Trials - PAS 100 compost in green roof substrates

Today's urban environment largely features paved roads and buildings, with small areas of green space that provide little potential for rain to soak away gradually. As a result, heavy rainfall can overwhelm existing drains, leading to surface water runoff and flooding.


Green roofs comprise one of the options to alleviate the pressure on drainage systems and are an established component of Sustainable Urban Drainage Systems. In addition to assisting with the management of storm water runoff, they also contribute to the insulation of buildings, improve air quality, protect buildings from UV and temperature fluctuations and offer the potential to reduce the urban island effect.

To achieve these qualities, green roofs must be lightweight, and both well draining and good at holding water and nutrients, while not leaching nutrients or decomposing over time. Typical substrates are made up of a mixture of aggregates and organic material at around 15 per cent.

The use of PAS100 compost as a component of green roof substrates can help to decrease rates and volumes of runoff while supporting the establishment of vegetation.  Trials took place to examine how substrates containing relatively large proportions of green compost could support healthy plant growth and to assess the potential for substrate shrinkage and the leaching of nutrients.

WRAP commissioned trials looking into the inclusion of PAS 100 compost in green roof substrates in order to provide robust, up to date information for anyone wishing to design, specify or install a green roof.