Green Roofs: what you need to know

Key points if you are considering a green roof.

This blog gives an overview of green roof systems before looking at:

  • Their benefits
  • Potential problems
  • Different green roof systems
  • The cost of installing a green roof

So, to find out whether this form of environmentally-friendly project is for you, read on!

Overview

There are a host of environmental reasons for creating a green roof project, and while many are built in urban areas, self-builders with biodiversity objectives in mind may consider including a sche me in their design. Planning permission may be required, however, some local authorities look favourably on schemes if they meet their sustainability criteria. A green roof can, of course, be constructed on a variety of existing roof structures including flat roofs, pitched roofs, roofs on commercial buildings and even garden sheds. When considering installing a green roof it is advisable to check with a specialist surveyor or chartered structural engineer to ensure that a roof is strong enough to take an increased load. A specialist architect may be needed to plan a green roof installation. The planning, installation and maintenance of all types of green roof comes under the UK Green Roof Code of Practice (GRO Code), a UK-specific document that provides a best practice guide (BSG Ecology, 2012).

Mat-forming species thriving on an innovative green roof.

The many benefits of a green roof

  • From a biodiversity aspect, intensive roof gardens provide a wildlife refuge in built-up areas, hosting foraging pollinators including many rare species of bees which are attracted especially to wildflower and sedum roofs. Extensive green roofs can also create habitats, forming networks across a city, encouraging greater diversity.
  • A living roof can improve a building’s thermal performance and environmental capabilities, creating an urban heat island effect. They offer financial benefits from savings on energy costs by creating an insulation layer (RHS, 2021), and improve storm water management by reducing the amount of run-off water. Green roofs benefit air quality in cities as vegetation reduces gaseous pollutants and filters dust particles from the air, and via photosynthesis, plants convert carbon dioxide into oxygen.
  • A green roof space helps 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, offering energy savings by reducing the need for air conditioning and providing insulation benefits in winter. Water evaporation from plants and soil also cools the air.
  • A green roof can help to blend a building into the landscape and in some cases, they may replace habitat lost in urban development (Bauder).
  • A living roof can improve water quality: it is possible to harvest rainfall from roofs.
  • Green roofing provides a waterproofing layer, helping absorb excess water.
  • Amenity space: some rooftop gardens in cities can create welcoming environments with ecological benefits. In some examples, they can support urban agriculture by creating habitats to grow food if the supporting roof structure is strong enough.
  • A living roof is ideal for both sun and wind tolerant plants especially dry habitat perennials.
  • Rooftop gardens can be used to blend buildings together for aesthetic purposes in urban environments.

Potential problems

  • A living roof is susceptible to wind damage and may need wind filtering screens.
  • Access is required to build and maintain a green roof.
  • 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 via a drainage layer; excess weight from water risks overloading a roof or killing plants.
  • Schemes can attract criticism for being scruffy; areas of unkempt grasses can be regarded as unsightly.
A living roof: grasses growing on a green roof system.

The different systems:

Extensive schemes: These are low maintenance and the benefits include water retention, insulation, biodiversity and low costs (Living Roofs.org, 2019). A 2–6-inch lightweight growing medium is needed along with light materials, waterproofing and insulation layers, a root barrier membrane, a drainage layer and a filter layer. Drains may also be needed to provide adequate drainage. Examples could include ready grown mats of sedum blanket inside geotextile pockets which are self-sustaining and do not require watering or weeding. Other types of suitable plants include mat forming species, moss, grasses, ferns such as Polypodium vulgare and common sedum such as sedum acre.

Semi-extensive green roofs: This category is suitable for garden green roofs and requires waterproofing, a root barrier, drainage and filter layers. Semi-extensive roofs are suitable for a wider variety of plants; they provide water retention, environmental benefits, biodiversity opportunities, insulation and amenity value. A 4–8-inch minimum depth of growing medium is needed to support perennial plants. Semi-extensive roofs are ideal on a sloping roof using a grid of cells containing compost. This model has low-maintenance requirements, needing just periodic tidying and planting, and can take up to two years to establish. Ornamental grasses are ideal, along with rudbeckia, sedum blankets, achillea, dianthus, herbs, bulbs such as muscari and small allium species.

Intensive roof gardens: 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 dry habit perennials, shrubs, plants suited to dry shady conditions, containers or raised beds and need 1 foot depth of growing medium. Compost can be lightened by adding perlite, but this can dry out, and containers need watering all year round. Drought-tolerant plants are well-suited to intensive green roofs such as ceanothus, Buxus sempervirens, ornamental grasses and hebe.

Blue/green roofs: These combine green roof and blue roof technology to maximise water storage and control rainwater run off.

Bio-solar green roofs: Solar panels can be combined with green roof schemes to provide renewable energy and biodiversity.

How much does a green roof cost?

This depends on the roof area, accessibility, roofing material chosen to grow plants in and the types of plants chosen. An intensive roof garden usually costs more than an extensive green roof due to the requirement for deeper soil. Costs rise if a landscape gardener is employed to design a scheme, and a structural engineer may be needed if a roof requires strengthening. The quality and quantity of the plants selected is another variable, and ongoing maintenance costs and plant growth should also be factored in.

As a guide, the cost of installing green roofs can be around £100 per metre square for an extensive green roof, and £150 per metre square for an intensive green roof. There are currently no grants available towards the cost of installations, however, there could be potential savings on fuel bills from the good insulation provided by the roof garden which will prevent heat loss. A rooftop garden carried out as a DIY project on an eight metre square site could cost between £500 and £800; employing an installer could increase the cost to around £1,500. Costs for commercial green roofs will be higher as a specialist designer and installer will be required and much depends on the site and accessibility.

Anyone planning a project should check with:

  • the council, to assess their green roof policy
  • their mortgage provider, who may have concerns about whether a green roof will affect the home’s value
  • their insurer, who may be concerned about weight being added to the house’s structure.

What do you think about green roof schemes?

If you have installed a green roof or have a view on how roof gardens can be used to boost nature in our urban environments, we would love to hear about it.

References

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)

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