Extensive green roofs
These generally require a 5-15cm (2-6in) depth of growing medium that can consist mainly of lightweight inorganic materials such as perlite, leca, sand, rockwool and crushed tiles or concrete.
The typical layers of an extensive green roof include waterproofing and insulation layers, a root barrier membrane, a drainage layer, a filter layer and then the growing medium above.
It may be necessary to incorporate drains into the system, which should be boxed in to prevent them clogging up.
For diagrams and further information, see Livingroofs.org.
Ready-grown mats of sedum grown inside geotextile pockets filled with substrate are often available from contractors who install green roofs. These are usually self-sustaining and should not require watering or weeding if the species chosen are appropriate to the climate.
Semi-extensive green roofs
These need 10-20cm (4-8in) depth of growing medium, enough to support Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.
Semi-extensive green roofs take 12-18 months to establish, during which time they will need maintenance. Hand weeding, watering and feeding are all necessary, and it may sometimes be necessary to replant areas that have failed.
Intensive green roofs
These are usually container or raised bed gardens. They need at least 30cm (1ft) depth of growing medium, much of which needs to be organic matter.
Built-in raised beds require planning and construction by suitable architects and/or contractors.
When choosing containers, consider using lightweight plastic, metal, or fibreglass pots and troughs. Make sure there are ample drainage holes in the base. Use lightweight growing media such as multipurpose Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.
Remember that the lighter the compost, the faster it will dry out and the less heavy it will be for anchoring plants. Plant supports such as wires and stakes may help prevent wind rock, and pots can be bracketed to walls or floors to prevent them blowing over.
Containerised plants need watering all year round, except in freezing or very wet conditions. Rain showers do not often penetrate pots well, so watering may still be necessary. In summer, plants may need watering twice a day, or more for small pots. It may be wise to install an irrigation system if you are unable to keep up with manual watering.
Make sure there is adequate drainage to deal with water run-off in storms, and stand pots on pot-feet in winter to allow for drainage of excess water.