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Jonathan Bourne talks perfect substrates for green roofs

Jonathan Bourne, director of Bourne Amenity, tell us why crushed brick is the right choice for green roofs

There are various methods of meeting the German FLL specifications for extensive green roof substrates as highlighted in the amended 2008 FLL guidelines. If we take a standard extensive green roof, which will predominantly consist of low maintenance sedum plants, the key parameters are drainage and percolation, nutrients and the all-important saturated bulk density for the one-in-100-year weather event. Bourne Amenity has worked with a variety of  primary materials over the past 10 years in order to locate the perfect substrate blend to not only suit the plant health, but to also tie in with the environmental credentials that these green roofs
seek to conform to.

The ideal material needs to be lightweight, permeable and non-toxic. Once we have found this ideal base material then the amendments of  compost, composted bark fines or in some cases reconstituted peat, would give us the range of green roof blends to suit standard sedum planting, along with low fertility and ericaceous planting schemes.

We have found that the premium material that ticks all of the necessary boxes is crushed  primary reject brick. Due to the intensive firing  process, around 20% of bricks are deemed  unusable due to minor faults that occur when they are processed under duress. We provide  a home for these rejects by crushing them and  screening down to a variety of grades  (depending on the performance criteria).

The crushed clay bricks provide the perfect substrate for managing water retention and  percolation, as well as weighing in at just under 1.00g/cm³ (as per FLL calculations) and providing the perfect habitat for green roof sedums. It is essential to use primary reject brick and not recycled to avoid the presence of concrete or any other potential contaminants.

Whilst some manufactures opt for the more expensive expanded clay products it has been proven that the crushed brick substrates have superior performance in the key FLL criteria over those made with similar lightweight materials. Another key advantage is from the recycling and environmental perspective, as the crushed bricks can be fed back into the construction industry through their implementation on green roofs and
help our brick manufacturers move toward a zero waste process.


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