Green roofs: key considerations
There are a host of environmental reasons for creating a green roof, and while many are built in urban areas, self-builders with biodiversity objectives in mind may consider including a scheme in their design. Planning permission may be required, however, some local authorities look favourably on schemes if they meet their sustainability criteria. Green roofs can, of course, be constructed on existing roofs and it is advisable to check with a surveyor or structural engineer to ensure that a roof is strong enough to take an increased load.
A specialist architect may be needed to plan the roof. The planning, installation and maintenance of all types of green roofs come under the UK Green Roof Code of Practice (GRO Code) a UK-specific document that provides a best practice guide (BSG Ecology, 2012).
The benefits of green roofs
- From a biodiversity aspect, green roofs provide a wildlife refuge in urban areas, hosting foraging pollinators including many species of bees which are attracted especially to sedum and wildflowers. They also create a network of habitats across a city, encouraging nature to flourish.
- Green roofs can improve a building’s environmental capabilities: they will save energy by creating an insulation layer (RHS, 2021), improve storm water management by reducing the amount of run-off water, benefit air quality in urban areas as vegetation reduces gaseous pollutants and filters dust particles from the air, and via photosynthesis, plants convert carbon dioxide into oxygen.
- They help mitigate sound pollution and reduce the risk of fire spread.
- Large building surfaces reflect and radiate solar energy and green roofs can absorb some of this heat, helping cool cities while offering thermal benefits, reducing the need for air conditioning and providing insulation benefits in winter. Water evaporation from plants and soil also cools the air.
- Green roofs help blend a building into the landscape and in some cases, they may replace habitat lost in urban development (Bauder).
- They improve water quality: it is possible to harvest rainfall from roofs.
- Amenity space: some roof gardens in cities can create welcoming environments. In some examples, they can support urban agriculture by growing food if the supporting structure is strong enough.
- Green roofs are susceptible to wind damage and may need filtering screens.
- Access is required to build and maintain green roofs.
- Whilst weight is obviously a consideration, structures which are too light might not provide sufficient depth of growing material to enable plants to survive during drought.
- Any scheme must allow surplus rainfall to drain away; excess weight from water risks overloading a roof or killing plants.
Types of green roof
Extensive: These are low maintenance and the benefits include water retention, insulation, biodiversity and low costs (Livingroofs.org, 2019). A 2–6-inch lightweight growing medium is needed, light materials, waterproofing and insulation layers, a root barrier membrane, drainage layer and filter layer. Drains may also be needed. Examples could include ready grown mats of sedum inside geotextile pockets which are self-sustaining and do not require watering or weeding. Other suitable plants include moss, grasses, ferns such as Polypodium vulgare and common sedum such as sedum acre.
Semi-extensive: This category is suitable for garden green roofs and requires waterproofing, a root barrier, drainage and filter layers. Such roofs provide water retention, biodiversity opportunities, insulation and amenity value. A 4–8-inch depth of growing material is needed to support perennial plants. Such roofs are ideal on a sloping roof using a grid of cells containing compost. This model requires periodic maintenance and can take up to two years to establish. Ornamental grasses are ideal, along with rudbeckia, achillea, dianthus, herbs, bulbs such as muscari and small allium species.
Intensive: Parks, gardens and urban buildings are ideal for intensive green roofs which have high maintenance costs. The benefits include water retention, biodiversity, insulation and amenity value (Living Roofs, 2021). They are ideal for perennials, shrubs, containers or raised beds and need 1 foot depth of growing material. Compost can be lightened by adding perlite, but this can dry out, and containers need watering all year. Drought-tolerant plants are well-suited such as ceanothus, Buxus sempervirens and hebe.
Blue/green roofs: These combine green roof and blue roof technology to maximise water storage and control its release.
Bio-solar green roofs: Solar energy and green roof schemes combine to provide renewable energy and biodiversity.
RHS Gardening. Green roofs. [Online]. Available here. (Accessed September 27th 2021)
Green Roof Organisation. What are green roofs? [Online]. Available here. (Accessed September 28th 2021)
Living Roofs. 2021. Types of green roof. [Online]. Available here. (Accessed 24th September 2021)
BSG Ecology. 2012. The GRO-green roof code. [Online]. Available here. (Accessed 24th September 2021)
Bauder.co.uk. The benefits of green roofs. [Online]. Available here. (Accessed 27th September 2021)