In Buildings.com Roofing News #63 some five years ago, we introduced the concept of protected membrane roofs (PMRs) as discussed in the Corps of Engineers document Commentary on Roofing Systems. So what has changed?
From a technical point of view, not much. However, the last few years have introduced new challenges you need to consider when installing a PMR system.
New Issues for PMR Systems
In a PMR system, the membrane is protected by an overlay (usually of medium-density closed cell extruded polystyrene [XPS] insulation), a filter fabric, and ballast as illustrated in the Corps commentary.
As we stated in Column #63:
By overlaying the membrane with opaque ballast and thermal insulation, the membrane is protected against accelerated oxidation, evaporation of volatile oils, ultraviolet degradation of organic materials such as bitumen, thermal movement (expansion and contraction), blistering, ridging, and stress concentration over insulation joints. Virtually all roof membrane systems may be used in a protected configuration. The slight cost increase (5-15%) is mitigated by much greater durability than with exposed membrane applications.
Fast forward to 2012. We now have the additional issues of photovoltaic assemblies, code changes calling for much higher R-values, unresolved issues of fire resistance with raised PV panels, emphasis on air barriers, finding ways to recycle roof components to gather LEED points, reflectivity, emissivity, and sustainability. In a sense, vegetated (landscaped) roofs are also protected roof assemblies except that the membrane is more like waterproofing than protected roofing.
Address Green Technology Concerns
Solar panels elevated above roof level by racks have been shown to generate considerable heat that could be transferred to a membrane located directly beneath the racks. Several membrane systems, such as TPO, have been reformulated to withstand this additional heat load, and ASTM specifications now call for higher oven aging temperatures of longer duration (for example, ASTM D4434 requires 56 days at 176 degrees F.). In a PMR configuration, the membrane is thermally protected by the insulation and ballast.
These assemblies must also withstand abuse caused by hail impact, roof traffic from installation, and maintenance activities. Higher compressive resistance to withstand such abuse is possible with the extruded polystyrene boards typically used in PMRs.
XPS (extruded polystyrene) systems can help meet the increased thermal resistance demanded by modern building codes. A good partner for PMR, XPS retains most of its R-value even when wet.
Air barriers help tighten the building envelope and lower the amount of conditioned air escaping, but they require thoughtful placement to properly fit a PMR. Remember that an air barrier differs from a vapor barrier. The vapor barrier (or retarder) needs to be installed on the warm side (high vapor pressure side) of a roof assembly. An air barrier, on the other hand, can be installed under the roof deck or anywhere else on the way up to the surfacing as long as it retards air flow from the building interior. Attention to the air barrier is paramount because migrating air carries much more moisture into a roof system than diffusion through a vapor retarder does.
If moisture does make its way into the roof assembly, ensure that it can exit without waterlogging. Synthetic non-woven mats placed between the insulation and the stone ballast helps keep grit from working down between insulation boards and interfering with drainage. Proper drainage is necessary so that the insulation does not become waterlogged. This filter fabric also serves to raft the individual boards together so that in a heavy rainstorm where the roof is flooded, the boards maintain their position.
Also consider the recyclability of insulation used with PMR. Extruded polystyrene (XPS) is the only thermal insulation that performs in a wet environment. Since the insulation boards are located (unadhered) above the roof membrane, they can self-dry as rainwater migrates to the roof drains or by evaporation at the insulation joints.
Producers of XPS will generally warrant the R factor so that you can reuse the boards in the next re-cover or re-roof project. Roof ballast can also be cleaned and reused, and composite boards of latex reinforced concrete and XPS (Lightguard or Heavyguard) are also reusable.
Weather resistance is no longer critical after the waterproofing membrane is physically protected. A relatively new method of finding breaches in a hidden roof membrane such as a PMR or a vegetative roof uses the principle of electrical conductance. An ASTM subcommittee is working its way through a new standard on this method, which will be applicable to both roofing and waterproofing systems. This method can pinpoint defects in the membrane so repairs can be quickly made. Note that the system is sensitive and won’t work for carbon-black filled EPDM.
Wind resistance, however, is still a major consideration in many areas. SPRI document RP-4 addresses wind uplift of ballasted roof systems. Interlocking Lightguard pavers can be upgraded by metal straps at roof edges or freeze-resistant concrete pavers.
PMRs Fit the Bill
While the suitability of PMRs in cold climates has been well documented, the above-mentioned issues present new opportunities to create roof systems that are indeed protected and sustainable.
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc., the Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, the William C. Correll award from RCI, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
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