Compost as a component of the substrate in extensive green roofs - Bioregional

This research provides an assessment of the potential for using BSI PAS100 compost as a component of the substrate used in green roof construction. It examines the performance of four different substrates in terms of plant establishment and water retention. 

The trial was designed as follow on research to an earlier project (OBF009-016), which identified a number of perceived barriers to the use of BSI PAS100 compost as a component of green roofs, such as the introduction of weed seeds. Raising awareness and demonstrating the quality and performance of BSI PAS100 compost was seen as key to helping address these perceived issues.

Based on preliminary trials under OBF009-016, the four green roof substrate mixes selected for testing were:

  • Crushed brick @ 90% and compost @ 10%
  • Crushed brick@ 80% and compost @ 20%
  • Aggregate fines@ 90% and compost @ 10%
  • Expanded clay @ 90% and compost @ 10%

The compost used in the substrates was sourced from TJ Composting and was certified to BSI PAS100.  The compost was less than 12 weeks old. Each mix was analysed against the BS3882:2007 standard for soils. The results of the chemical analysis were used to inform the assessment of each treatment mix through plant growth measurements and water holding capacity. The chemical analysis showed that the crushed brick and compost mixes were most suitable for plant growth, and this was borne out by the comparison of the levels of plant growth achieved in all four substrates. Similarly, water holding capacity was superior in the crushed brick mixes.

The trial was located in Scrutton Street, London and used recycled plastic trays to contain the trial replicates. These were sown with equal quantities of a seed mix containing wildflower perennials and cornfield annuals suitable for calcareous soils. Additional irrigation and fertilisation were applied as required and, at the end of the trial in October, each tray was watered to run off to establish the volume of water held at field capacity.


The trial demonstrated that compost which conforms to BSI PAS 100 provides a suitable and sustainable source for the organic fraction for the vegetation support course in extensive green roof installations in the UK when combined with appropriate mineral fractions such as crushed brick.