Benefits of roof greening

Environmental Benefits

The benefits include:

1. Storm Water Management

reduce storm water run-off by 75% by retaining water and in pore spaces in the substrate, absorbent materials in the substrate or by the plants
slow down the draining rate to prevent localized flooding during rainy season
absorb storm water before it drains through the streets and picks up pollutants, thus reducing contamination
reduce downstream flood risk
reduce cost of drainage schemes because smaller pipes are possible

2. Lessening the urban heat island effect

warming up with the difference of 0.4°C per decade between temperatures in urban and rural areas can be attributed to the effects of urbanization
dissipate some of the city heat through the process of evapo-transpiration

3. Improve air quality

absorb carbon dioxide for photosynthesis, thus reduce carbon content in the air
lower disease rates such as asthma
absorb pollutants such as heavy metals may help to improve air quality
trap up to 95% of cadmium, copper and lead and 16% of zinc, removing 0.2 kg of airborne particles per year per square meter of green roof
intensive green roofs (large three-dimensional surface leaf area) will have a much greater effect than extensive green roofs (flat, ‘two-dimensional’ vegetation profile)

4. Sound Absorption

10cm thick green roof at Frankfurt Airport, Germany reduced sound transmission into buildings by a minimum of 5 decibels
general reduction in ambient environmental noise levels

Economics Benefits

1. Reduced energy use

Improve thermal performance of the roof by retaining more heat in the winter and reflecting solar radiation in the summer, thus reduce energy needed for cooling and heating
26% reduction in summer cooling and a 26% reduction in winter heating

2. Increase roof lifespan

Green roofs can protect the waterproofing membrane from UV radiation and physical damage
Reduce periodic maintenance cost of roofing material
Extend the lifespan of a roof by over 200%

Amenity & Aesthetic Benefits

important in a dense urban environment such as Hong Kong
both intensive and extensive green roofs offer visual aesthetic benefits to the city, however intensive systems have potentially greater visual benefit than extensive ones: large trees and shrubs offer a three-dimensional greening effect in intensive systems, whereas only a thin two-dimensional ‘skin’ of green which may not be visible unless viewed from above is provided for extensive system

See the original paper

Social Benefits

There are many activities that can be done on an accessible roof, it will be much more attractive to people to visit the roof if it is greened and planted like a garden. Researches showed that roof greening can improve the relationship of the building occupants by providing a place for them to socialize. However, for extensive green roofs when compared to the intensive green roof, there is not much social benefits as basically these extensive green roofs are inaccessible to public and do not have much asthetics effects.

We visited the rooftop farm on Runme Shaw Building in the University of Hong Kong. Let's have a look on their garden and feel the benefits brought about by the farm!