Over the last few decades, PVC (polyvinyl chloride) thermoplastic roofing membranes have become a primary choice for flat and low-slope roofs. They are often used for roof recovers and attached over old roofs. PVC is a strong material and naturally a reflective white color. PVC comes in rolls and is either mechanically attached with fasteners into the deck or fully adhered using glue and adhesives. The seams are overlapped and welded together with heat. Typically, PVC welds hold strong over time. It’s supplied in 60 mil (1.5 mm) or 80 mil (2 mm) thick sheets, though a good portion of the thickness is taken up by the reinforcement scrim. There is PVC cure polymer material, which is the actual waterproofing, above and below this scrim. In the case of GAF’s PVC membranes, there is only 24 mils (0.6 mm) of waterproofing above the scrim.
Some manufacturers, such as Duro-Last, are even selling 40 mil thick sheets. In this instance, they state in their literature there is only 17 mils of PVC polymer above the scrim. This means you only have 17 mils of waterproofing to withstand UV, ponding water, and the elements before the scrim is exposed. There’s a reason these cheap 40 mil sheets aren’t lasting more than a few years.
PVC became popular for many reasons, mostly economical and environmental. It is lightweight, safe to install, durable, and energy efficient. No open flames, and no hot kettles. You could lay insulation board or hard board over an old roof and then roll out a PVC membrane for a completely new, warranted system. However, most of the time this requires thousands upon thousands of fasteners to go through the old roof and the old deck in order to hold it down. Every seam must be welded perfectly, every flashing done correctly, and everything sealed with caulk in order to prevent the creation of a water bed.
The photos below I took in Las Vegas recently on a 14-year old Carlisle PVC roof. You can see where the UV rays had been reflecting off of the parapet wall onto the field. What happens is a magnifying effect. The UV that bounces off the wall is more intense and deteriorates the PVC membrane faster. Remember, there is only 15-23 mils on average of material before the scrim is exposed. The scrim can then absorb water and moisture can actual wick through the scrim. This not only causes leaks but accelerates degradation of the rest of the sheet.
What they found a long, long time ago with PVC roofs is that under ponding water, the plasticizers “migrate”. This means the plasticizers that comprise the PVC polymer structure actually move out of their originally intended position. The result is brittleness and loss of stability. Ponding water also intensifies UV rays, deteriorating the PVC material rapidly. In the instance of this particular roof, the scrim was exposed in both a ponding area and a non-ponding, very dry area. My estimate is this issue probably started 5 years ago, meaning this 60 mil PVC roof didn’t even last 10 years before it needed extensive repairs and maintenance. I also want to note aged PVC is difficult to weld new PVC patches to. The welds typically don’t hold.
Some of these issues led to the rise of TPO (thermoplastic polyolefin) membranes. TPO sheets are easier to weld, both when new and after it ages, and they eliminate the issue of plasticizer migration in ponding water areas. Plus, they are much cheaper! It would make sense that TPO is the way to go, right? Unfortunately, you get what you pay for. TPO membranes are struggling to last 5 years in many instances as the TPO polymer materials are not as strong, durable, and long-lasting as PVC. Many of you have likely experienced this firsthand.
PVC membranes do have their place in the market, despite the common issues and maintenance requirements to ensure watertight performance. However, many manufacturers are retracting a bit and going back to traditional two-ply systems on new construction assemblies. There is definitely value in redundancy and having more than one ply on a roof. You can now find “hybrid” PVC roofs with a base layer of modified bitumen, either torched down or self-adhered with a peel-off backing. Then a fleece-backed PVC membrane is adhered on top, resulting in two-plies. The long-term performance is still unknown, as this is a very recent shift in the market.
All of this proves why liquid-applied membranes are continuing to rise and gain a much larger footprint in the roofing industry. For restoring an existing roof membrane, a liquid membrane like ours makes the most sense. Unless, of course, the existing insulation is saturated or the old roof is completely shot and a tear-off is the only option. But even on new construction assemblies, I believe you will start to see more and more liquid alternatives. As these PVC and TPO roofs show signs of failure, a lot of money is being saved by being pro-active and restoring them with our TritoFlex 2K system.
If you’re interested in learning more about our smart, sustainable, common-sense approach to roof restoration, reach out to us anytime.