Sustainable buildings: benefits and practical issues of green roofs and walls
26 September 2016
Sara Wilkinson introduces RICS guidance that will help to familiarise members with the benefits and practical issues of green roofs and walls
The first edition of the RICS Green Roofs and Walls guidance note is a timely publication that places chartered surveyors in a strong position to provide clients with professional advice. A multidisciplinary international team comprising building surveyors, a valuation surveyor, a property management surveyor, a green roof installer and an engineer prepared the guidance.
It looks at green walls and roofs from the surveyor’s perspective, encompassing technical factors, value and community impacts. It is aimed at commercial and medium- to high-density residential buildings, and provides information for:
- property managers
- builders and developers
- project managers
- facility managers
- services consultants
- local and national government.
Green roofs and walls are coming
Many cities around the world are adopting policies to encourage or mandate green roofs and green walls. Green infrastructure is an essential component of liveable, healthy cities. Roofs represent up to 32% of horizontal surfaces in urban areas, and a 2009 study found the Melbourne Central Business District (CBD) could retrofit around 17% of these. A further study this year estimated that more than 16ha of CBD wall space has potential for greening.
Figure 1: Rooftop garden at MacQuarrie Bank and Qantas, Sydney
Benefits of such green infrastructure include:
- improved air quality – plants emit oxygen and absorb carbon
- large-scale reduction of the urban heat island and negative human health impacts
- attenuation of stormwater flows
- improved thermal performance, reduction in carbon emissions and heat loss to upper floors of buildings, and walls where provided
- lower energy costs
- extended lifecycle for roof coverings
- increased biodiversity habitats enabling, for instance, more pollination of plants by insects
- potential recreation space for workers and residents
- potential spaces for urban food production
- potential spaces for start-up businesses such as rooftop cafés, cinemas or farms
- lower energy consumption by provision of shade
- improved, water-sensitive urban design
- reduced noise pollution
- improved health and wellbeing for occupants
- aesthetic appeal
- enabling water recycling.
Types of green roof
Green roofs can be extensive, intensive, or vegetated rooftops or rooftop gardens. Extensive roofs have substrate depths of less than 200mm, require minimal or no irrigation and are generally planted with low-growing succulents and stress-tolerant herbaceous species. Intensive green roofs, on the other hand, have greater variation in substrate depth, although typically they are deeper than 200mm.
Many cities around the world are adopting policies to encourage or mandate green roofs and green walls
Rooftop gardens are characteristically small-containerised garden beds (see figure 1) using varying depths of substrate and higher organic components than extensive or intensive rooftops, and also feature recreational spaces. This enables them to sustain a wider variety of plant species, including fruit and vegetables, and provide amenity and recreation space for occupants. They offer the environmental and economic benefits of intensive green roofs without being physically incorporated into the structure, and costs fluctuate according to site access and use of cranes or goods lifts.
Types of green wall
Green walls refer to vegetation grown directly onto a facade, or to vertical vegetation grown on a separate structural system that can either be freestanding and adjacent or attached to the wall. (see figure 2) Vegetation may be grown in planter boxes and trained on a trellis system with mechanised watering.
Generally, there are two types of green wall: soil-less and modular. Soil-less green walls occur when vertical gardens grow on the surface of built structures. They mimic the growing conditions found where green walls occur in nature and are sometimes called living walls, green facades, bio-walls or vertical vegetation. Modular green walls use pockets of plants and climbing plants in prefabricated sections; the cost is generally about half that of a soil-less green wall.
Figure 2: Green wall at Central Park, Sydney
The guidance note offers the world’s first template for owners to adopt when licensing rooftops for commercial uses. Rooftops can be income-generating spaces for owners, and the note also proposes an approach to valuation.
A barrier to third-party use of rooftops has been the need for owners to access them for maintenance or other purposes, and here the guidance note proposes a licence, which does not confer exclusive possession of the roof. The owner and their representatives can access the roof whenever needed.
In the licence, commencement and termination dates are agreed, as is an ongoing schedule of condition and a commitment to make good at termination. These measures reduce owners’ concerns and provide surveyors with a comprehensive checklist to follow. However, all parties should check the relevant laws in their territory to ensure such a licence does not inadvertently create a lease.
One section covers assessment of buildings’ potential for retrofit with a useful checklist. It considers access, maintenance, structural, drainage and membrane issues, with a further section on maintenance and property management issues for surveyors. Using the Australian Green Star scheme as an example, the guidance note shows how green roofs and walls can increase sustainability ratings.
With this guidance note, surveyors can be confident that they have evaluated all issues necessary to provide comprehensive professional advice to clients, and in this way we can enable our cities to become more liveable.
Sara Wilkinson FRICS is Associate Professor at the School of the Built Environment at the University of Technology Sydney
- Images © MacQuarrie and Qantas; Frasers
- Green Roofs and Walls
- S. J. Wilkinson & T. Dixon, Green Roof Retrofit: building urban resilience (forthcoming)
- T. Croeser, ‘Biological potential in cities: An estimate from Melbourne’, Urban Forestry & Urban Greening, 16 (2016): 84–98
- Related competencies include Sustainability, Property management, Leasing/letting
- This feature is taken from the RICS