Living roofs or green roofs
Living roofs come in many colours, depending on the plants chosen. Foliage and flower colour change during the seasons, and they can be haven for butterflies and other wildlife.
I have been fascinated with living roofs since childhood. I never forgot how Laura Ingalls Wilder describes her father making a sod house, in her classic book "Little House on the Prairie". The roof was made with grass turves, which blended into the landscape, and it was shady and cool in summer. This form of turf roof, still used in scandinavian countries, has been adapted to other climates and is currently undergoing a revival of interest, compatible with new technologies and with contemporary lifestyle.
Green or living roofs are, simply put, roofs with a layer of growing medium - soil and/or other material - sufficient to support vegetation. Living roofs are an ecological benefit in a city, bringing vegetation and wildlife into urban centers, and reducing pollution and dust levels. They act as a sort of sponge, soaking up rainwater which would otherwise run off, contributing to erosion and increasing pressure on sewer systems, and as a basic cooling mechanism. Water is absorbed by the plants and then gradually released through transpiration, cooling the air above, and the humid mass of planting medium and plants adds insulation to the building below - thereby reducing the need for air-conditioning and the heat generated by air-con motors. On a wide scale, green roofs are promoted as a way to reduce water run-off and heat islands over extensively built-up areas.
Green roofs can be built everywhere. Anyone who has observed vegetation flourishing on abandoned rooftops, dry-stone walls or gravel roadsides can testify that the right plant will find its way into the most inhospitable-looking place. Designing a green roof relies on extensive research in the field, planning a structure adequate for the building below and the growing medium on top, and sourcing the adequate plant material. As in any other aspect of garden design, it's a combination of art, empirical observation, and careful adaptation to the site. Each roof is custom made for the building and garden it inhabits, and for its unique microclimate.
In extensive green roofs, the layer of soil is comparatively thin, up to 20-25 cm in depth, and supports only herbaceous perennial plants and some grasses. By comparison, 'intensive' green roofs, with much greater planting depth, can support trees or large shrubs, but strictly speaking they fall under the gategory of roof gardens. Taking into account the weight of wet soil and plants, and other factors such as wind resistance and need for irrigation, a roof garden will require a much stronger building to support it, or considerable engineering work, than an extensive green roof.
Extensive green roofs are cheaper and far more adaptable than roof gardens, and are probably a more ecological solution. Some states offer incentives to companies who want to green their roofs, and there are several companies that offer green roof solutions on an industrial scale. It's possible to buy ready-made mats of plants that can be simply unrolled onto prepared roofs, many existing industrial buildings are already suitable for green roofs, with little modifications, while new industrial buildings and warehouses can have the load of a green roof easily integrated into their deisgn at a planning stage. On a smaller scale, and without the load-bearing capacities of reinforced concrete, green roofs are much more made-to-measure. The chief requirement is not to damage the waterproofing which protects the building below, and to take into account the weight of the structure itself plus planting medium once wet. It take planning and skilled builders.
In central Italy, on a domestic scale, living roofs are still not standard practice. The benefits are obvious, because they reduce the ecological footprint of any existing or planned building, and integrate it into the surrounding landscape. They do however require careful planning, to consideration many variables. The load-bearing capacities of each roof, the local ecosystem, local planning regulations and building traditions, all have to be taken into account before a green roof can be designed. It's always best to consult an architect or engineer, and have the structure built by expect builders. The range of plants is determined by depth of soil, exposure, and drainage - the angle of the roof and the planting medium will affect how fast it dries out. The planting medium is a mix of different materials - not common garden soil, but a combination of peat or peat substitute, whenever possible, pumice stone, sand, or other inert materials. It's important to calculate both their dry and wet weight, and their water-retentive or draining capacities, as this will affect the structure of the roof itself and the choice of plants. The depth of planting, which ideally should be between 7 and 20 cm., affects the plants which can be employed.
A final layer of gravel tops off the planting, to hod down the lighter soil below, and provide a mulch. This protects the soil from drying out and a reduces self-sown weeds. Once established, the roof should not require additional watering, if the right balance of drought-tolerant plants is found.
Initially upon planting, the roof still looks sparse.Over time the plants will knit together and cover the gravel mulch. In the early months the roof should be watered occasionally to ensure root establishement, but an irrigation system is unnecessary and completely against the whole water-saving philosophy of green roofs.