PVC Roof Trim
A coastal builder’s technique for durable and weathertight fascias and soffits is based on years of experience with plastic trim.
Synopsis: Author John Spier has spent the last thirty years building houses on Block Island, R.I., where wind, rain, sun, and salt spray all take their toll on the exterior elements of a house. Based on his long experience, he always recommends using PVC exterior trim, because it doesn’t rot. This six-page Master Carpenter article details his techniques for trimming the roof line with PVC: how he handles the long lengths, his methods for fastening and gluing it, and how to plan for its thermal expansion and contraction. He discusses tricks for straightening out the underlying frame and his preferred methods for installing the soffits and fascia. Also included are a sidebar on the benefits of painting PVC. Video extra: How to Install Exterior PVC Trim. Photos: Charles Bickford
Framing defines the bones of a building, but a well-executed trim job highlights its design for the world to admire. If done properly, the individual trim components blend together, their arrangement is smooth and harmonious, and the trim looks good for years to come. Trim installation is also a very satisfying part of the job that marks the transition from rough frame to fine finish.
For almost thirty years, I’ve been building and renovating on Block Island, R.I., a place where houses are routinely blasted by wind-driven salt, sand, debris, and precipitation, occasionally all at once. I’ve learned to have my clients invest in quality materials to resist these forces of nature, and for the past decade, this has included PVC exterior trim. As with using any new material, installing PVC has a learning curve, but after working with it for a number of years, I feel like I’ve developed techniques that allow the material to perform at its highest potential.
Recently, I built the house shown here and trimmed it in PVC. The design includes an upper roof marked by almost rounded gables and a pair of A-style dormers. A lower roof shelters a porch encircling the first floor. With their longer lengths and variety of joints, the house’s roof soffits and fascias provide good examples of the methods I use to work with the material.
Good or bad, PVC isn’t wood
Although PVC trim is weatherproof, its plasticity makes it harder to work with, especially if you’re used to working with wood. First, because of its flexibility and long lengths, it’s difficult for one carpenter to move efficiently. It took a couple of jobs for me to find an efficient method for carrying and cutting the floppy material.
Its flexibility also means that PVC will telegraph every bump and ripple in the framing, rather than bridging and hiding them, so the framing needs to be straight before I start to trim. I use shims, saws, a grinder, and an old power plane to smooth things out.
The other tricky aspect of PVC is its finish. PVC crosscuts, rips, and routs smoothly, but the exposed core doesn’t have the same smooth surface as the factory faces and edges. When I look back at my earlier PVC-trim jobs, it’s usually the ripped edges and cut ends that I notice first. In the early years, the material’s porous interior structure tended to capture mold and dirt. Since then, manufacturers have refined the material’s composition. Even so, I’ve learned to plan carefully so that only factory edges are exposed.
When that is not possible, I smooth ends and edges that will be visible with a slow pass of a sharp plane. I also use a block plane to ease sharp corners. Some professionals advocate wiping down cut edges with acetone to seal the material, but that’s an additional level of toxicity that I prefer not to have on the job.
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