SPRI and NRCA continue to disagree
I would like to address each concern Graham voices in the article.
Growth media size: There is no gradation of growth media. There are no requirements for gradation in the RP-14 standard as has been shown by data compiled in the January 2010 study "Wind Uplift of Pre-Vegetated Mats," which was presented to NRCA, as well as by the data NRCA generated. NRCA's small-scale testing demonstrated that with short bursts of 140-mph wind and two-hour tests at 110-mph wind, little or no growth media dislodged from the system in question. The studies show when growth media is fully covered with vegetation, the growth media remains in place.
The wind tunnel and wind impingement tests may not show any correlation to the wind-uplift resistance called for in building codes, but they demonstrate vegetative roof systems perform as expected in resisting the removal of growth media.
Typical maximum size for growth media is 3/8 of an inch. Most growth media is expanded shale or expanded clay, and the typical weight of the material ranges from less than 40 pounds per cubic foot to about 70 pounds per cubic foot. Most material is relatively fragile. A good comparison can be found between growth media and hail stones. Growth media is smaller than a 1/2-inch hailstone and often lighter. There are few products used on buildings' exteriors that are vulnerable to 1/2-inch hail. This is not meant to imply there are no concerns about growth media blowing off roofs; this is only offered to put Graham's concern in perspective.
Systems are untested: Graham's claim that performance has not been demonstrated clearly does not mesh with Graham's other article in the same issue, "Testing vegetative roof systems," page 28. That article states fully vegetated roof systems withstood 140-mph winds. In addition, tests in Europe, explained in the study "Calculations of Pressure Equilibrium Underneath Loose Laid Flow Permeable Roof Insulation Boards," demonstrated soil does not move when protected by vegetation.
There are hundreds of vegetative roof systems in Chicago and elsewhere in the U.S. and thousands in Europe. Despite all the designs that may or may not conform to RP-14's requirements, there are no reports of ongoing or serious wind-related problems.
Nominal coverage: A standard that relies on the coverage of vegetation requires a definition of what coverage is and a measurement of the size of the area that can remain free of vegetation. This resulted in a definition of nominal coverage that provides a readily measured space. Directly exposing growth media to the impingement of wind results in scour (as would be expected).
The standard's commentary provides information about commercially available products and existing methods that can be used to provide wind stability when vegetation is less than the defined nominal coverage.
The wind test results cited clearly depend on specific parameters of the test procedure. The data available is used as a guide to how products will perform in a wide variety of conditions.
All ballast requirements for higher winds largely depend on having vulnerable materials well inboard from the wind's effects at a roof's edge or having adequately large parapets. In all cases when a design wind speed of 140 mph is used, the standard does not allow for the use of loose stone, growth media or modular roof trays in corner or perimeter areas. This would require a setback equivalent to 40 percent of the roof's height. RP-14 has been adapted from RP-4, which made it possible to run wind tunnel tests to determine the positive effects of higher parapets and uplift-resistant perimeters. These wind-uplift-resistant qualities do not change when the media of a roof's interior are changed.
SPRI and Green Roofs for Healthy Cities have made a diligent effort with input from contractors, suppliers, wind experts and others to provide a standard that reflects state-of-the-art roofing practices and presents conservative requirements for a wind-resistant vegetative roof system. If this is not adequate, what resource can contractors or designers possibly have?
NRCA'S Technical Operations Committee responds: The comments Ennis provides do little to alleviate NRCA's concerns with ANSI/SPRI RP-14.
The January 2010 study Ennis cites does not address growth media size and gradation as he implies in his letter. Instead, the study tests vegetative roof systems that use pre-vegetative mats as plantings, which is a continuation of the wind research NRCA participated in at Southern Illinois University Edwardsville (SIUE) (see "Testing vegetative roof systems," September 2010 issue, page 28).
NRCA's concern with vegetative roof systems' growth media size, gradation and density is based on the importance of these same factors for aggregate used as ballast for loose-laid, ballasted single-ply membrane roof systems. SPRI claims one of RP-14's fundamental premises is vegetative roof systems' growth media and plantings act similarly to large-diameter aggregate ballast.
RP-14 defines growth media as "… heat-expanded clays, slates, shales, aggregate, sand, perlite, vermiculate and organic material including but not limited to compost worm castings, coir, peat, and other organic material." Clearly, these materials are not of the same size, gradation and density as aggregate ballast.
RP-14 permits areas of exposed growth media up to 5 inches in diameter and allows the use of vegetative roof systems in areas with basic wind speeds up to 140 mph in some instances. NRCA conducted wind tunnel testing at SIUE of vegetative roof system specimens with areas of exposed growth media up to 5 inches in diameter—such as is permitted by RP-14. The tests showed scouring failure of the growth media at wind velocities of only 90 mph.
The European tests Ennis cites address pressure measurements underneath loose-laid roof insulation boards, such as those used in protected membrane roof systems. We fail to see how this study is substantiation for RP-14 particularly when the study about the tests doesn't address the potential for vegetative roof systems' growth media to scour and RP-14 doesn't require above-deck insulation, which the study required.
Although, as Ennis mentions, there are hundreds of vegetative roof systems in Chicago and thousands in Europe, Chicago is in a 90-mph basic wind speed region and most of Europe uses design wind speeds less than 90 mph. This information provides better support for the data from NRCA's wind tunnel testing conducted at SIUE than it does RP-14, which allows the use of vegetative roof systems in areas with basic wind speeds up to 140 mph in some instances.
NRCA acknowledges the efforts of SPRI and Green Roofs for Healthy Cities in attempting to develop standards for vegetative roof systems. However, based on the available information, NRCA maintains its stance that roofing professionals should not rely on RP-14.
A disclaimer on the first page of RP-14 indicates: "This standard is for use by architects, engineers, roofing contractors and owners of low-slope roofing systems. SPRI, its members and employees do not warrant that this standard is proper or appropriate under all conditions."
This type of disclaimer is somewhat typical for most standard-setting organizations; however, it appears SPRI and its manufacturer members have a need to disclaim any responsibility for RP-14.
Designers, component manufacturers and suppliers, and installers of vegetative roof systems should realize RP-14 is not currently referenced in the model building codes and most building codes include wind load and resistance provisions applicable to vegetative roof systems that are more stringent than RP-14. For example, the International Building Code places restrictions on the use of aggregate roof surfacing—based on concerns with wind scour during high winds—that, in many instances, are more restrictive than RP-14's.
Ennis concludes his letter by asking: "… What resource can the contractor or designer possibly have?" NRCA recommends consulting component manufacturers and suppliers of vegetative roof systems, as well as the applicable building code.