Not All Green Roofs Are Equal, Here's Why
Despite what you may think, not all green roofs are good for the environment...
The benefits of Green roofs are many, but their design and location can reduce this dramatically.
Firstly, looking at the public benefits of locating green roofs in our cities, there is the potential to reduce
the effects of dust and smog in polluted air.
The right types of plants can filter particles of dust and absorb the nitrates and other harmful materials that
come from rainfall.
Green areas and parks can absorb up to 80% of energy coming from the immediate environment through
transpiration, and landscaped roof areas will have a similar result. These roofs can also act as a heat shield
reducing indoor temperatures with vegetation. The benefit here is to reduce demand for air-conditioning and heating
in these buildings.
Another feature of modern city landscapes is the large areas of sealed paths, concrete, steel and glass. These
sterile spaces are lost to biological populations of animals and plants, and any step towards reversing this should
be encouraged. As bees and butterflies return, so do birds and other wildlife that feed on them.
The opportunity to create a new ecosystem on a small scale can have a positive result, but only if proper
planting of appropriate species for the climate is done.
Green roofs without a good subsoil and structure can dry out in summer and turn rapidly to deserts, adding to
Also, without collection and storage of rainwater for irrigation, the green benefits of roof gardens can easily
be negated by the large demand for water and energy to be pumped up to the high points on a building. This is one
imporatant factor that you need to consider if you are thinking of landscaping for a green home or intending to take up such a
green home project.
Storm water retention can reduce the immediate effects of high rainfall episodes by reducing runoff by up to 50
per cent. This depends again on how well planted and structured your green roof is. In worst case situations,
gravel and soil washing into drains and sewer systems can actually cause more immediate flooding by blocking these
Make sure that storage tanks, filters and trenching to facilitate more water retention is part of the system.
The idea is to ensure most of the water is trapped in the substrate to be released by evaporation and transpiration
over a longer period of time.
For public buildings there is an added benefit that tax incentives apply in some cities if your system meets
appropriate levels of storm water management.
For the individual, aside from the environmental benefits of a greener outlook and cleaner city, a green roof
for your home can save you money on energy demand reduction.
However, older homes that have been planted out if the construction allowed the extra load, may not meet today's
energy rating levels. Thermal insulation may in fact be more effective unless your green roof is combined with a
ceiling extraction fan to keep the roof cavity dry.
Certainly you will benefit from a cooler home and reduced noise levels from surrounding industrial sources.
Another benefit of a well-constructed roof is that it will require less maintenance cost over time, and last longer
than the average 15-25 years of a 'naked roof'.
But installation on this critical load bearing part of your home must be done with all engineering precautions
taken, and quality materials used. Skimp on material costs and load a roof incorrectly, and you will find major
structural problems developing in a very short time.
However if you proceed cautiously, a green roof can save you energy and make your home both more appealing to
look at and to live in.