Waterproofing a green roof

In my last post, I identified factors that should be considered when installing a green roof (i.e. desired function, type of roof, whether structural modifications must be made to accommodate extra weight, composition of the roof system, proper substrate formula, appropriate mix of vegetative species, and desired time to maturity/planting method).  In this post, I will begin to investigate the “guts” of the roof system (i.e. roof membranes, root barriers, insulation, drainage mats, filter fabric, and water retention pads) and identify local (ideally, within Brooklyn) suppliers of these elements.

As I mentioned in my last post, before addressing the composition of the system and locating suppliers it is crucial to understand the type of roof to be installed, and the functions that are desired from it.  In addition, it is important to determine whether structural modifications will be necessary to safely install the roof.  For the current project, we have assessed the structure and determined that it is sufficiently strong to withstand the added load of the new roof.  We therefore plan to install an extensive roof (with some intensive portions) that will insulate the Eco Brooklyn show house and buffer stormwater by retaining precipitation.  In addition, the roof will be accessible from the top floor of the house.  Since visitors are expected, it is important to maintain a reasonably attractive space and to incorporate an area that will accommodate foot traffic.  Lastly, since the home is directly connected to neighboring buildings, it is important that the roof does not retain excessive amounts of dry biomass that could become a fire hazard.

Though structure of a green roof can differ slightly depending on type of roof, climate, materials used, etc., the basic elements of an extensive green roof system include interior structural elements, waterproof roof membrane, root barrier, insulation, drainage layer, filter fabric, water retention fabric, growing medium, and, lastly, vegetation.  See the Study on Green Roof Application in Hong Kong for an excellent analysis of green roof elements, including waterproofing membranes, root barriers, drainage layers, and filter fabrics.  In this post, I will address the most crucial element of any roof: the waterproofing membrane.  I will focus on the remainder of the roof’s structure in later entries.

It is absolutely critical that the roof membrane function as designed, namely to prevent water from entering the structure and compromising its integrity.  In short, if the roof membrane fails, it doesn’t matter whether the remainder of the green roof works — you will probably have to tear it up to fix the membrane, anyway.

As part of its analysis of green roof components, the Study on Green Roof Application in Hong Kong assesses the fitness of five different types of waterproofing material for green roof applications.  Based on the report, several membrane types would be suitable for an extensive roof retrofit: (1) asphalt roofing felt/bituminized fabrics, (2) SBS modified bituminous membrane sheets set in SEBS polymer modified bitumen and coal tar pitch/polyester built-up systems, (3) fluid applied membranes, and (4) single-ply roof membranes. The National Roofing Contractors Association’s recommendations largely coincide with those of the Hong Kong researchers.  NRCA’s suggestions include: (1) hot-fluid-applied polymer-modified asphalt membrane, (2) APP- and SBS-polymer-modified bitumen sheet membrane, (3) EPDM membrane, (4) PVC membrane, and (5) one- and two-component, fluid-applied elastomeric membrane.

Further analysis of the membrane options suggested above, however, reveals that organic waterproofing treatments (i.e., those that incorporate asphalt/bitumen) are more highly susceptible to root penetration and decomposition than those made from inorganic materials.  According to the Study on Green Roof Application in Hong Kong, it is unclear whether fluid applied membranes are root-resistant.  Because they are made from inorganic materials, single-ply membranes are inherently resistant to root damage.  Different sources present conflicting views on whether root barriers are necessary when using single-ply systems, though the EPDM Roofing Association recommends installation of an independent root barrier layer even when using an EPDM membrane for waterproofing.

The Study on Green Roof Application in Hong Kong notes that single-ply membranes “have a long proven track record in the green roof industry.”  The major hurdles to successfully utilizing these products are associated with installation (e.g., ensuring that seams are properly treated, and that surfaces are thoroughly cleaned and dried prior to application), not inherent to the products themselves.

For argument’s sake, let’s assume that installation is not a concern.  In this case, I believe that the factors weighing for single-ply membranes — endorsement by the NRCA and a thoughtfully conducted study on green roofs, a long and proven history, and an ability to prevent root penetration and withstand rot — are sufficient for us to select this type of membrane.  Now that we have made this decision, there are several types of single-ply membrane for us to choose from, including TPOs, PVCs, PIBs, and EPDMs.

The ASTM has established standards for TPO roofing.  Relevant qualities for our purposes include the following: TPOs contain no chlorine, are recyclable, come in widths of up to twelve feet and can be attached to the roof mechanically (i.e.with plates and fasteners that penetrate the material and anchor in the decking) or using an adhesive.  Individual sheets are overlaid and fused using hot air to render the membrane waterproof.  TPOs, however, have only been used in roofing for approximately 10 years and are not inherently flame resistant.

PVC roofing possesses benefits relevant to our project as well, as it is inherently fire resistant and is especially good at withstanding ponded water.  PVC, however, has had varying degrees of success in maintaining its integrity after installation; in testing the effectiveness of PVC roofs, for example, the U.S. Army found that it was susceptible to shattering and splitting.  In addition, PVCs do not stand up well to foot traffic in weather less than 50 degrees Fahrenheit and release bioaccumulative toxins, including dioxins.

PIBs were used as roofing and waterproofing materials as early as the late 1920s but became more common throughout Europe and the U.S. in the late 1970s and 1980s.  PIBs perform well under thermal stress and will resist UV degradation but are not intended for situations in which water may pond.

To be sure, then, TPOs, PVCs, and PIBs present benefits that make them good alternatives for waterproofing a roof.  EPDM, however, appears to present the best option for our roof.  In an examination of roof longevity EPDM roofs were identified as outperforming both PVC and PIB membranes.  Another study of the durability of roofing materials identified EPDM as the single most durable roofing material tested.  A third favorable comparison between EPDM and other single-ply membranes can be found here, in the Manual of Low Slope Roof Systems.  In addition, EPDM is recyclable, durable, and will withstand ponded water (it’s used in pond liners, after all).  Fire-retardant EPDM is also available.  Based on this review of information found online, then, Gennaro’s suggestion that we use EPDM roofing seems like a good one.

Now that we have decided that EPDM is the best type of waterproofing material for our job, we need to find a local supplier that can provide the material at a reasonable cost.  The NRCA has a good resource for this purpose — they have assembled a helpful online buyer’s guide that provides contact information for suppliers of a variety of roofing materials.  Otherwise, I’ve found that Google searching “building supply Brooklyn, NY” has been an effective method for identifying potential suppliers.

I called around to 17 different building supply companies in the Brooklyn area and found 7 that carry 60 mil EPDM roofing.  Of these suppliers, the cheapest quote was provided by S & J Sheet Metal Supply ($0.45/sq. ft., or $225 per 10’x50′ roll), followed by Marjam Supply Company ($0.52/sq. ft., or $260 per 10’x50′ roll), and Allied Building Products ($0.53/sq. ft., or $266 for a 10’x50′ roll).

So, now that I have identified an optimal waterproofing membrane and Brooklyn’s lowest-cost supplier, I will shift focus for my next post to the protective layer and root membrane.  Stay tuned.

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