Teaching guidance:
Substrate workshop 2 – maximum water storage competition!
Purpose: The substrate workshops helps students to gain understanding about
the different characteristics of different substrate materials. This workshop
focus on what kinds of material are beneficial for water storage capacity as well
as making sure the drainage capacity on the green roof is ensured. The balance
between good drainage and water holding should be highlighted as well as the
fact that most extensive green roofs substrates are made to be as light weight
but not too expensive (!) – how do the market meet all these demands?
Time: 2 hours divided into 2 days with 26 hours in between.
Note: The idea behind dividing the workshop into two days is that we follow the FLL
standardized recommendations for testing a substrates maximum water holding capacity within.
If it is not possible for you to do the workshop with 26 hours between, changes can be made.
The goal is not to do a standardized test, the goal is to get an idea of substrate considerations.
Materials: Suggested materials divided into four categories (for extensive roofs)
1. Organics Compost / green waste compost, bio-char (dry!)
2. Recycled: Crushed bricks 0-2 mm (fines), crushed bricks 2- 10mm (or
similar sizes), crushed ceramics
3. Light weight and porous aggregates: Pumice, lava, perlite, vermiculite,
zeolite
4. Minerals: Gravel, sand, silt, clay
Equipment: Scale with 0,1 gram precision, buckets with each substrate
component in dry condition. Wessels and a mesh / net (not finer than a
mosquito net). 10 liter buckets, a small peace of filter fabric and a weight (like a
stone) as additional equipment for each wessel.
For standardized maximum water holding capacity test a cylindular wessels with
a diameter of 150 mm is used with 5 mm in diameter wholes in the bottom.
It is not necessary for the success of the workshop to access this equipment, but
if possible choose a plastic wessel that is cylindular and about 1 liter big. In the
bottom of the wessel drill plenty of 4-5 mm wholes, so they cover the whole
bottom. In the bottom place the mesh / net so they cover all the wholes.
Location: Indoor with access to tables.
Requirement: The substrate workshops requires that you have time to start a
workshop in the beginning of the day and then continue later during the same
day and/or the following day.
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Substrate workshop part 2
The following recommendations for water storage ability and organic content for intensive
and extensive green roof layers (as a hand out or projected at a screen during the workshop)
exist within the FLL guide-line.
General recommendations
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
Maximum water storage capacity:
o
o
Intensive roof greening
Extensive roof greening
> 30 % by volume
> 20 % by volume
First - Let’s discuss these recommendations
 Regarding clay and silt content. Why are there recommendations for the amount that
is aloud? Should clay be mixed into an extensive substrate?
 Regarding water storage capacity, why are extensive green roofs recommended to
store less than intensive green roofs?
Mix a 1-2 liter batch (depending on the size of your wesells). The goal is to make an extensive
green roof substrate which doesn’t contain more than 65 g / liter of organics and have a good
water storing capacity.
All of the ready (and dry) batches of substrate must note weight more than 1 kg / liter and, as
part of the “competition” today - not less than 0,8 kg!
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Test procedure
1. Place each wessel in the bottom of a 10 liter bucket. The net should be placed in the
bottom of the wessels. The ready batches (1 or 2 liters) are then poured down in one
wessel each. Place the textile and the stone (or other weight) at the top of the
substrate to make sure the substrate doesn’t rise when you fill the wessel (and the
bucket with water).
2. Fill each bucket and wessel with water so that the water surface is about 0,5 – 1 cm
above the substrate.
3. Leave the buckets where they are for 24 hours (if possible).
4. After 24 hours: gently lift the wessels out of the buckets. Empty the water in the
buckets and place the wessels in such a way so that the remaining water within the
wessels will be able to drain (none of the wholes in the bottom should be covered)
5. Let the wessels drain for exactly the same period of time; about 1-2 h (2 hours in the
standardized test).
6. Remove the stone and filter fabric on top of the substrate. Weight each substrate
batch with the scale.
Take away the weight of the wessels from the result and the substrate batches (which each
weighted 1 kg / liter in dry condition) should now weight somewhere between 1000 mg and
1300 mg. Winner of the competition is the group with heaviest substrate batch!
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