Extensive green roofs can be specified to provide a low maintenance, lightweight aesthetically-pleasing and/or functional ecological roof covering. Not intended for regular access or use, extensive roofs convey numerous benefits on the building owner and wider community (e.g. stormwater reduction, climate change mitigation etc).
Specifying an Extensive Roof
Vegetation is typically in the form of drought tolerant plants including hardy succulents, grasses, herbaceous perennials, annuals, native wildflowers, alpines and bulbs. A specially-formulated growing media and drainage board should be specified to provide the air, water and nutrients appropriate to the selected plants.
The choice of planting has important implications for both the cost of the installation and the in-use performance and appearance. Typically comprising a selection of Sedum species in one of the following forms:
A vegetated mat consisting of a biodegradable base layer, a specially formulated substrate layer and a 90% mature plant cover comprising 6-8 species (predominantly Sedums) randomly sown and grown to maturity.
A large and diverse range of young plants grow to provide greater coverage over 1-2 growing seasons. Selecting from a range of co-habitable species offers diversity of colour, shape, height and flowering time.
A mix of Sedum cuttings, seeds, mulch and fertiliser 'hydroplanted' onto the growing medium is the most economical method of planting. The vegetation germinates, roots and grows to cover the initially-bare surface over 1-2 growing seasons (depending on the time of year of installation).
The substrate composition must be derived from the choice of planting and geographical location - providing the air, water and nutrients that are appropriate to the plants' needs. A special blend of natural inorganic and organic matter ensures the required permeability to water, whilst retaining sufficient moisture for the plants' requirements.
Substrate selection must also consider the implications for the building structure's capacity to withstand structural loads (i.e. dead loads in a saturated condition) and the potential for wind erosion and uplift (in dry substrate conditions).
A drainage layer performs two key functions:
- Retaining excess water that has infiltrated the highly-permeable growing medium, for subsequent diffusion into the substrate for consumption by plants in drought periods; and
- Maintaining an air supply to the plant roots, even when the growing medium is water-saturated.
Extensively Designed by Blackdown
At Blackdown, we have extensive experience in tailoring our plants and components into a whole range of extensive roof schemes. Our Design Support Team will be happy to work with you to deliver your project's requirements. For further details on our design service, please contact us.