Yesterday, right after finishing my last post about waterproofing membranes for green roofs, I got an interesting e-mail from a friend that I went to graduate school with.  She is professionally involved in the green building industry and I contacted her for information on waterproofing membranes for green roofs when I first began investigating options for the show house a couple of weeks ago.  In her latest e-mail, she pointed me towards the GreenPoint guidelines for multifamily residential homes, which include a section on vegetated roofs (see .pdf pp. 122-125).  In their analysis of green roofing options, GreenPoint notes that “Roots can penetrate asphaltic, bitumen or EPDM rubber roofing, so projects using these membrane types must also use a root barrier. In contrast, most single-ply membranes are root resistant.”  The authors continue: “Avoid using EPDM for green roofs because the adhesives used for sealing seams tend to degrade from the constant presence of moisture, shortening the membrane’s lifespan.”

This analysis directly contradicts other evidence I’ve seen.  For example, these sites:

http://www.sustainindy.org/assets/uploads/4_01_GreenRoof.pdf – “PVC, EPDM, and thermal polyolefin (TPO) are inherently root resistant.”

http://translate.google.com/translate?hl=en&sl=no&u=http://www.toppbygg.no/miljovennlige.htm&ei=yAQwSsvdIZCmM9_xjIYK&sa=X&oi=translate&resnum=6&ct=result&prev=/search%3Fq%3DFirestone%2BRubberGard%2BEPDM%2BFLL%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official – A Norwegian site that states that Firestone RubberGard EPDM roofing has been FLL-certified as root-resistant and “an ideal system for Sedum roof.”

http://www.usgbccc.org/documents/StormWaterManagement.pdf – “Other suitable materials for waterproofing and root protection include rubber membrane (EPDM) or hypolan (CSPR). The disadvantage with EPDM is that seams need to be bonded with adhesives or tape (glued together), which might present a higher potential risk for leaks.”

Firestone Building Products apparently believes that RubberGard is fit to withstand green roofing applications, given that they both use it in their own green roof system and extend their warranty to customers who install a green roof on top of the EPDM.  After seeing this, I called Roofing Specialties, the closest authorized Firestone Building Products sales rep. to New York City, and asked whether Firestone RubberGard 60 mil EPDM membranes are FLL-certified for root resistance.  The woman that I spoke with was initially confused by my question but called back promptly and reassured me that the product is, in fact, FLL-certified.

The sense that I get is that EPDM is a very good, proven roofing material.  It is durable and weathers well.  It holds water well and can even be used as a pond liner.  It is recyclable.  It doesn’t leach toxins like PVC.  It is made from inorganic material, which means that it cannot be broken down by bacteria.  The downside to EPDM is that it comes in sheets that must be bonded together by the individuals installing the roof.  These seams, if not sealed well, can be penetrated by roots (and rhizomes), which could result in problems with leaks.  One of the keys to avoiding this type of situation is to test the membrane by flooding it after installation.  Periodic inspection is also crucial, to ensure that water is draining properly and not ponding on the roof.  Flashings and exposed seams should be checked on regularly as well.  And, last, one can opt to install an independent root barrier.

The other important point to take from this situation is that although the American green roof industry is growing very quickly, there seems to be confusion surrounding appropriate construction materials and design.  Although some work has been done, American organizations have not developed comprehensive guidelines like those produced by the FLL that govern German green roof design and construction.  In the end, builders should understand the crucial functions of a green roof, do as much research as possible, and be cautious and conservative until more is known about components’ performance.