Green roofs are vegetated soil filter where the naturally loosened root zone creates an enhanced degradation and retention of pollutants. The older the green roof the better the performance - considering the green roof was designed, constructed and maintained according the worldwide accepted FLL Guideline. Based on higher temperatures in summer the release of nutrients can be higher but will be still far below the strictest German drinking water ordinance as different studies showed in long term studies between 1999 and 2003 in Germany (W. Kolb, G. Mann, M. Köhler). However most run-off is more or less stained by humic substances that are typically caused by organic matter including releasing substances of the plants.
Since this is just a visual issue (no overall quality problem) the run-off water can be easily stored in cisterns. The water can be used for toilet flushing, irrigation, car washing and other purposes. Colored water is not recommended for doing the laundry, it might stain white fabrics. Research from Köhler (1993) and Liesecke & Borgwardt (1995) also showed that green roof have the ability to filter airborne pollutants efficiently, like lead, copper, zinc and formaldehyde.
To minimize run-off water coloration the initial organic content of the growing media can be reduced. Ideal are green roof layers with high mineral content (drainage and green roof soil). Within the FLL guideline Green Roof Technology is able to enhance the removal of pollutants and run-off coloration at the same time. Specially engineered green roof growing media will perfectly customize for our clients need and based on leading research performed in Germany over the last 15 years.
Since extensive monolithic green roofs retain as an average 50-75% the storage of the run-off (cistern size) has to be adjusted accordingly and is typical larger than without green roof.
Data from any Run-Off Quality research done in the United States or Canada can be just used as a rough speculation. We don't know any research performed with standardized components (e.g. growing media) and further more there is no uniform standard for compost or organic ingredients going into the growing media. Without uniform and standardize research all results are only speculation. Independent research (not sponsored by manufacturers) in Germany did consider these important facts and therefore the results are useful and easy to implement on real projects.
Any kind of plastics (drainage boards, chemical substances or modular systems) doesn’t support the cleaning power as much as perfectly engineered wall-to-wall green roof systems. More and more experts starting researching on how much chemicals are released by plastic components and water proofing materials since it well know that plastic components affect sea life - where all run-off ends up.
On request Green Roof Technology will translate any information about leading green roof research done in Germany over the last 25 years.