PVC Roof Shattering

Jeff Evans, CRC

During winter months in northern climates, cold weather conditions have created special demands on roofing systems. Some non-bituminous materials are especially vulnerable when they become brittle with age. Such a material has been some of the polyvinyl chlorides (PVC) membranes, which have been known to embrittle during their service life. In winter conditions particularly, they are very susceptible to damage by foot traffic.

PVC membranes owe their flexibility to the addition of plasticizers. Yet in both warm and cold regions, these plasticizers may migrate or evaporate out of the sheet causing it to embrittle and shrink.

The sudden and catastrophic shattering of a PVC roof membrane is a story being repeated with saddening frequency. The number of roofs that have been shattered has not been well documented. However, the frequency has been high enough for the National Roofing Contractors Association and the Single Ply Roofing Institute to jointly issue an alert. The communique describes the potential for roof failure, which generic types of PVC membranes are affected, and some precautionary measures that should be taken with specific PVC roofs.

Not all PVC roofs have these problems. In fact, there are some reinforced PVC roofs on the market that have performed very well for over 20 years, and are often considered to be the most appropriate single-ply membrane for many applications.

Roofs most often afflicted are non-reinforced PVC gravel ballasted membranes. There are non-ballasted PVC roofs that have experienced sudden failure, but due to the relative popularity of ballasted roofs over non-ballasted, the majority have been ballasted. Glass or polyester reinforced PVCs do not experience the same shattering tendencies. Non-reinforced membranes that have shattered have been found to exhibit shrinkage and loss of flexibility at all temperatures below freezing.

Are all non-reinforced PVC roofs at risk? We believe that the eventual failure mode will be shattering, or the roof membrane will become so puncture prone, replacement will be necessary. Whether failure occurs during year seven or 17 is difficult to predict, but some of the symptoms we have observed over the years are the following:

• Pulling away of baseflashings or wood nailers from their anchorings, particularly at the roof perimeter
• Sunbronzing or discoloration of the membrane in areas exposed to sunlight
• Wrinkling or gathering of the roof membrane around the base of soil pipe and drain flashings
• An increasing susceptibility to puncturing
• Tension on the membrane
• Elliptical tears in the membrane or flashings

In testing several membranes seven to 10 years old, we found the cold temperature flexibility to be greatly decreased. In the case of a membrane taken from a recently shattered roof, the membrane failed the cold bend test at 22 degrees Fahrenheit, while "as new" the membrane passed at -40 degrees Fahrenheit. This would explain why the failures have occurred in cold weather. The membranes are under tension from membrane shrinkage, and have poor low temperature flexibility.

What can building owners with a PVC roof system do then to prevent shrinkage and shattering from occurring? The NRCA/SPRI document does not recommend rooftop traffic when the temperature is below 50 degrees Fahrenheit. While this is not a severe restriction to some, roof traffic on most roofs is unavoidable. Since this is most often the case, roof traffic should be minimized and walkways provided to reduce the potential for damage.

During warm weather, regular periodic inspections should be conducted to check for any signs of membrane tension, shrinkage, tears or discoloration. If these items are encountered and the roof is under warranty, we suggest the manufacturer become involved. Several manufacturers have made good on material defects, and could reduce the cost of repairs or improvements.