According to a new study, green roofs could actually cause damage to the environment by degrading water quality.

Green roofs have long been extolled as one of the most handy and convenient of sustainable building measures, making use of exposed surfaces which would otherwise sit idle and having them capture airborne pollutants in vegetation and soil while also providing much needed patches of natural scenery to urban environments.

A new study by  a team of scientists in the UK, however, claims green roofs may simply be passing the pollution buck from one environmental sphere to another.

Research conducted by scientists from the University of Manchester and the University of Leicester suggests that rainwater runoff from green roofs is taking the pollutants seized by urban vegetation from the atmosphere and transferring them to the surrounding environment.

The study, the results of which were published in the journal Environmental Pollution, examined differences in the performance of green roofs and conventional roofing.

According to Dr. Andrew Speak, lead author of the study from the School of Environment and Development at the University of Manchester, the rainwater runoff from the green roof turned out to be green and yellow in appearance when collected.

yellow Green roof An analysis of rain water samples from the green roof found that they contained a high concentration of heavy metals, some of which were in excess of environmental quality standards. These metals include copper, lead and zinc.

An analysis of samples of soil from the green roof found that they too contained high concentrations of lead.

Researchers believe these concentrations accumulated in the years prior to the gradual removal of leaded petrol from the market, given that the roofs were built in 1970. They noted that modern green roofs would not contain such large amounts of the toxic metal.

The paper nonetheless points to green roofs being a quid pro quo for the environment as a whole, with a reduction in air pollution achieved at the expense of the quality of water runoff.

The team has offered a number of recommendations for preventing green roofs from compromising water quality in urban settings.

These include refraining from installing the fixtures in locations where vehicular pollution is particularly intense, such as congested motorways, as well as the mixing of biochar - a type of charcoal - into the substrates of green roofs, in order to reduce the seepage of chemicals from the soil.

Speak also pointed to the potential development of new breed of "super plants," which will possess an enhanced propensity for pollution removal and retention.

  • The main feature of the study is that the roof is 43 years old so was subjected to high inputs of lead and other metals from before the phase out of leaded petrol. Therefore applying the results to modern green roofs subjected to different air pollution impacts should be done with caution.

  • When I first read the headline, I screamed “Oh no, this can’t be true” but after reading the whole article, it made sense to me. I’m surprised so-called green roofs haven’t been studied earlier as they are all the rage among advocates of ‘green buildings’.

  • Green Roofs are natural environments located on top of human made structures that took away a piece of nature in the first place.
    All natural and living systems (including human bodies) take-up pollutants, ideally transform them into less harmful natural elements, partially accumulated them or release them back into nature. With the end of the lifespan of any living organism or when oversaturated with pollutants, they will be released eventually. Some natural elements (sun, wind, rain) might be able to break down pollutants but most likely transform them into other – sometimes more harmful – elements.
    To reduce the release of pollutants it is crucial that the source of pollution is treated or as a temporary solution treatment for the polluted natural bodies. For human there is health care when affected by pollutants. Since all green roofs are connected to a sewer system, the run-off from green roofs is treated in a wastewater facility.

  • This is a great example of “Green Bashing”, allowing sceptics of Green Roofs to join the band wagon.

  • There is only one thing a contractor needs to aware of: hire a competent installer. My mother-in-law purchased a home with a green roof, as green roofs are mandatory in Toronto. The trouble is that the installer hired was incompetent and the roof is leaking after about three years. That mean that the new home warranty period is over and she has no recourse but to do the work at her own cost.

    Therefore, insist that the subcontractor is able to provide you with letters of reference that can be checked prior to awarding the contract.

    Thus green roofs are a valid means of controlling rainwater, but the applicator must be certified by the vapour barrier manufacturer. Be careful whom you hire to install one, as these have three main benefits.

    One, they control rainwater runoff, like storm water pounds. Two, they reduce heating and cooling requirements as the soil acts as an insulator. Three, they extend the life cycle of the vapour barrier, as they absorbs the UV to convert chlorophyll to oxygen.

  • At the risk of piling on a bit, and perhaps I missed something, but a couple of perhaps obvious questions and comments…

    1. Who funded the study?

    2. Why was a 43 year old green roof tested? Even in the UK I would think that would be a statistical outlier. Were there newer roofs tested? Is the 43 year old roof the entire study subject or one data set?

    3. A link to the study maybe?