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Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield

Moisture retention characteristics of green roof substrates Jörg Werdin1, Dr Virginia Stovin2, Dr. Tobias Emilsson3 1

Junior Researcher, ZinCo GmbH / University of Sheffield, joergwerdin@gmail.com 2 Lead Academic, University of Sheffield, v.stovin@sheffield.ac.uk 3 Senior Researcher, ZinCo GmbH, tobias.emilsson@zinco-greenroof.com

Introduction Green roofs are exposed to more extreme weather conditions than comparable ground level plant systems, which results in higher evaporation rates and quicker drought stress for the plants. Therefore good green roof performance regarding plant survival depends on a high reserve of tightly bound, plant available water within the substrate. In order to evaluate the moisture retention characteristics in the range between pF 2.5 (Field Capacity) and pF 4,2 (Permanent Wilting Point) of a range of mineral and organic substrate components, green roof substrates and promising substrate amendments, the pressure plate method was used. By characterising the moisture retention behavior, valuable information could be gathered to get a deeper understanding of substrate performance in a range of other tests performed within the project such as phytometer and evapotranspiration testing. Knowledge of the moisture retention characteristics of the tested components, amended mixes and substrates may lead to a quicker selection process of promising green roof substrates regarding plant survival and give some indication, amongst other parameters, on the storm water retention behavior.

Overview of Methodology

Figure 1 Relationship between pF-value and pore diameter (adapted from Scheffer/Schachtschnabel, 2010)


Marie-Curie IAPP ‘Green Roof Systems’ Project

The Green Roof Research Conference 18-19 March 2013, Sheffield A pF-curve or moisture retention curve shows the relationship between the volumetric water content (VWC) and the suction force by which water is held within the pores and around solid substrate particles against gravity. The smaller the pores are in diameter and the closer the water is around the solid soil particles, the stronger is the suction force which retains the water (Figure 1). Due to the specific properties (such as particle coarseness and low capillary capacity of some of the tested components, mixes and substrates) the test method described by the pressure plate manufacturer (Soilmoisture Equipment Corp.) had to be adapted.

Key Findings A valid and repeatable method to produce moisture retention curves applicable for a wide range of mineral and organic substrate components, green roof substrates and promising substrate amendments in the range between pF 2.5 and pF 4.2 could be developed and an extensive set of data has been produced to characterise the moisture retention characteristics of these materials. It is believed that substrates with the ability to contain and release a high reserve of tightly bound, plant available water can support green roof vegetation for a longer period of time without artificial irrigation under drought conditions. This will not only reduce maintenance costs but make green roofs more sustainable regarding the use of water. Besides that it opens up the possibility for shallower green roof build-ups that result in lower weight and therefore make green roofs more applicable for roofs that have lower load-bearing capabilities.

pF 2.5

Plant available water

pF 4.2

26 24 22

Substrate 1

VWC in %

20 18

Substrate 2

16

Substrate 3

14 12

Substrate 4

10 8 6 4 0

1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16

Pressure in bar

Figure 2 Moisture retention curves for 4 different green roof substrates

Moisture retention characteristics of green roof substrates - Joerg Werdin  

Green roofs are exposed to more extreme weather conditions than comparable ground level plant systems, which results in higher evaporation r...

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