Build-Smart? Plastic Trim Performance Problems

Builders like cellular PVC trim for a lot of reasons. It’s lightweight, durable, and waterproof. You don’t have to paint it; but it holds paint well. But there is one draw-back: plastic trim doesn’t move because of moisture the way wood does, but it does move because of temperature. And unlike wood (which mostly shrinks crosswise), PVC trim grows and shrinks along its length—a lot. A long run of PVC fascia on your eave or rake easily could shrink an eighth of an inch between summer and winte —enough to be noticeable, and enough to break a glue joint.

If you nail the material up in the mild spring or fall, it will shrink when the weather gets cold, and expand when the weather gets hot. In that case, since you started out in the middle, you may not see trouble. But if you install the trim at 95 degrees on a July day, big gaps can open up when temps go to zero in January.

The solution? Do what you do with any material that moves: Use planned movement joints so that the movement happens where you want it to happen. Nail the trim securely to a solid substrate in visible locations, glue the joints that you want to keep closed, and provide unglued movement joints in less noticeable spots (such as masked behind a corner board).

1. Shrinking Away

Cellular PVC trim has a big “coefficient of thermal expansion”—meaning that annual temperature swings can make long boards grow and shrink an eighth of an inch or more, opening up unsightly gaps. In extreme cases, the shrinking plastic exerts enough force to break a glued joint or even snap a board.

2. Filling the Gaps

If you’re trying to fix an existing problem, a very flexible and resilient caulk can be used to fill gaps that open up. Painting to mask the spot can help (PVC trim holds paint beautifully). But it’s only a partial fix: expansion and contraction will squeeze and stretch the caulk.

3. Controlling the Action

As with masonry expansion and contraction, the best solution to PVC trim movement is to allow it, but control it. Use glued half-lap joints at the seams where you don’t want movement; leave the joints unglued to allow movement where you choose to allow it (such as behind corner boards).