MythBusters

MYTH 2:  The most common reason for green roof failures is inadequate moisture

Without doubt, when green roofs fail to thrive, the most common culprit is too much moisture. 

Proper drainage is one of the most important considerations in the design of Roofmeadow® green roof assemblies.  Even irrigated green roofs must include provisions for rapid drainage of excess water during heavy rainfalls.  Chronic high moisture content in the growth media layer will lead to the rapid decline of most green roofs (exceptions include wetland green roof landscapes).   It is not uncommon for green roof designers to focus on moisture capture and retention to the detriment of maintaining good drainage and aerobic conditions in the media. 

Common mistakes made in green roof design include: 

• Installing peg-type drain panels or sheets in an ‘inverted’ configuration.     When a peg-type drainage panel is installed in the inverted (“cups up”) position, water percolates less easily to the drainage zone of the panel than if the panel had been inserted with the cups facing down.  Inverted, “cups up,” installations actually compromise the drainage function of the panel.  When the goal is to capture water and the drain panels are installed with the cups facing up, then a layer of coarse-grained drainage material should be installed to cover the panel prior to installing the growth media to restore drainage properties to the profile. 
• Incorporating drainage layers with inadequate water carrying capacity.    Standard procedures for predicting the drainage capacity of both synthetic and granular drainage layers have been published by ASTM.   

For more information on moisture management consult our FAQs