Substrate Properties

The design of a green roof growing substrate must include specification of the following physical properties, outlined in table 1. Many of these measures are used in the development of growing media for containers and landscaping soils, making this information relevant to development and testing of growing media for use with some green wall and facade systems. Referenced standards used to develop this table are AS 3743 2003, Appendix B, and the Singapore Centre for Urban Greenery and Ecology Standards: these are listed in the References section. It is valuable to obtain independent testing and verification of the properties of substrate mixes on large scale commercial green roof projects.

Table 1: Properties that should be specified in growing substrates

Property Definition What it means in relation to green roof substrates Unit of measurement
Saturated bulk density The mass of a known volume of a material under fully saturated conditions. The maximum possible weight of the growing substrate (assuming no ponding water sitting above the top of the substrate) as it includes the weight of the material and of water that drained freely from it under test conditions.  This parameter is set by the roof weight loading. g/cm3 or kg/m3
Air filled porosity (AFP) The percentage of air held by a known volume of fully saturated growing substrate, just after it has stopped draining. AFP is measured by watering and draining a known volume of substrate under a set of standard conditions described in AS 3743 2003, Appendix B. AFP provides an indication of the likely aeration of the growing substrate and flags any potential for waterlogging, in conjunction with water-holding capacity (see below).AFP should be 10-20% % (of known substrate volume)
Water-holding capacity (WHC) The total amount of water a known volume of growing substrate can hold after it has been watered and drained under standard conditions described in AS 3743 2003, Appendix B. WHC indicates how well the growing substrate retains water.WHC should be >35% or greater for an extensive green roof, and 45% for an intensive green roof (up to a maximum of 65%); WHC will vary inversely with AFP (above) and an acceptable balance of the two must be achieved. % (of known substrate volume)
pH The measure of the acidity or basicity (alkalinity) of a solution, expressed as the negative log concentration of free hydrogen ion (H+) it contains. Determination of pH of an aqueous extract of a growing substrate indicates whether any nutrient deficiencies are likely, as pH affects the availability of the nutrient supply to plants. Most plants show optimal growth in growing media with pH of 6 to 7.Optimum pH range may vary with substrate composition.

pH 6 to 7.5 for mineral mixes;
pH 5.5-6.5 for organic mixes;
pH 5.5-8.0 should provide acceptable growth for most species.

 

No unit; range from 1 (acid) to 14 (alkaline, or basic).Solutions with pH below 4 are extremely acid, and solutions with pH above 10 are extremely alkaline
Electrical conductivity (EC) Determination of the ability of salts dissolved in solution to conduct electricity. An indication of the degree of salinity of a growing substrate: high salt content can impair plant growth.Growing substrates should have EC of less than 2 dS/m. deciSiemens per metre (dS/m)or

measure total dissolved salts in ppm

Cation exchange capacity (CEC) The ability of growing media to attract and hold positively charged ions (cations) thus making them available for plants to take up Cations such as calcium, magnesium, potassium and ammonium (a source of nitrogen) are crucial for healthy plant growth. Decomposition of organic materials such as pine bark and coir increase the cation exchange capacity of growing substrates. Colloidal materials such as clay also provide CEC but these are usually present in very low proportions in green roof growing mixes.Analytical laboratories determine CEC by measuring the amounts of different exchangeable cations in growing media. Growing substrates should have CEC of 50 – 100 mEq/l. milliequivalents or ppm
Infiltration rate A measure of how rapidly water soaks into a growing substrate, growing medium or soil. For an extensive green roof growing substrate the infiltration rate should be 1.0 mm/min or 60 l/m2/h.For an intensive green roof, infiltration rate s should be 0.6 mm/min or 36 l/m2/h. mm/minor

l/m2/h

Australian Standards also specify testing requirements for growing substrates and media to ensure they are not toxic to plant growth.

Green roof growing substrates are designed to have a mix of large and medium-sized particles to create an open, porous structure inside which smaller particles can fit. The sizes and proportions of small particles and their packing inside the large pores make a major contribution to the amount of water the mix can retain. Water is held by capillarity between small, tightly packed particles in the growing substrate, and it is this water that is available to plants. A mix with a high proportion of fine particles will retain more water than a mix composed of few fine particles.  

The report, The Matter of Landscape: Sustainable Design Strategies for RMIT City Campus, provides further information on different elements growing mediums and their properties.

There are no Australian specifications for the composition and physical properties of green roof growing substrate, and often international specifications are used or adapted. Table 2 outlines the properties of green roof growing substrates as specified by Germany’s FLL and Singapore’s Centre for Urban Greenery and Ecology.

Table 2. Properties of green roof growing substrates

 

  Extensive Intensive
Clay and silt content <15% by mass <20% by mass
Proportion of particles > 4mm in diameter < 50% by mass < 40% by mass
Organic matter < 65 g/l  (FLL)10-25% (CUGE) < 90 g/l (FLL)5–10% (CUGE)
Settling No more than 10% of nominal depth Average of < 5 cm for substrates at least 50 cm deep
Water permeability 0.6 – 70 mm/min 0.3 – 30 mm/min
Water storage capacity > 35% by volume > 45% by volume
(maximum of 65%)
Air-filled porosity > 10% > 10%
pH 6.0 – 8.5 6.0 – 8.5
Total soluble salts 1.5 – 3.5 g/l 1.5 – 2.5 g/l

 

Table 3 shows a range of components commonly used in green roof growing substrates and reasons for using them, along with indications of their saturated and dry weights.  This represents a very small number of components that could be used in growing mixes.  Comparative values for topsoil and sand are also provided for reference, but their high bulk density often limits or precludes their use.

Table 3. Comparison of materials used in green roof growing substrates and their saturated bulk density

 

Material Component Why used Saturated bulk density (kg/m3) Dry bulk density (kg/m3)
Recycled waste products Ash Waste product 1160 – 1310 640 – 900
Organic materials
(coir, pine bark, compost)
Waste product, water retention, CEC 930 – 1100 50 – 360
Crushed ceramics
(brick, roof tile)
Longevity, recycled 1090 – 1300 1640 – 1720
Quarried products Scoria aggregate rock Longevity 1290 – 1560 670 – 1000
Scoria non-descript
crushed rock (NDCR)
Longevity 1530 – 1730 1030 – 1270
Inert volcanic products Perlite Light weight 500 – 800 40
Pumice
(Hekla, Iceland)
Longevity; porous and light weight 540 –753 260 – 490
Synthetic additives Hydrocell Inert, water-retention 660 27
Loamy topsoil 1700 – 2400 1000 – 1900
Sand 1800 – 2200 1440 –1650
Water 1000  
Burnley extensive green roof medium 1370 850

 

Values compiled from Handreck and Black, University of Melbourne Green Roof Infrastructure Group research data, Weiler and Scholz Barth, Dunnett (Small Green Roofs) and JEI International.