Work Packages

Research objectives and description of research projects

The principle aim of the programme is to enhance the qualities of extensive and intensive green roof systems by revisiting the fundamental basis of green roof system design. This will provide a profound understanding of green roof system performance and of the options for optimizing those systems to meet the new challenges described previously. The main programme will be realised through two complementary strands: The first strand will focus on vegetation selection, substrate characterization, and research into green roof components such as drainage elements, filter sheets and protection and water retention mats, with the objective of improving water retention and supply in the different green roof layers.  The second strand will focus on characterizing the performance of new combinations of vegetations and components, with emphasis on optimizing green roof performance in terms of hydrological efficiency and runoff reduction.  This will result in the development of new products and green roof systems with commercial potential.



Work Package 1 – Plant Selection and Screening Programme


Plant selection for extensive green roofs at an international level is still largely dependent on lists produced from research in Germany in the last 20 years and on the use of for example of standard lawn mixtures and perennials for intensive green roofs. The resultant vegetations are similar from country to country.  Moreover, most extensive green roof vegetation does not resemble any naturally occurring plant communities, but is instead a mixture of different Sedum species from different geographical origins. Research is needed to identify suitable plant species from different climatic regions, and to use co-evolved assemblages from suitable habitats such as rock outcrops, cliffs, dunes and heathland. In addition to suitability for growth in the harsh exposed environment of a green roof, additional factors for plant selection may include such facets as the ability to withstand shaded conditions, or the ability to remove contaminants from stormwater, or provision of pollen and other resources for native insects. For intensive green roofs there is the need for research on a lawn replacement type of vegetation,  and of vegetation that fits into a flowering roof garden but with the use of drought resistant low water demanding species.


Research Objective:

To develop and implement a rigorous and standardized plant screening programme for green roofs, to test potential plant species for diverse, self-sustaining, and low-input green roof vegetation – both extensive and intensive, and focused on survival of moisture stress at different depths of growing medium.  This will be the largest international plant screening and development programme undertaken since the original development of green roof in Germany.



The research will identify optimal growing media depths and degree of tolerance to prolonged drought for different species and will be undertaken using small-scale experimental green roofs.  A cross gradient design will be used, with one gradient being substrate depth, and the other gradient being moisture availability (through applications of different amounts of water at differing intervals). Each subplot or cell within the grid experiences a different combination of substrate depth and moisture availability.  Each cell is planted/seeded with the same plant species, and the experiment is monitored over several growing seasons. Each experimental unit is replicated 5 times to provide the necessary experimental rigour for robust, reproducible research on natural systems. Two contrasting growing media will be employed, with a standard nutrient regime will be standardised across the experiments. The output from this work will be the identification of suites of species that survive well with minimal supplemental irrigation at different depths of growing medium.  Plant species for screening will be selected according to a set of rigorous criteria, and taxa will be drawn from both suitable reference habitats in both Europe and North America.


Plant screening methodologies, protocols and plant selection will be developed at USFD.  However, the screening programme will be undertaken at both USDF and ZinCo, using the same protocols and the same core set of species.  Plants will therefore be tested in a continental European climate zone (ZinCo) and a maritime European climate zone (USFD).

Workpackage 2 – Product development to optimise hydrological performance


Typical green roof systems are composed of a number of distinct layers above the waterproof membrane of the roof.  These layers include a) water retention and protection mat: an absorbent and strong mat that holds moisture but which also protects the underlying waterproof membrane from damage by the green roof elements and the installation operations; b) drainage elements: typically plastic cellular layers that both enable excess water to be removed from the substrate and vegetation layers, but which may also act as reservoirs for water storage beneath the substrate, for use by the green roof vegetation; c) substrate or growing medium.  This layer can be of varying depths according to the type of roof buildup.  Green roof substrates are currently derived from a small number of aggregate or mineral materials, together with organic matter. Materials include lava, shale, light expanded clay granules (LECA) and brick as well as bark compost, peat moss and green waste compost. Each of these layers has a role to play in green roof hydrology, with the substrate layer traditionally being seen as the most important. 


Overall Workpackage Objective:  Development of green roof system components to optimise the hydrological performance of green roof systems, with particular focus on water retention and supply, whilst not significantly increasing system weight. 


The workpackage has two components: Component 1 – the substrate or growing medium, and Component 2 – the drainage elements and other non-substrate components.


Component 1.  Substrate Development

Management of water supply is at the heart of successful green roof design.  This relates to the demand of the vegetation for moisture retention, and the need for efficient discharge of water during large storm events.  The goal is to eliminate the need for irrigation, and this is achievable in all but semi-arid climates.  A key factor in eliminating or reducing water requirements is appropriate selection of green roof growing media.  Green roof growing media must provide a suitable basis for plant growth, and for successful moisture management must efficiently absorb and retain water, be readily drained, and offer a high void ratio (pore space or air volume).  Green roofs are typically composed of mineral granular components, with a small fraction of fines, and a low percentage of organic matter. 


Research Objective:

  1. to investigate green roof substrate formulations which optimise moisture supply and retention.
  2. To characterize the properties of these materials on stormwater runoff from green roofs (in terms of peak flows, and total runoff


Methods  A range of new mineral materials and substrate amendments will be tested and evaluated.  These materials will, where possible, be locally sourced and have low embodied energy e.g secondary aggregrates and other waste or recycled materials.  A standardized phytometer testing regime will be developed to assess their suitability for supporting plant growth, both on their own and when mixed with differing volumes of organic matter and fines, using shallow trays in glasshouses.  Indicator plant species will be sown onto the substrate materials, and their subsequent establishment and growth success recorded, at both high and low moisture stress, using replicated factorial experimental design.  Promising combinations will be used in outdoor green roof test rig trials, again using dependable green roof indicator species, at high and low moisture stress.


Detailed physical and chemical characterization tests will be carried out, using standard techniques, following accepted techniques for measuring physical and hydrological properties of substrate employed by the FLL (the body responsible for setting and monitoring green roof standards in Germany) Guidance Manual, with focus on moisture-related tests such as maximum water-holding capacity, field capacity, bulk density, porosity.  The specific objective of the substrate evaluations will be to investigate water availability to vegetation (the pattern of water uptake and release from the substrate or growing medium).  Sorption curves (relating moisture content to the capillary potential of the substrate) will be produced.  These curves indicate moisture availability and are particularly useful in determining the ‘resistance’ to moisture removal by vegetation as the substrate dries out.   However, there is a need to develop a new sensitive test to measure water availability to plants in highly mineral-based substrates and this will be developed as part of the programme.  In addition, work in controlled climate chambers will evaluate evaporation rates from the substrate surfaces, and evapotranspiration from vegetated systems.


Capillarity and evaporation effects can be particularly crucial in the early phases of green roof establishment before vegetation cover is complete – potential evaporative moisture loss can be very high.  The programme will test the value of temporary moisture retaining additives on moisture availability in the early phases of vegetation establishment.  All new materials and combinations will be compared with the properties of standard commercial green roof substrates. 

Component 2. Drainage element, water retention and protection mat + filter sheet research


To date the focus on the drainage elements of a green roof has been on their efficiency in maintaining the overlying substrate in a drained condition (preventing ‘drowning’ of the vegetated cover), and providing an efficient mechanism for discharging stormwater runoff.  Although many systems include water reservoirs in their design which retain water following irrigation or rainfall events, there has been little intensive research into the functioning of these layers in relation to water supply, or into how they can be optimized to aid vegetation survival under moisture stress.  Relying on the substrate only to retain moisture is the usual approach, but retaining water in the substrate makes the whole system much heavier.  Potentially, the drainage and protection layers can have a high water retention capability, without imposing great extra weight.  Mechanisms need to be investigated for delivering this moisture to the vegetation in an efficient and controlled manner (either by vapour or capillary flow).  At the same time, the runoff characteristics must be equally efficient so that the components retain their drainage role. 


Much of the research work on green roof hydrology has been mainly based on the substrate. There has been virtually no work on the influence of the different green roof components.  Also whole roof situations have not been taken into account.  Moreover, green roof hydrological models focus exclusively on runoff and there has been little consideration of the inputs necessary to sustain efficient green roof function.  There is a need to shift the focus from storm-based hydrological performance to the inter-event moisture retention and availability characteristics.  In order to understand these we need to be able to characterize the performance associated with individual system components, as well as assess the integrated system as a whole.


The hydrological influence of the different layers needs to be determined, on their own and in combination, and models constructed to enable a more accurate description of the performance of the complete system.   There has also been little consideration of the different drainage situations of roofs (drainage into a linear eaves gutter versus drainage into a roof outlet as a punctual drain). 


Research Objectives

  1. To understand the actual and potential contribution to green roof hydrology of the different physical, non-substrate green roof components.
  2. To develop and test materials and components that maximize moisture retention and availability to green roof vegetation (for either extensive or intensive green roof types), whilst fulfilling the practical requirements of green roof system components.  The research on this will help to improve the drainage layer and understand which method needs to be used for either extensive or intensive green-roofs systems.
  3. To characterize the properties of these materials on stormwater runoff from green roofs (in terms of peak flows, and total runoff). 




Materials and prototype products will be identified by ZinCo in collaboration with USFD.  Materials will tested on small-scale (1m x 500mm) indoor green roof test rigs, which take the form of trays with drainage outlet that allows runoff to be collected in a barrel, using a pressure transducer to record (at one-minute intervals) the depth of collected runoff.  This is then converted to runoff depth for comparison with measured water (simulated rainfall) input.   Different drainage materials can be inserted into the trays – the same test rigs can be used to test many different materials under different simulated rainfall conditions (using overhead irrigation gantries).  Drainage materials will be tested in isolation and also in combination with a standard commercial substrate – this is necessary in order to simulate water availability to vegetation using the different drainage materials, under extreme wet and dry conditions.  More detailed laboratory and green house tests will be undertaken to determine the infiltration and through-flow (horizontal drainage) effect of the materials and components to determine the specific water storage capacities and runoff delay capabilities of the components.  New protocols will be developed to evaluate the possibility of zero additional water input to maintain the efficiency of different green roof systems, using the studied components.  Furthermore new test rigs will be developed to determine the influences of flow length and different drainage situations (punctual versus linear).


Workpackge 3 – Integrative Whole System Hydrological Modelling


Work packages 2 and 3 will have delivered, over the first two years of the project, suggestions for new plant mixes, substrate compositions and engineered drainage layers.  This will have resulted from a mix of larger scale work on simulated green roofs, and small-scale laboratory tests and experiments.  This second phase of the programme will characterize the performance of complete systems derived from the proposed materials and plant mixes, and will test their performance against several standard and widely used commercial systems with typical green roof vegetations. 


Research Objective

  1. To characterize the performance of new green roof systems and configurations resulting from the product development phase of the programme. 
  2. To develop new, generic hydrological models of green roof performance
  3. To make recommendations for system configurations, substrate compositions, drainage elements, and vegetation compositions that reduce or eliminate the need for irrigation, whilst retaining stormwater attenuation and other benefits, and maintaining aesthetic value. 




Small-scale microcosm experiments will be undertaken in climate chambers with multi-layered green roof systems (using components from workpackage 2 in differing combinations) in small trays to determine water movements through green roof components – water retention and time-elapsed for runoff will be recorded.  The most efficient combinations will then be taken outside into practice.


Work will be undertaken on 20 small scale external flat green roof test rigs (1.0 x 3.0 m) with drainage outlets, using different configurations of substrate depth and vegetation type, depending on whether extensive or intensive type.  The runoff quantity characteristics of the roofs will be characterized, as a function both of the individual rainfall events, but also as a function of the antecedent dry weather period (ADWP) and the moisture content of the substrate and drainage elements (monitored through moisture sensors).   The water demand of the different vegetations throughout the year will be monitored through cross-comparison of water input and output, and the tolerance of vegetation to both high and low rainfall periods assessed.  In addition, controlled hydrological tests will be undertaken using an indoor rainfall generator to study in detail the response of the new configurations to design rainfall events of different intensities. 


The following modelling approach will be adopted, which will build upon data from all three workpackages.  We will develop a generic, physically-based model to describe the green roof rainfall-runoff response.  The model will be parameterised in terms of fundamental physical properties of the system, including roof configuration details, substrate material properties and plant types.  It will provide a time-dependent (5-minute resolution) runoff prediction in response to time-series rainfall inputs, and be responsive to both long time-series rainfall inputs as well as extreme event design storm inputs.  Antecedent conditions, including retained moisture and evapotranspiration rates, will form critical state variables in the model.  The modelling tool will have transferable predictive capabilities that will allow hydrological performance characteristics for other locations and/or other configurations to be simulated.  It will therefore be feasible not only to model performance in other contexts, but also to optimise configuration options to meet specific local constraints and opportunities.  As part of the present proposal, the modelling tool will be utilised to refine/optimise the design of new green roof systems to maximise water retention efficiency whilst maintaining well-understood stormwater retention performance and aesthetic values.  The second way in which the modelling tool will be used is – in simplified form – through its integration with commercial urban drainage network modelling tools to demonstrate the catchment-wide impacts associated with the widespread introduction (new build and retrofitting) of green roofs into urban catchments.  Studies will be undertaken relating to local case study catchments to provide new insights into the policy-level implications of the widespread implementation of green roofs as part of broader urban stormwater management strategies.