crushed ceramics, hasopor (or similar)
3. Light weight and porous aggregates: Pumice, lava, perlite, vermiculite,
zeolite and light expanded clay aggregates (sealed and non-sealed LECA)
4. Minerals: Gravel, sand, silt, clay, macadam
Before the workshop, prepare two well mixed batches (B1 & B2), 1 liter each:
 B1 with 40 % compost, 40 % sand and 20 % gravel
 B2 with 40 % compost, 30 % sand, 15 % gravel and 15 % clay
Do this without the students. The batches are going to be used in a mini-test
in the first section of the workshop.
In addition to this, gather a ½ liter sample (S1) of soil from the garden or
green areas nearby outdoor (Can be done with the students if you like).
Optional: Also use 1-2 different green roofs substrate from a 1-2 green roofs
companies (G1 & G2).
Equipment:
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Scale (or scales) with 0,1 gram precision
Buckets with the substrates
1 liter plastic vessels / containers (one per group of 2-3 students)
Large drinking glass or large transparent plastic cups
Optional: one 10 liter per group of 2-3 students
Location: Indoor with access to tables and tap-water.
Requirement: -
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Instruction
Divide the students into groups of 2-3 persons in each group.
Start the workshop with the mini-test. Bring the batches and sample B1, B2 & B3 (and G1 &
G2). Do not tell the students what the two batches you prepared contains, neither what
material or percentage. Pour about 0,5 dl of each batch into a large glass / plastic mug filled
with water (one glass for each batch, sample or substrate. Mix it well. Leave these glasses
until the end of the workshop – make sure they are not moved and left perfectly still.
The rest of the workshop is divided into two parts.
Part 1 – group discussions
All groups of students should sit at one table each. Provide them with the “Substrate
workshop 1 answers sheet” (appendix below). Place all materials in containers or bowls on a
large table / desk, without the categorization above but with nametag “pumice”, “compost”
etc. Each group should take a sample of every (or of most) of all materials. The questions are:
 Does this material hold or release any nutrients?
Why? Why not?
 How would you describe this material out of the following words?’
Porous
fragile
compact
lightweight
Now, out of all these materials you are going to suggest one extensive and one intensive
green roof substrate. Compare with the soil from outdoor.
 How would you by %, in volume, suggest the composition (Fill in the form).
How many of the materials do you exclude?
 How much organic do you choose? How much by % in volume and how much by gram
per liter of is this? (Fill in the form)
Discuss within your group:
 How much differ your solution from the soil from the green area outdoor nearby?
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Show the the general recommendations below first in part 2.
General recommendations from the German FLL green roof guide line
Organic content:
o
o
Intensive roof greening
Extensive roof greening
< 90 g/l
< 65 g/l
Maximum amount of clay and silt content (< 0,063 mm):
o
o
Intensive roof greening
Extensive roof greening
20 % by mass
15 % by mass
Part 2 – classroom discussion
In the part two of the workshop have a classroom discussion where all groups briefly describe
how their two substrate mixes look like by % in volume. Discuss with them how they thought
and you may also bring up these questions related to the general FLL recommendations:
 Regarding the organic content, why are you aloud to have nearly 50 more
organics in an intensive substrate than extensive?
(suggestion: discuss fire risks and type of vegetation)
 Did you think of what the plants need when you designed your substrate?
(suggestion: address nutrient needs, air-content, water holding capacity)
Now! Look at the large drinking glasses with the B1, B2, S1 (+ G1 and G2). Compare them, is
there any difference between them? Are some water transparent? (all particles smaller than
clay will have fallen to the bottom. If there is clay in the substrate you will see)
 In the recommendations there is a limit to have much silt and clay particles you
should have. Why is that?
Optional: Let the students mix 10 – 100 liters of substrate which you later use at your green
roof installation in the course.
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Appendix 1.
Name of material
Does this material hold or release any
nutrients? Discuss why or why not.
How would you describe this material out of
the following words?’
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Yes
a little bit
no
Porous
fragile
compact
lightweight
Now, out of all these materials you are going to suggest the content of one extensive green
roof substrate and of one intensive green roof substrate.
Material
% in your intensive substrate
% in your extensive substrate
1.
2.
3.
4.
5.
6.
7.
8.
Discuss within your group:
 How much do you think your suggestions differ from the soil gathered from the green
area outdoor nearby?
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