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Construction Practice

number 46Who should install green roofs?

Installing a green roof system is completely different to the installation of landscapes over natural ground. Ideally only people who have been trained in the installations of green roofs should undertake this task. However, if contractors who are untrained in green roof installation do attempt to install one, they should take particular care working at height, and not to damage the waterproof membrane, amongst other considerations.

There are increasing number of contractors who have received specialist training in green roof installation and care from organisations such as GRO (The Green Roof Organisation) and BALI (British Association of Landscape Industries).

number 47How do different green roof systems affect construction practice?

a ) Mat-Based Green Roofs Typically consist of pre-grown mats of sedum or meadow flowers placed over water retentive material (e.g. horticultural rockwool), or a thin layer of substrate. Pre-cultivated mats are a good solution for wind-exposed areas, as they hold additional substrate in place. Biodegradable nets can be secured over the whole green roof area to hold the mats in place. These materials tend to be supplied on pallets and simply require rolling out like turf, cutting to size and watering in. The rolled up vegetation mats will suffer if they are stored in rolls for more than 48 hours. Aim for these to arrive on site the day they are to be installed. b ) Substrate-based green roofs Typically consist of layers of growing medium which support plant growth. The materials for these roofs come to site as separate elements, the moisture mat, drainage board and filter layer will usually be supplied in rolls or large sheets. The substrate will be delivered in bulk bags and be craned on to the roof to be spread or blown on to the roof using hoses Where strong winds may cause substrate to be dislodged, biodegradable nets can be secured over the whole green roof area. Over time the vegetation will bind the substrate surface together and the net will degrade into the roof. c ) Biodiverse / Wildlife Locally sourced aggregates and natural features are used to recreate habitats for a specific species or a variety of flora or fauna. Successful biodiverse and wildlife roofs need a high level of design input from specialists if they are to provide suitable habitats for the target species. Biodiverse roofs are normally left to vegetate through natural colonisation but may also be seeded with a locally appropriate wildflower seed mixture or planted with suitable wild-flower plugs. The placing of unscreened demolition waste and building rubble on roofs is unlawful as it may leach pollutants into the water table. All green roofs which include intentional vegetation should be watered-in for between 4 and 6 weeks after installation, depending on the weather at the time.

number 48How should the delivery and storage of green roof materials be organised?

a ) Programming Ideally the green roof should be the last element of constructions on any development. Elements of the roof which are living products i.e. plants, must be delivered in a just in time manner. b ) Vegetation Care and Preperation Ensure a good pressure of water is available at roof level. All plants and pre-grown mats should be well watered while in storage; prior to installation and after installation. Depending on weather conditions, 6 or 8 weeks of watering maybe required. Where strong winds may cause substrate to be dislodged, biodegradable nets can be secured over the whole green roof area. Over time the vegetation will bind the substrate surface together and the net will degrade into the roof. c ) Site Storage Where materials come on to site that need to be stored, care should be taken that the risk of damage is kept to a minimum. Bulk bags of substrate should not be stacked more than two bags high and care should be taken that substrate does not get mixed with other aggregates being used on site. d ) Delivery to the Roof Materials should be stored at ground level and carried, craned or pumped up when required. Ensure that materials are not stored, even for a short time, in one position on the roof whilst awaiting installation - although a building may be designed to take the additional load of a green roof. The loading is usually based on even distribution across the roof.

number 49Are there seasonal considerations for the implementation of a green roof?

a ) Planting Seasons Spring or autumn are the best times to install a green roof and ensure successful establishment. It is recommended that green roofs are not installed during the summer months - this needs to be factored into the construction programme. b ) Exceptions Biodiverse roofs, which are not intended to have plants on them from an early stage can be installed at any time of year.