If You Know TPO, You’re Most of the Way There
If you’re adept at working with TPO roofing systems, you’re well on your way to being so with PVC systems, too. Both are thermoplastic membranes, so many of the guidelines and techniques are the same. There are, however, some key differences. This blog post outlines those differences and shares other PVC best practices.
Please note that these are general guidelines. Always consult the manufacturer’s product data sheet and installation/application instructions for detailed information about the specific product being used.
Attachment Options
Like TPO, PVC systems can be fully adhered, mechanically attached or induction-welded.
Fully adhered systems will typically provide the longest and highest manufacturer wind speed warranty options and the highest FM (Factory Mutual) ratings.
Induction-welded systems, in which specialized equipment is used to weld the membrane to PVC-coated plates installed directly underneath, are fast and cost-effective. They also allow the crew to quickly dry in a roof; once the membrane has been secured, the crew can weld the seams to keep out the weather and then go back to weld the remaining plates.
Mechanically attached systems are often the fastest and lowest in cost to install.
Fully Adhered Systems
Like their TPO cousins, PVC membranes can be fully adhered using water-based or solvent-based adhesives.
Most solvent-based adhesives for both TPO and PVC are contact adhesives, meaning the adhesive is applied to the back of the membrane and to the substrate. All contact adhesives must be allowed to “flash off” – the time required for sufficient water to evaporate from the adhesive so the membrane can be adhered to the substrate – but the sign that the process is complete often is different. With most TPO adhesives, the adhesive will feel tacky when it has flashed off. With some solvent-based PVC adhesives, however, there will be little tack, or the adhesive may even feel dry.
As when using any adhesive, the “open window” – the time after flash-off when the membrane will “grab” the substrate and adhere properly – must not be exceeded when installing PVC. The time it takes for a solvent-based contact adhesive to flash off and the length of the open window depend on the particular adhesive as well as such environmental factors as ambient temperature and sunlight. If the open window is exceeded, a new coat of adhesive can simply be applied to the back of the membrane and allowed to flash off.
Two-part urethane adhesives used to install fleece- or felt-backed TPO are often also compatible with fleece- or felt-backed PVC. The available methods of applying the adhesives, such as using a roller or a sprayer, also mirror each other, though they will be dictated by the specific adhesive type and the manufacturer’s specifications.
To ensure quality welds, it is essential that adhesive be kept off seam areas. If spray-applying adhesive, we recommend covering seam areas with a slip sheet, piece of wood or similar item that will not move in the wind. Applying adhesive with a roller provides greater control, but we still recommend using a slip sheet or having a crew member hold some type of barrier to protect the area.
Mechanically Attached Systems
Mechanically attaching PVC is nearly identical to mechanically attaching TPO. The plates and fasteners are the same, and perimeter sheets are required for both system types. The primary difference is in sheet widths.
Perimeter sheets are often referred to as “half sheets” because they are narrower than adjoining sheets further in the field of the roof. Because the sheets are narrower, the rows of fasteners and plates along the membrane seams are closer together, securing the membrane more tightly along the perimeter where the effects of wind are more intense.
Induction-Welded Systems
Induction welding a PVC system also is very similar to induction welding a TPO system. The major induction welders on the market work for both TPO and PVC and use the same process. Though, as with welding seams, it generally takes more heat to weld PVC than TPO.
The fasteners and fastening pattern for TPO and PVC systems are often identical, but the welding plates have different coatings on them and are not interchangeable; a plate designed for a TPO system will not weld to PVC and vice versa.
Test welds, like those done when welding seams, should be completed to verify that the membrane is successfully welding to the plates. Delamination of the membrane from the scrim reinforcement on the full top ring of the plate signifies a successful weld.
The cooling magnets that come with the induction welder should be placed as quickly as possible after a weld has been completed. Resting on top of the membrane, they absorb the heat rising from the plate, keeping the membrane from overheating. Crews should consult the welding machine manufacturer’s instructions for guidance on how long the magnets should be left in place.
It also is essential that the cooling magnets be clean. The magnets can pick up metal shavings and other debris from the membrane. If that debris is not removed from the magnets regularly, it can become singed into the membrane.
Seam Welding
As with TPO systems, PVC sheets are welded together to create strong, waterproof seams. The process is similar, but the following should be kept in mind.
It takes more heat to weld PVC than TPO. The recommended starting temperatures for the welder – whether a hand welder or an automatic welder – will, therefore, be higher.
Test welds are essential and should be done at the start of each work day and whenever the welder has been shut down for a significant period. Cold welds are a common cause of leaks in TPO and PVC systems, and test welds can prevent them. Test welds should be completed on the rooftop to replicate the environment. For most manufacturers, a successful seam test weld for both TPO and PVC is indicated by at least 1.5-inch delamination of membrane from the reinforcement scrim (starting at the front edge of the seam).
When hand-welding TPO seams, bleed-out – when the darker bottom ply spills out of the lighter top ply slightly – is a sign that the welder is too hot. With PVC, however, it is a helpful indicator of a good weld. When using an automatic welder, having bleed-out on every seam is not necessary; successful test welds are the best gauge that the welder is set correctly.
Weight is particularly important when using automatic welders to seam PVC membranes. Generally, two weights are provided with the machine, and it is highly recommended that both be used.
Unlike when working with TPO membranes, cut edge sealant is generally not required when installing PVC. Most PVC membranes have a wick-resistant scrim that prevents water from being absorbed, making the additional sealant unnecessary.
Tape Products
Most PVC membrane manufacturers do not offer pressure-sensitive tapes for use with their membranes, and those that do typically limit their use. Stripping in edge metal is the most common application, as it allows for the use of Kynar®-coated metal, which opens the door to more color options than are available when welding membrane to PVC-coated metal.
Separator Required
Unlike TPO, PVC is not compatible with EPS insulation, XPS insulation or asphalt and cannot come into direct contact with these materials. An appropriate separation layer must be placed between the components. This caveat is commonly overlooked, particularly in shingle tie-ins, where the PVC membrane is installed directly underneath asphalt shingles.
Whenever a project includes PVC and EPS insulation, XPS insulation or an asphaltic product, the manufacturers’ specifications should be consulted for guidance on selecting the appropriate separation layer.
Note, too, that certain tape products used with air and vapor barriers also are not compatible with PVC membrane. The manufacturers of the PVC membrane and the tape should be consulted to confirm that they can be used together.
Managing Grease
PVC is highly chemical-resistant, making it the ideal choice for restaurants, manufacturing facilities and other buildings where the roof will regularly be exposed to grease or other chemicals. It is not, however, chemical-proof and will not withstand unlimited exposure. Where higher levels of chemical exposure are expected, such as around rooftop vents, the membrane should be protected with a grease trap or with a second, “sacrificial,” layer of membrane.
Cleaning
If a PVC membrane is dirty, it should be cleaned prior to welding. As with TPO, a scrub pad may be required to clean older or excessively dirty membranes, but a rag is usually sufficient, particularly with newly installed membranes. When cleaning aged membranes, only the manufacturer-specified cleaner should be used, and it should be used judiciously. Use of cleaner is not recommended on newly installed membrane that has not been exposed to dirt or weather.
As with TPO, the membrane should be allowed to dry before welding to ensure all excess cleaner has evaporated. A membrane that feels dry to the touch is not necessarily completely dry. Excess cleaner may remain in the membrane, which can result in poor welds. Approximately 20 minutes of dry time is generally ideal, but guidance from the membrane manufacturer should be followed.
Repairing Aged PVC
When repairing aged PVC, it may be difficult to weld a patch to the top, weathered surface. In these cases, an under weld – in which the patch is placed below the old membrane and welded into place – is a good alternative. The bottom of the aged membrane must be cleaned well first, particularly if there is adhesive there.
PVC roofing membranes have a track record of success spanning 50 years and millions of square feet. And if you’ve learned the ins and outs of installing high-quality TPO roofs, you’re well on your way to adding PVC to your arsenal. Just keep these few tips, tricks and differences in mind.
Learn more about Mule-Hide’s PVC product offerings here.
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