Green Roofs - Living Architecture Monitor - Fall 2008 - (Page 6)

LETTERS TO THE EDITOR ORGANIC DEBATE CONTINUES DEAR EDITOR Though not explicitly stated, it seems clear that the author is advocating an organically based growing medium in green roof construction (please see “Going for Organics," by Rick Buist, Winter 2008, now available online at This of course, is a departure from the generally accepted practice in the industry. I would suggest Mr. Buist entirely fails in making his case and that the advice given is not in the best interest of the green roof industry. There are a number of lines of evidence to support this. Presently in Europe there are over 200 sq. km of green roofs. (I don’t know how many projects that represents). A great number of them are in excess of 30 years old and all of them are constructed on a mineral based fll approach to growing medium design which stresses that green roof growing media must be mineral based. These roofs are all over Europe in a wide range of climatic conditions from Scandinavia to the warm and dry climates of Greece and Spain. This combination of incredibly large areas installed in a wide range of climates over extensive periods of time represents ironclad credentials to the success of mineral based growing medium design. It’s not like the Europeans have not tried organically based growing media. One line of research conducted over a 10-year period concluded that green roofs with high organic content growing media experienced a wide range of problems. Long term decomposition of the growing media resulted in loss of depth and lack of sufficient air resulting in plant death due to root damage and the creation of anaerobic growing conditions in the growing medium. (Prof. Dr. Hans-Joachim Liesecke, Hanover University.) The strict limits on the organic content of the growing media in the fll are based on this and similar research. Mr. Buist notes that not all organics are the same and their chemical and biological nature and structure vary depending on the source material. No doubt that is correct; however, what they all have in common is that they rot. For green roofs that is the point. Organics decompose, whatever their source, and when they do they disappear, that is a problem for a green roof that is supposed to last the life of the building (generally accepted as being 50 to 75 years). The author points to the extensive use of organics in the horticultural and nursery trade. The implication is that this proves the merits of organically based growing media. However, green roofs that should last many decades are clearly different than the nursery trade with potting mixes that are intended to be used for only a few seasons. This point is made very eloquently in the classic green roof text, Roof Gardens by Theodore Osmundson. In the discussion on growing media design, Osmundson notes that potting mixes developed by the nursery trade should not be used in green roof applications. He dramatically illustrates this with two sites in Oakland, California completed in the 1960s. The site with the organically based growing media eventually failed and had to be rebuilt while the mineral based site continues to this date. Current research as reported by numerous papers at the annual grhc conferences also strongly supports the fll mineral based approach. To date all 6 LIVING ARCHITECTURE MONITOR FALL

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Fall 2008

Green Roofs - Living Architecture Monitor - Fall 2008
From the Founder: A Robust Economy
Strata - People, Products & Projects: Austin's Hotspot for Habitat & Baltimore Hilton Goes Green
A Green Roof for a County Courthouse
Research Grant for Design Tool
On the Roof With...
A New Vue on Downtown Open Space
Sound Transmission Loss of Extensive Green Roofs
On Target
Optimizing for Sustainability
First GRHC Green Roof Symposium in Florida
Welcome New Corporate Members
Professional Calendar
Experts Reflect on the Value of the GRP

Green Roofs - Living Architecture Monitor - Fall 2008