Green Roofs: Substrates SUBSTRATE SELECTION By Julian Thurbin, Wallbarn Director. Precise quality, speed of installation and known performance are key factors that have made modular construction products and systems an attractive proposition. And applying this thinking to green roofs has delivered huge advantages, not least that specifiers and installers can rest assured that the system they choose will live up to expectations and the sedum or wildflowers will thrive for years to come. At Wallbarn we were an early adopter of the modular approach, developing our high performance M-Tray system to offer roofing contractors an easy, quick and clean-to-install solution that delivers an instant living roof with the bonus that the 500 mm x 500 mm trays can be lifted if access to the roof deck is required. Our multiple award-winning M-Tray is a green roof in a box, combining all the elements of a traditional roll-out solution – including substrate, plants and drainage – into carefully designed trays that click together on site. Every element of the system has been meticulously thought through, including the all-important substrate. The substrate – the material into which the green roof is planted – holds the key to a successful installation. At Wallbarn, working closely with partner company Sedum Growers and in conjunction with renowned soil scientist Susie Holmes, a specialist in horticultural growing media and soil management, we have invested heavily in developing and field testing different formulations to identify the substrate composition that works best, both in enabling plants to flourish and delivering fire performance. The result is a ‘gold standard’ compost that is made predominantly from materials sourced close to the our growing nursery in Hampshire, and matured onsite for around a year, producing a super high quality, low organic and peat-free growing medium that meets fire ratings and exceeds GRO Code guidance. It has been designed to deliver an optimum blend and supply of slow release nutrients coupled with great moisture control, enabling plants to flourish throughout the life of the roof. The mix represents a fine balancing act between the basic elements of the substrate – compost, lightweight aggregate and coir – and the need to achieve compliance with BS 8616:2019, Approved Document B, the GRO Code and the B ROOF t(4) fire rating. Composting Producing this BS-compliant substrate relies heavily on the quality of raw materials and it’s here that the greatest of care is taken, as Sedum Growers’ joint Managing Directors David Holloway and Keith Pegren know. As David explained: “For us, a lot of the commercially available Pas 100 compost is of unknown quality. It might contain weeds or hormonal herbicides, which don’t breakdown in the composting process. We therefore manufacture our own on site using local, known sources of raw materials. We control the whole process from start to finish, which can take in excess of a year.” One ingredient used in the compost is discarded matter from a nearby business producing chamomile and other essential oils. This waste stream meets David and Keith’s exacting standards because they know its provenance and it has been heated to high temperatures, killing weeds etc. Other carefully selected agricultural waste is combined into the mix, which is then left to compost. “It’s quite a complex process,” explained Keith. “It has to be warm enough to kill weeds etc. so we continually check the temperature, and the chemical composition of the final compost is key. Chemicals released during the composting process create salts, which can be harmful to plants so they have to be allowed to Above: M-Tray farm sedum harvesting. Left: Substrate components. leach out. We carry out electro conductivity tests to measure salts content and only release the compost for use when it has physically cooled down sufficiently and salts are at a safe level.” To complete the Wallbarn substrate, coir and Lytag and/or Leca are combined with the compost: Coir: a lightweight, natural by-product of the coconut industry used to lighten the substrate and introduce fibres that lock in moisture and nutrients. Lytag: a lightweight ballast produced from power station waste. Leca: an expanded clay which is also a waste product and super lightweight/bulk forming. The roots of sedum and wildflowers grip the particles to bind into the substrate and support the whole green roof. The care invested in the M-Tray substrate demonstrates how at Wallbarn we go above and beyond legislation and guidance. We have been at the forefront of many industry changes and always strive to physically test ingredients to ensure they perform as we wish, using Warringtonfire for fire tests and STRI for growth tests. M-Tray exceeds all regulations by a significant degree. Our ethos is to provide customers with industry-leading, healthy green roofs that thrive throughout their lifetime, meet regulatory and industry standards and are as eco-friendly as possible. Contact Wallbarn www.wallbarn.com 020 8916 2222 24 TC DECEMBER <strong>2022</strong>
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