This column will focus on the thermoplastic or weldable single ply systems. We will cover PVC, copolymers, TPO’s and Hypalon® (CSPE).
The first single ply roofing systems introduced into North America back in the 1960’s were based upon plasticized Poly (vinyl) Chloride (PVC). These sheets gained their flexibility through the addition of various chemical plasticizers, some of which were fugitive. As with the vinyl-topped automobiles, the plasticizers evaporated, resulting in shrinkage of the membrane as well as embrittlement. Non-reinforced sheets sometimes failed in a catastrophic way, shattering from ordinary foot traffic, hail or other impact.
Resolving early product failures
Much of the problem was due to poor choice of plasticizers, and the lack of reinforcing fibers to limit damage spread. By September 1990, the problem had gotten so serious that a joint bulletin was published by NRCA and SPRI addressing the issues— Shattering of Aged Unreinforced PVC Roof Membranes.
The bulletin described the shattering phenomenon, membrane identification, early warning signs, and cold weather precautions.
The solution was to remove the entire membrane and replace it with something else. Not all membranes were having this problem, as some products were clearly superior to others. In some cases, less volatile polymeric plasticizers were used, with the products sometimes called copolymers, copolymer alloys or interpolymers.
The PVC industry essentially abandoned ballasted roof systems, as silt associated with the ballast was blamed for extracting plasticizer, much the same way that Fuller’s Earth (Kitty Litter) is able to absorb an oil spill. Separator sheets were introduced to prevent contact between plasticized PVC sheets and underlying polystyrene insulation, as that too could accelerate plasticizer loss.
To limit the concentration of stresses, virtually all the currently used weldable single ply systems use fabric reinforcement. This is especially helpful in mechanically fastened roofing systems subject to concentrated stresses around the fasteners.
Warning to maintenance personnel
The SPRI/NRCA bulletin cautioned that early signs of failure included displaced wood nailers, base flashings and metal flashings due to sheet shrinkage. Because the PVC was getting more brittle with age, it was recommended that traffic stay off the roof in cool weather (below 50°F), when the sheet was especially brittle. In areas subject to severe hail (2” diameter or worse), it was best to avoid the plasticized sheets.
Evolved products and performance
One result of these early product failures was the reduction in the number of products and vendors. The products currently available have an excellent track record, with at least one unchanged since its introduction in the 1970’s.
Use of polymer-clad metal components
Because thermoplastics are subject to creep, even when reinforced, many system producers utilize polymer-coated galvanized steel for gravel stops and base flashings. This solves the compatibility problem with substrates, effectively isolating the polymer from asphalt-contaminated walls, for example. The metal is securely installed by fastening to wood curbs or nailers, and the single ply membrane is applied to overlap the metal by several inches. Since we are joining polymer to polymer, solvent or heat fusion achieves a durable weld, one that is totally resistant to water attack. Due to the competitive nature of the roofing business, all-polymer systems are sometimes used as the lower cost option, but the clad metal is generally superior. Since the metal is stronger than the sheeting, stresses are uniformly distributed along the weld, rather than at fastener attachment points for the polymeric systems. If there is a down side to polymer coated steel, it is that overzealous maintenance crews may be tempted to paint the steel when they are coating railings, etc. This will render the metal flashings unweldable.
Modern thermoplastic systems
With the introduction of thermoplastic polyolefins (TPOs), another weldable thermoplastic—Hypalon is declining in market share. Hypalon is durable and weather resistant, but very difficult to maintain. Some Hypalon sheets suffer from excessive chalking or from algae attack. Most Hypalon suppliers now offer both TPO and PVC as alternatives. Another similar product was chlorinated polyethylene (CPE) but this has been out of the market for quite a while.
Since all these products were generally offered in white or other light colors, it is difficult to identify in the field what polymer you have. Search your records, or look for identifying brands on seams or on the flashings. Laboratory analysis can identify the sheets, but the repairs may be the same for many products: Clean with detergent and heat-weld a patch of compatible membrane material (if still available). Since the Hypalon becomes unweldable with age, that product would require an adhesive patch.
Repairing weldable single ply systems
The ARMA/NRCA/SPRI manual lists a 12-step procedure for repairing punctures. The procedure is identical to patching a vinyl inflatable toy or back-yard swimming pool. Clean, cut a piece of patch material that is larger than the hole in each dimension by about 4 inches or so, round the corners of the patch, and weld to the existing membrane. If the surface of the membrane is excessively degraded, slip the patch through the hole, and weld it to the underside of the membrane. When reinforced patching material is used, apply seam sealant or recommended caulk (generally just membrane material dissolved in solvent such as tetrahydrofurane) in order to prevent wicking into the exposed fabric.
Let’s talk about that welding step. The welding device uses hot air to soften the membrane and patch. These welding tools are available from roofing material distributors. One caution is that the voltage drops with the length of electric cord. The roofing contractor generally uses a portable generator and short cords to insure adequate voltage. This protects the welding device from burn-out and furnishes adequate and consistent heat to the weld area. The other tool used during welding is a silicone hand roller, to press the surfaces together as they are melted together.
Both of these tools are readily available. If you are working with an experienced single-ply contractor, these are well-known tools of the trade.
Ridges and Buckles
Because single-ply membranes are so flexible, wrinkles or ridges may be found on the finished membrane. Unless they are raised so high as to interfere with drainage, they should be left alone, or huge wrinkles could be pulled down into multiple small wrinkles.. Since the membrane is welded, there will be no leakage. The membrane material is weather resistant, not relying on surfacing for UV protection. Blisters also are not likely, since there is only one layer of material. However, if a fully adhered membrane is ballooning, this may be an interface failure, where the facer is peeling off the underlay board, or the adhesive was inadequate. The membrane could be split open, readhered and then patched, or fasteners could be driven through all components into the deck, with the fasteners then covered with a membrane patch.
Obtaining Repair Manuals and Materials: The joint NRCA/SPRI/ARMA Repair Manual for Low-Slope Membrane Roof Systems can be obtained by contacting NRCA at www.nrca.net.
Repair materials and tools can be purchased from roofing supply houses. However, a recommended procedure would be to have the membrane supplier train your crew on repair procedures and how to use the tools. Most membrane suppliers also have a pocket-sized Foreman’s Manual that is very helpful. Also visit the SPRI web site at www.spri.org