What kind of waterproofing do I need?
Charlie says: Many types of waterproofing are compatible with green roofs and optimized for different types of roof decks and substrates (i.e., steel or concrete decks). Worldwide, polyvinyl chloride (PVC) and polymer modified bituminous membranes are the most common. Many of these installations have now been in place over 30 years and continue to perform as designed.
Waterproofing membranes should be compatible with frequent or perennial ponded water conditions and be resistant to degradation by soil bacteria and fungi. Additionally, Roofmeadow recommends avoiding waterproofing materials that require supplemental root barriers; unfortunately, few membranes in use in the US have been successfully tested for root-resistance. Those that have include: Sika Sarnafil’s PVC, Fibertite, Hyload, and Decothane (now Sikalast) liquid applied membrane. Many (including us) believe that caution should be used when using TPO or PVC membranes that are fabricated using the laminated/extrusion process. These membranes typically require caulking at cut edges, and no data exists on the long-term durability of these caulked seams. We feel much more confident with spread-coated membranes (which include Sika Sarnafil’s PVC and Fibertite).
What should I consider when deciding between Conventional versus Protected Membrane Roofing (PMR) waterproofing system configurations?
Charlie says: The answer to this question varies, depending on whether it is informed by the green roof perspective or the waterproofing perspective. We advocate the green roof perspective and therefore usually prefer the waterproofing in a conventional configuration. When the insulation is located below the membrane (convention configuration), the insulation remains dry, resulting in maximum thermal performance. Additionally, moisture is kept in the root zone where it is available for evapotranspiration which cools the roof and surrounding environment. On the other hand, PRM systems place the thermal insulation above the waterproofing membrane, allowing moisture that should remain in the green roof to drain away, down through the insulation. Solutions to this moisture problem include the (a) Type V assembly or (b) installation of the root barrier membrane on top of the insulation layer.
When the waterproofing system is installed in the conventional configuration, wind uplift pressures on the insulation and green roof assembly are minimized or eliminated. Alternatively, when water-impermeable layers such as insulation are added above the membrane, they allow air pressure gradients to develop. In a PMR configuration, the green roof must be treated like ballast to prevent the insulation from lofting. While it is possible to overcome some of the design issues associated with PMR configurations, we believe a conventional configuration is the best option for most green roof assemblies.
What concerns do leaks pose?
The green roof protects the waterproofing, thereby reducing the likelihood of leaks. In the event of a breach in the membrane, the location and repair of a tear is much easier if the insulation is located beneath the membrane (conventional configuration). Electric leak detection methods, such as Electric Field Vector Mapping® (EFVM®), rapidly and accurately pinpoint even minute holes in properly designed green roofs, with minimal disturbance to the green cover. EFVM® is more reliable than the older flood testing method; usually requires no upfront capital investment, and can even be used on steeply-sloped surfaces. Repairs to waterproofing are quick, and disturbance of the green roof is minimal. The presence of a root barrier, usually required in roofs with a PMR configuration, complicates the use of EFVM® and undermines its success.