Green roof retrofit: reducing run-off during heavy rainfall

Runaway success

26 March 2015

Green roof retrofit on a large scale might offer a partial solution to reducing run-off from buildings during heavy rainfall. Sara Wilkinson, Jessica Lamond and David Proverbs discuss the latest research

Weather patterns are changing globally. Australia is getting hotter and drier, and is likely to experience more heavy rainfall patterns and increased flooding (IPCC 2013). In 2012, Queensland floods damaged buildings with remediation costs varying from AUD$9-$20bn. The UK also experienced extreme weather during the winter of 2013-14.

Local economies weaken as many businesses fail to recover after flooding, and the real cost is long lasting and difficult to measure. Could green roof retrofit reduce the impact of flash floods and be more cost-effective than upgrading below ground drainage systems? An RICS research project Modelling the reduction in stormwater run-off through green roof retrofit in the CBD examines the potential.

Unused rooftops constitute 40-50% of the impermeable areas in cities. Green roofs are a living vegetated alternative, providing environmental and social benefits including improved thermal performance, increased biodiversity, improved air quality, carbon sequestration, and space for people to enjoy. They absorb rainwater by:

  • delaying the initial time of run-off
  • reducing total run-off
  • distributing run-off through a slow release of water.

Materials comprise:

  • a roof structure
  • a waterproof membrane
  • insulation
  • a root barrier protecting the membrane (i.e. made of gravel, impervious concrete, PVC, thermoplastic polyolefin, high-density polyethylene or copper)
  • a drainage system
  • a filter cloth (non-biodegradable fabric)
  • a growing medium (soil) consisting of inorganic matter, organic material (straw, peat, wood, grass, sawdust) and air; and plants.

Extensive roofs provide space for people, the substrate layer depth varies from 50-200mm and requires artificialirrigation. Intensive roofs require deeper planting mediums exceeding 150mm. Keeping plants alive requires active maintenance and irrigation. Standard soils are too heavy for roof structures and a calculated ratio of aggregate (e.g. shale, vermiculite), organic materials, air and water is used. The correct growing medium is critical and may be challenging because of climatic conditions, particularlyexcessive seasonal or minimal rainfall (see figure 1).

Modelling reductions in run-off in Brussels through green roof retrofit showed that non green roofs experienced between 62% and 91% run-off whereas intensive green roofs experienced run-off from 15% to 35% and extensive green roofs 19% to 73%. Run-off was much lower in summer, 52% versus 80% in winter for green roofs compared to 70% versus 86% for gravel roofs. During wet winter periods, roofs become saturated and are unable to absorb much additional rainfall (Mentens et al, 2006).

Extensive green roof Intensive green roof
Shallow growing medium (<150mm) Deeper growing medium (>150mm)
Lightweight structure to support roof Heavier roof structure required to support roof
Cover large expanses of rooftop Small trees and shrubs feature
Requires minimum maintenance More maintenance required
Lower capital cost More expensive
Not usually recreational More common in tropical climates
Accessible or inaccessible Accessible or inaccessible
Does not usually require irrigation
Minimum structural implications for existing buildings

Figure 1: Attributes of extensive and intensive green roofs

What a surveyor should do

Retrofit suitability depends on roof type, size and slope. The roof structure and covering influence the type of green roof that may be selected, for example the load-bearing capacity. Roofs on mid-sized and larger commercial buildings in the UK and Australia tend to have concrete slab construction, which may be able to bear green roof loads. A typical load for an extensive green roof structure varies between 1.6kN/m2-2.4kN/m2 and for intensive green roofs 2kN/m2-15kN/m2 (Andras, 2010). Depending on the load-bearing capacity of the roof and weight of the growth media, additional structural support may be required.

The intended use of the roof and size are also issues. For example, is public/user access possible? The roof may be too small to warrant the cost. Typical costs for a 100m2-1,000m2 roofs vary, based on the specification (see figure 2). Other works may be triggered, such as upgrading access, which may render projects prohibitively expensive.

Sedum blanket only
Sedum blanket with drainage layer and filter fleece
Sedum blanket on filter fleece and drainage layers, capping layer and vapour barrier £80-£115/m²
Extra for insulation
Extra for waterproof membrane and vapour barrier
250mm thick growing medium on drainage board, root membranes and insulation; turf £85–£100/m²
225mm thick growing medium on filter fleece and LDPE drainage core; plug and hydro-seed planting
Figure 2: Indicative costs of green roof UK 2006

Green roofs require a minimum slope of 2% less than this and the roof will require additional drainage measures to avoid water logging. Additional requirements include good drainage and waterproofing. In some locations, rainwater harvesting and the use of drought or heat-tolerant plants is desirable. Planting should be specified with maintenance in mind; watering and irrigation can add to total costs.

Stormwater retention depends on the depth and absorbency of the substrate, exposure, prevailing wind conditions, and the amount of evaporation, which varies according to external temperatures and humidity (Blanc et al, 2012). The surveyor has to identify the additional loads the existing roof may safely bear and evaluate the design to ascertain how much reduction in run-off might be achieved.

Orientation affects the amount of exposure to sun, and thus the type of plants that will flourish, for instance where overshadowing cuts access to sunlight. Finally, the height above ground influences exposure levels to high winds.

The longevity of the structure, drainage and waterproofing system is essential because replacement costs are high. Green roofs can last more than 50 years (Koehler, 2008); approximately twice the life cycle of a roof covering such as bituminous felt, which may present a good economic argument. Where an existing roof covering is approaching the end of its useful life, it may be cost effective over a 50-year life cycle to retrofit. The criteria include:

  • load-bearing capacity
  • roof pitch
  • water supply for irrigation
  • preferred planting
  • orientation
  • height above ground
  • sustainability of componentsmaintenance.

A minimum 5-year maintenance contract is recommended to ensure the correct processes are undertaken and that planting is established. All green roofs have different water retention characteristics depending on their location, orientation and exposure. Stormwater roofs should have enhanced water absorption qualities.

Finally, there is the budget, including how much the owner is willing to pay. A whole life-cycle costing approach is useful to determine overall costs and may offset higher initial costs. In some cases green roofs add to capital and rental values and environmental ratings.

What are the risk areas?

The problems with green roof retrofit cover design, workmanship and maintenance and include:

  • access for installation
  • installation damage to membrane
  • overloading and structural defects
  • inadequate drainage and blockages caused by substrates leaking
  • lack of plant care causes plants to die.

Returning to correct defective workmanship is time consuming, disruptive and expensive. Overloading a roof and causing structural defects may render parts, or all, of the building unusable during remedial works. The costs of work is borne either by the owner, contractor or designer depending on the cause.

Case study: St Bartholomew's Hospital

Redevelopment of St Bartholomew’s Hospital in the City of London was part of a PFI with London Hospital NHS Trust and Skanska. Phase one was completed in 2010 with two green roofs in the FM yard and Energy Centre. Some 1,100m2 of roof area was greened with an extensive sedum blanket with a 300mm substrate depth. The build cost was £106/m2 of which £36/m2 was for the green roof. The drivers were achieving an excellent Healthcare BREEAM rating and the opportunity to re-introduce greenery. Finally, the green roof was essential for planning approval because of a high risk of surface water flooding. The barriers were accommodating both greenery and plant equipment on rooftops and one building was initially considered unviable because of this. Other restrictions were accommodating St Paul’s Cathedral view controls. Stormwater has been successfully attenuated, reducing flood risk and surface run-off in the local area.

Sara Wilkinson is an Associate Professor in Property and Construction at the University of Technology Sydney

Jessica Lamond is a Senior Research Fellow in the Centre for Floods Communities and Resilience at the University of the West of England, UK

David Proverbs is Professor and Head of the Department of Architecture and the Built Environment at the University of the West of England, UK

Further information

  • The research examined the potential for city scale retrofit and recommends that modelling can determine the efficacy of incentivising owners to retrofit for stormwater run-off. Any views or comments are welcome.
  • Related competencies include Design and Specification, Sustainability
  • This feature is taken from the RICS Building surveying journal (December 2014/January 2015)