In the last 10 years or so, cellular PVC products have been a game changer for exterior trim. Like most products, PVC trim has advantages and disadvantages. As a carpenter, I don’t love the material—I much prefer wood sawdust to plastic sawdust. And so does my wife—especially when I come home after working and the fine, white dust statically clinging to my clothing eventually finds our furniture. But dust notwithstanding, you can’t beat PVC for its longevity and stability.
One trick that I picked up a couple of years ago while working with PVC trim is miter-fold post wraps. We do a lot of deck and porch work, so I’m often asked to wrap posts with finish trim. I’d heard of another contractor who was making his own miter-fold wraps, so I decided to give them a try. At first, I didn’t know what to expect and went through the requisite trial-and-error period before working out the technique.
Each post wrap is a four-sided assembly made out of PVC sheet stock. Using a router, I cut V-grooves nearly all the way through the material to create matched 45-degree bevels, which then fold up to create 90-degree miters.
Although miter-fold post wraps are available commercially, I find it less expensive and more fun to make my own. One local company, Intex Millwork Solutions, makes them in several sizes, cutting the miter joints on a CNC machine.
In my formative years as a carpenter, I was taught never to miter exterior trim, whether you’re turning a corner with a fascia or wrapping a post. Even rot-resistant wood such as cedar will eventually cup and cause the joint to open up. Square-edge joinery was always a better course of action with any type of wood.
But with PVC, mitered joints seem to be a better choice. Because PVC doesn’t cup or shrink much across its width like wood does, and because of the greater surface area of a mitered joint and the molecular bond of the glue, PVC miter joints stay together. And they’re also less visible than square-edge joints.
Laying Out the Post Wrap
On the project shown here, I wrapped nominal 4x4 porch posts to create a finished dimension of about 5 inches square. Instead of using thinner 3/8- or 1/2-inch stock and packing out the post, as I have done when finishing larger posts, I opted to use 1-by PVC material and apply it directly to the 4x4. I aimed to make the inside dimension of each side of the wraps 3 5/8 inches. (In hindsight, I should have made that dimension 3 3/4 inches. I ended up having to do a bit of on-site planing of the yellow-pine posts to remove some dimensional irregularities).
Knowing that the distance between the groove edges had to be 3 5/8 inches (the inside dimension of each side) and that the PVC stock was 3/4 inch thick, I could easily calculate the outside dimension of each side. That worked out to be 3 5/8 inches plus 1 1/2 inches (the width of a groove cut in 3/4-inch stock using a 90-degree V-grooving bit), or 5 1/8 inches. So I laid out the grooves to be cut at 5 1/8 inches on-center.
I make the cuts by running a router against a straightedge. To lay out the position for my straightedge, I first took into account the width of my router base. The 2.25 HP Bosch router that I use has a 6-inch base. Then, I simply added half the width of the base, or 3 inches. This put the straightedge layouts at 3 inches, 8 1/8 inches, 13 1/4 inches, 18 3/8 inches, and 23 1/2 inches. I noted these numbers so that I could lay out the rest of the sheets that I’d need to make all six post wraps.
Router Set Up
As mentioned above, I make the grooves for the miter folds using a 90-degree V-grooving bit. If you are going to make your own post wraps using a router bit, I recommend getting a large V-grooving bit with a 1/2-inch shank. The heavier shanks are much safer to use, and the cutter is large enough to make miter-folds in material up to 1 inch thick.
Before I start cutting the post wraps, I always check the depth of the router bit using some scrap of the material that I’ll be working with. Setting the depth of the cut exactly right is critical. The objective is to cut almost—but not completely—through the stock, creating a joint that is flexible enough to be folded by hand. I generally leave less than 1/16 inch of material. Held up to the light, the bottom of the groove is translucent. Once I’ve set the depth of the bit, I label and stow away the test scrap with the hope that I’ll be able to find it the next time I have a similar job using that stock thickness.
Cutting the Grooves
Routing PVC is dust-intensive, so I always wear protective glasses, and if I’m cutting a large number of sheets, I also don a dust mask—and sometimes even a Tyvek suit to keep my clothes dust-free and my wife happy.
Once the router is dialed-in and the first sheet is laid out, I clamp the straightedge on the layout. Router bits always rotate clockwise, so keep the straightedge to the left of the router when cutting. That way, the direction of the rotation pushes the router base against the straightedge and keeps the router from walking away from the straightedge.
As I make a pass with the router, I make sure that dust doesn’t build up between the router base and the straightedge. Any buildup can push the router away from the straightedge, leaving you with a groove and a miter joint that is not straight and true.
As with most carpentry projects, this process goes much more quickly and smoothly with someone there to help. I was lucky to have one of my carpenters, Justin Cline, available to give me a hand with setting the straightedge, tending the router cord, and sweeping off the dust between passes.
After I’ve cut the grooves, I carefully sweep the dust off the grooved sheets to get them ready for glue. Because we needed to wrap posts that were already in place, the most successful strategy was to install three-sided wraps, leaving the final side loose to be installed later. We sliced the sheets into sections of three sides by running a razor knife down the base of the appropriate groove.
To glue the wraps together, I use Christy’s Red Hot adhesive in caulking tubes. I squeeze out a bead of adhesive in the second and third grooves of my three-side sections. Then I wait a couple of minutes. The adhesive actually heats up the PVC and makes the material in the groove more pliable.
After letting the glue warm up, I fold one side up to make the first miter joint, starting in the middle of the wrap. The material sometimes makes cracking sounds, but it should stay together as long as I’ve made the groove the correct depth. After folding up the opposite side to make the second miter joint, I clamp the three-sided sections together using plywood blocks on the inside of the wrap to hold the assembly square.
On that cold and nasty February day, we fabricated the wraps for all six posts in my warm shop and let them cure overnight. The next day, we broke down the clamp assemblies and ran a sharp chisel along the inside of the miters to clean off any excess adhesive that had oozed out. Cline cleaned up the exposed miters with a block plane to remove the small bit of material that was left by the router when the grooves were cut.
Once the weather decided to cooperate, we headed to the jobsite to install the wraps. I started by sliding the three-sided assemblies onto all the wood posts. I’d covered the beam that supported the porch roof in PVC, making the bottom part 5 1/2 inches wide, which let the post wraps fit in nicely below.
I fastened the PVC assemblies to the posts with 15-gauge stainless-steel nails. After running beads of adhesive down the exposed miters, I fit in the last sides, fastening them with 18-gauge stainless-steel brads. To finish up, I eased all the corners with a 1/8-inch round-over bit in a mini-router.