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 to 15 percent) is mitigated by much greater durability than with exposed membrane applications.
Vegetated roofs have the same potential for increased durability, but issues such as root attack, clogging of the drainage paths, and increased weight from slowed drainage need to be addressed. Special waterproofing techniques may be required since leak detection will be extremely difficult and repairs will be expensive.
In the traditional PMR configuration, extruded polystyrene foam (XEPS) is installed loosely over the installed roof membrane. Most rainwater flows to the drains along the surface of the membrane. Drainage channels (kerfs) approximately 0.5 inches by 0.5 inches on the bottom side of the insulation boards facilitate water flow. Porous fabric (filter fabric or “stone-mat”) is laid loosely above the insulation board to keep silt and fine stones from getting between down between the boards. The fabric and overlying ballast (stones or pavers) serve to “raft” the buoyant boards together during heavy rains so that they will not be displaced. Pavers should be placed on pedestals or have ribs on the bottom surface to facilitate drainage and drying potential.
PMRs can be used in all climates, but have found particular success in extremely cold climates. Reasons include:
- Once the membrane and flashings are installed, the rest of the installation (insulation, filter fabric, and ballast) can be done in cold or wet weather.
- By relocating the insulation from beneath the membrane to on top, the dew point is shifted from below the membrane to above. The membrane now also serves as the vapor retarder for the system, eliminating the need for a separate vapor retarder.
- The above-membrane thermal insulation keeps the membrane well above freezing, eliminating ice damage and keeping the drains open. Any melt water that reaches the membrane remains as a liquid in route to the drains (see figure 3-2). Snow that accumulates on the PMR surface adds to the winter insulation value of the assembly. (NOTE: This advantage is diminished in a cold, wet [maritime] climates where cold melt water flows across the warm membrane for extended periods of time, bypassing the thermal insulation.)
- · By substituting factory-formed, cement-faced panels (Lightguard®) for stone ballast, installation of a PMR can take place in the winter, even if local quarries are frozen. Interlocking pavers weigh as little as 4.5 psf compared to ballast stones or concrete pavers weighing 10 to 20 psf.
First-generation PMR systems had problems with flotation and silt build-up between insulation boards. Since XEPS only weighs 2 pounds per cubic foot while water weighs 62.5 pounds, a 2-inch layer of polystyrene has a buoyancy of 10 pounds per square foot (2/12 of [62.5-2.0] = 10). If just 10 pounds per square foot of ballast has been used, individual boards could float up during a heavy rain, allowing displacement of the insulation boards or having small stones dropping between and under the boards.
Non-woven synthetic fibers such as polypropylene or polyester mat have been successfully used to help with both problems. The mat serves to “raft” the boards together so that, if they float, they will not be displaced and, as the water drains, they will nest together again as installed. The “filter” in filter fabric also serves to keep fines from clogging the drainage channels or wedging between the polystyrene boards.
While there is no ASTM specification that describes suitable filter fabric, major suppliers of PMR systems have a list of approved materials. These generally consist of non-woven mats with UV treatment for greater durability. The mat is laid over the insulation boards with about a 12-inch side lap between sheets. At curbs and wall flashings, the mat is turned up vertically to the approximate height of the ballast.
Thus far, the only suitable thermal insulation for PMR systems is extruded (not expanded or molded) polystyrene. Extremely low water absorption and very high vapor resistance are needed, properties which only the extruded (XEPS) possesses. While molded (bead board or MEPS) boards have been used for flotation devices, they succeed only because there is no significant vapor drive from side to side. However, in a wet roof configuration, water pick-up will significantly reduce thermal performance.
At one time, XEPS was a Dow Chemical patented product (Styrofoam®), as was the concept of a PMR (Inverted Roof Membrane Assembly or IRMA); however, these patents have long expired and there are several sources both of XEPS and roof system manufacturers that offer similar warranted PMR systems.
Sustainability and Recyclability
PMR systems offer unique possibilities for recycling. Since the XEPS is both water-resistant and is protected against UV degradation by the filter fabric and ballast/pavers, it can be reused. Warranties as to retained R-value are offered even when reinstalled on a new roof installation. This is also true of the cement-topped interlocking lightweight pavers. Roof shims and pedestals and standard roof pavers can also be reused if undamaged.
If the roof membrane is single-ply sheeting (e.g. EPDM, PVC, or TPO) and has been loose-laid, the sheets can be most likely be cleaned-up enough to be sent back to the factory and reformed into lower-performance products such as parking curbs or roof walk-pads.
While this column has been focused upon protected membranes, flashings can be (and ought to be) protected as well. For walls and curbs, a layer of XEPS can be placed vertically against the flashing, and covered with metal counter-flashing. This both holds the boards in place and protects them against UV deterioration (see figure 3-3).
Wind Up-Lift Resistance
When loose-laid membranes are installed under a PMR, it will be necessary to incorporate an air barrier so the membrane does not bubble or billow from air leakage under the membrane. While poured concrete decks are fairly easy to make airtight, a steel deck might require a mechanically fastened underlayment board followed by a self-adhering air barrier.
For paver systems, double layers of pavers can be installed in roof corners and perimeters, if needed. This has been found to be a satisfactory solution to wind-scour of conventional stone ballast. ANSI-SPRI document RP-4 discusses ballast designs for various building locations and parapet conditions.
Roof-edge metal must always be at least as high as the ballast layer or tops of pavers. Perforated metal screening may be needed to keep ballast from migrating into scuppers, gutters, and roof drains.
When interlocking lightweight pavers are used, they may be ballasted by a course of standard (20 psf) pavers in corners and perimeters, or metal straps may be mechanically fastened into the cement topping to bind the units together (see photo 64-1). When the interlock is disturbed such as at valleys or curbs, straps may be used in these areas at will to prevent flotation.
Many references on green or vegetated roofs have appeared since the release of the Corps publication. I highly recommend the website of the National Research Council of Canada’s Institute of Research in Construction (www.nrc.ca/irc/ircpubs).
American Society of Civil Engineers
T Clear Corp.
Whole Building Design Guide (PDF)
Single Ply Roofing Industry