Cellular PVC trim has become quite popular since its introduction in the late ’90s, and for good reason. It won’t rot. You can easily do things with it that are difficult with wood, such as bending around tight curves. The lack of grain reduces the chance of splitting and cracking. And if properly installed, the material is indistinguishable from wood trim and holds its looks over time. The key phrase is properly installed. Yes, the installer can use the same tools and many of the same installation techniques with PVC as with wood—that’s another reason for its popularity. But once in place, the material behaves differently than wood, the main issue being expansion and contraction. Fortunately, carpenters have developed a number of best practices to deal with this, most of which have to do with choosing the right joints, adhesives, and fasteners.
“The first thing to realize is that while wood responds to changes in humidity, PVC moves with changes in temperature,” says John Pace, president of Versatex. He adds that while wood expands and contracts mostly in width (that is, perpendicular to the grain), PVC’s greatest movement tends to be lengthwise. Because of this, most of the discussion among installers revolves around how to properly join boards end-to-end.
While some installers use scarf joints, the best technique is to make ½-inch to ¾-inch shiplap joints and glue the overlaps. On long runs—say, three or more boards—you may also need to leave expansion gaps at these joints. When you’re working in temperatures above 80 degrees, the board joints should be tight. In temperatures of 40 to 60 degrees, leave 1/8 inch of space. And when it’s less than 40 degrees outside, provide for 3/16 inch between boards. Fill joints with a flexible sealant.
Outside corners can be prefabricated by gluing and nailing the miter. The leg on either side of the miter should be less than 8 feet, but the best length depends on the job. You want end joints in an inconspicuous place so the legs can be shortened accordingly. If the backing is solid, as when attaching a skirt board to a band joist, you can further inhibit movement by gluing the trim to the joist with construction adhesive.
Pace, who spends much of his time in the field with installers, finds that the key to getting a good bond between PVC boards (at those miters, for example) is to use adhesives that are solvent-based. That’s because solvents will cause some chemical breakdown of the PVC, effectively welding the boards to one another. A good choice is PVC pipe glue or a product specially formulated for PVC lumber. For bonding PVC to wood, use a subfloor or heavy-duty construction adhesive.
The same principle holds for sealants: Solvent-based products are better. One way to tell whether a sealant is solvent-based is to look at the bottom of the tube where it inserts into the caulking gun. If the tube has a metal base, the product contains solvents; if it’s plastic, it doesn’t. Avoid silicone sealants because they won’t form a solid bond.
Stainless steel nails and screws are best. Nails should be annular or smooth, and thick enough so that you can’t bend them with your fingers. They should be long enough to penetrate at least 11/2 inch into the substrate.
Screws are even better. Stainless steel trim screws work well, but many experienced installers like a fastening system that consists of screws, plugs, and a special bit that attaches to a drill/driver or impact driver. The bit automatically sets the screw to just the right depth, and a plug is then placed in the hole and tapped snug with a hammer. Plugs come with smooth or faux-grain faces and are brand-specific to match all major PVC trim brands.
Place pairs of nails or screws every 16 inches as well as on either side of each joint. On long runs, try to reduce spacing to every 12 inches.
Following the above recommendations should result in a stable trim job that stands the test of time. The only remaining question is whether to paint it. The answer is that the material doesn’t need to be painted but can be. Any good 100% acrylic paint should do the job; in fact it should last longer on PVC than on wood, since it won’t be subjected to moisture in the board.
One caution: Avoid very dark colors, especially where boards will be exposed to direct sunlight. This can raise the temperature of the board and make movement more likely. Some paint companies make coatings specifically formulated for PVC lumber.
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