Using compost and digestate - Royal Horticultural Society green roof study

Green roofs offer a range of environmental benefits, including:

  • Reducing rainfall run-off (aiding storm water management)
  • Insulating buildings
  • Increasing building lifespan (protecting from UV and temperature fluctuations),
  • Potentially reducing the 'urban heat island' effect and improving air quality (Dunnett and Kingsbury, 2008)

Green roof substrates have a number of specific requirements:

  • Lightweight
  • Well drained
  • Good at holding water and nutrients
  • Not readily leaching nutrients or decomposing over time

Organic matter can be incorporated into the substrates, but only around 15% organic matter is considered suitable due to problems with decomposition and nutrient leaching.  Compost has potential to be the organic matter component (and is used routinely in substrate mixes in the UK) but research is needed to establish a quantitative basis to support further use of compost.

A number of issues need to be addressed, including:

  • Constraints on the range of suitable plants able to thrive in growing media containing significant proportions of green compost.
  • The risk of leaching from green roof substrates containing significant proportions of green compost.
  • The capacity for green roof substrates containing relatively large proportions of green compost to support healthy plant growth in the medium to long term due to decomposition causing substrate shrinkage and general loss of organic matter.

This project investigates the potential for maximising inclusion rates of BSI PAS 100 certified green compost in green roof substrates through the addition of biochar - a biomass-derived charcoal. 

Biochar is a fine-grained porous material, high in organic carbon and largely resistant to decomposition in most environments. It can be used as a soil conditioner and can enhance plant growth and retain nutrients in soil by improving the soil's physical and biological properties.

The investigation will address the following hypotheses:

  1. Green compost can be included in green roof substrate mixes without detrimental effects on plant community establishment, leachate quality and substrate structural stability.
  2. Potential constraints on maximum inclusion rates of green compost in green roof substrates can be addressed by addition of biochar, which will increase the range of plants it can support, improve water holding capacity, nutrient retention and structural integrity.

Using a range of green roof substrates with varying proportions of green compost and inert material, the RHS are carrying out work at a field research facility at Deer’s Farm to address concerns within the green roof industry and allow practitioners to obtain research data on the effects of quality compost application within green roof substrates with respect to shrinkage, runoff composition and influence on plant communities.


Now in its second year, early results look positive showing reduced surface runoff compared to the control trial where no compost was used which in turn elevates the pressure on drainage infrastructure. There has also been good plant establishment on the sites again compared to the control sites. These trials will continue to be monitored until the end of 2010. The final project report will be due for publication in Spring 2011.