Work Smart With PVC Trim
For good results with this rotproof material, you have to understand how it moves.
Synopsis: PVC trim has many assets, not least of which is that it is rotproof. Contributing editor Rick Arnold offers his advice on the best ways to work smart with PVC trim. The first steps involve the proper handling, cutting, and storage of PVC trim, keeping in mind that heat can cause the material to warp. It’s important to use the right number of fasteners; nailing schedules can differ from wood trim. Control movement in running trim by using the appropriate connection: shiplap joints, mitered corners, butt joints, or scarf joints. Casings are assembled either with pocket screws or biscuits. This article also includes a sidebar on additional gear for the PVC tool kit, including sealant, filler, PVC cement, antistatic spray, and cleanser.
Here in New England, it seems like we’re installing more and more PVC trim every year. And why not? It doesn’t rot or need paint, although paint is not a bad idea (more on that later). I typically recommend PVC trim for two particular applications. First, it’s the perfect material for a customer who wants white trim and never wants to paint. Second, it’s the best choice when there are unavoidable moisture problems resulting from the location of the house or the weather.
Installing PVC trim close to the ground, a deck, a roof, or a driveway doesn’t carry the same risks as doing so with wood or fiber-cement trim. When PVC is painted, the paint will last longer than when it’s applied to wood installed in moisture-prone areas.
PVC trim is available in many thicknesses, lengths, and profiles, and for the most part, it cuts and shapes like wood. Like other building materials, PVC expands and contracts with the ambient temperature. It’s important to know how to work with that movement. With proper joinery and fastening, PVC trim can be virtually trouble-free. However, I’ve seen carpenters try to install it like wood and then find themselves revisiting the job for repairs. Here’s why: In contrast to wood, PVC moves along its length, not its width.
When I have to install lengths over 12 ft., I pay attention to the temperature and plan for its effect on the material’s movement. This is especially important because the standard stock length is 16 ft. to 18 ft., so it’s always tempting to use one board instead of two.
The ideal temperature range for installing PVC trim is 60°F to 70°F. That’s about midstream for board movement. When the temperature cools, the boards shrink. When it gets warmer, they expand. If I’m installing long PVC boards and it’s 90°F in the shade, I make the joints tight because I know there could be a 3⁄16-in. gap by the time midwinter rolls around. Of course, the reverse is true for winter installations.
The key to minimizing seasonal movement is to employ a strategy involving the right joints, fasteners, and adhesives. I can arrange and install a sequence of joints and choose which end of the board will remain stable. The other aspect of this strategy is a recognition that the material has to move, and that the installer’s job is to pick the best place for the movement, then compensate for it with a combination of joinery and flexible gap fillers that will look good while protecting the underlying structure for many years to come.
For more photos, illustrations, and details, click the View PDF button below: