In response to stricter volatile organic compound (VOC) emission standards enacted in many parts of the country, roof system manufacturers now have a plethora of bonding adhesives for use with TPO and PVC single-ply roofing systems.

While it’s great that roofing contractors now have more choices when it comes to bonding adhesives, the sheer volume of new products on the market can make it confusing for roofing applicators.

Each of the major single-ply roofing manufacturers now offers “standard” solvent-based bonding adhesives for jobs where VOC emissions are not an issue; one or two lower VOC, solvent-based bonding adhesives that meet various emission standards; and water-based adhesives used primarily with fleece-back single-ply membranes. One must also consider that some products are restricted for use with smooth-back TPO only, while others are designed specifically for PVC or fleece-back membranes.

Depending on the specification, the distributor, the roofing contractor, and—most importantly—the roofing crew may have to double-check a lot of labels when going through buckets of bonding adhesive prior to application.

First, the roofing foreman needs to ensure that the correct bonding adhesive has been delivered to the jobsite for the single-ply membrane(s) that are being installed. Second, he must determine if the buckets of material have been stored properly and protected from freezing. Third, the bonding adhesive must be applied at the correct temperature, using the coverage rates recommended by the roof membrane manufacturer. Coverage rates can vary dramatically, depending on the bonding adhesive and single-ply membrane being installed.

For example, one popular low-VOC bonding adhesive for smooth back TPO membranes features a coverage rate of 500-600 square feet per five-gallon bucket. But a similar bonding adhesive from the same manufacturer that includes more solvent only yields 60 square feet of finished mated surface per gallon. In addition, the low-VOC product features a fast-drying solvent system, while the second bonding adhesive will be slower to dry.

Water-based adhesives for smooth-back TPO will generally yield about 100 square feet of finished, mated surface area per gallon. However, woe to the roofer who uses this same water-based adhesive to adhere smooth-back PVC membrane. Water based adhesives for non-fleece-back PVC may or may not be acceptable, depending on the roofing manufacturer’s specification.

After checking the bonding adhesive, the application should begin only when the adhesive, substrate, membrane, and outside temperatures are above 40 degrees Fahrenheit (4.4 C) and rising. Application temperatures above 50 F (10 C) are heartily recommended to allow easier adhesive application. Again, special care must be used when installing water-based adhesives in colder climates.

The applicator also needs to keep in mind that solvent-based adhesives must be allowed to dry until tacky to the touch before mating flashing membrane. Water-based adhesive must be allowed to dry completely to the touch. Typically, the flashing should be installed within one hour of applying the water-based adhesive. However, this may vary depending on ambient temperature conditions.

As can be seen, there isn’t much room for error when using bonding adhesive to adhere a TPO or PVC single-ply membrane. When one combines this with the typical controlled chaos of a low-slope roofing job, the onus is really on the roofing foreman to read and follow the labeling instructions on each bucket of bonding adhesive delivered to the jobsite.