Cutting and Installing Plastic Trim

Learn the secrets of successful PVC trim installation.

PVC trim looks and cuts like wood and it lasts forever, but there are a few special installation techniques you need to know about to install it successfully. Tools RequiredMaterials Required100% acrylic exterior paintAcrylic caulkExterior fillerLong-setting PVC cementPVC trimStainless steel trim-head screws

Tired of scraping and repainting your trim? Or disheartened when you find that your trim has rotted and will no longer hold paint? Then step back and consider the advantages of replacing those problem boards with PVC azek trim—your worries will be over.

Cellular PVC board (polyvinyl chloride) azek trim is a close cousin of the familiar white plastic plumbing pipes. (The “cellular” part just means it’s filled with zillions of tiny air bubbles to make the material lighter and less expensive to manufacture.) But unlike its cousin, PVC azek trim is specially formulated to make it resistant to sunlight, hold paint well and easy to work with.

PVC azek trim looks just like wood (well, flawless wood) and lasts virtually forever. PVC azek trim impervious to rot and insect attack and doesn’t absorb water. It holds paint well because water can’t penetrate the material behind the paint. And if you like the look of white trim, you may not have to paint it at all (more on this later). It’s an especially good substitute for wood trim in areas that are highly exposed to water, such as corner trim boards and door trim that’s near the ground or unprotected by an overhang.

Although you install cellular PVC azek trim almost like wood, there are a few crucial differences. In this story, we’ll show you those special cutting, joining and fastening techniques so your PVC azek trim will perform flawlessly for the life of your home. Don’t worry—there aren’t any fancy tools or skills required. If you’ve cut and installed wood trim before, you have the moxie to work with PVC trim boards.

PVC trim is sold in 5/4 (1-in.) and 4/4 (3/4-in.) thicknesses in the common widths found with wood. But the selection may be limited. You’ll probably have to special-order some thicknesses and widths. PVC trim boards are sometimes only sold in 18-ft. lengths, so you may want to have your order delivered. Most companies offer material that has an embossed wood grain side and a smooth side, so you can choose the look you want. Don’t confuse cellular vinyl trim with polyurethane-core, vinyl-coated products. While they too are highly durable low-maintenance products, their installation techniques are different.

PVC trim isn’t cheap; expect to pay about the same price you’d pay for clear, knot-free wood. Other PVC products are also available, including preformed outside corners, tongue-and-groove boards, and sheet goods that you can carve up and work just like plywood.

Find PVC trim by contacting lumberyards in your area. Project step-by-step (9)

Step 1Cut with carbide-tipped saw blades and bits

Cut PVC trim boards with the same hand and power tools that you use for wood. But use only carbide-tipped saw blades; plain steel ones will dull quickly. In general, the more teeth a blade has, the smoother the cut edges will be. We recommend standard combination saw blades. You can easily rout decorative edges or grooves with routers or shapers, but use carbide-edged bits as well.

Sawn edges aren't as easy to sand as wood. So if possible, plan your work so that newly cut edges will butt against siding, soffits or other trim, and let the smoother factory edge show wherever possible. Sanding isn't always necessary, especially if you plan to paint. But sand exposed edges that are highly visible, like near the front door. Use a random orbital sander with 100-grit paper. Belt sanding doesn't work well because the friction from the belt melts the plastic rather than smoothing it.

Step 2Fasten PVC trim with stainless steel screws or hot-dipped siding nails

When it comes to choosing your fasteners, don't scrimp by using fasteners that won't last as long as the vinyl trim. Stainless steel screws with small finish heads are the best choice because they'll never corrode. You can also use hot-dipped galvanized nails, but they may corrode and stain the trim over time. If you want a flawless finish, choose screws, countersink them slightly, and then use an exterior filler to hide the screw heads. In areas that are completely protected from water, like under a soffit, you can use a 15-gauge air nailer with galvanized nails and fill the holes with paintable caulk or filler.

PVC tends to expand and contract with temperature changes, so fasten it well. Drive screws or nails into framing only, never just to the sheathing. Select fastener lengths that will penetrate at least 1-1/2 in. into the framing. No predrilling or countersinking is necessary if the temperature is over 40 degrees F when you install the trim. But lower temperatures call for both drilling and countersinking; otherwise the vinyl may split. Place fasteners every 16 in. at both sides of the trim, spacing them about 1/2 in. from the edges. If you're using 10-in. or wider trim, add another fastener in the middle.

Follow all of the flashing techniques that you would with wood trim. You still want to prevent water from penetrating between the PVC trim boards and the wall sheathing behind. Sign Up For Our Newsletter

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Step 3Cement seams for sturdy, watertight joints

One advantage of PVC is that you can “weld” joints to keep them tight and prevent water from penetrating behind the trim. Manufacturers recommend a special type of PVC cement that has a longer “open time” than the type of cement that plumbers use on plastic pipes. You can buy this cement wherever you buy the trim. You'll have about five minutes of working time to clamp and fasten the joints before the cement sets. Smear a little cement on both surfaces and then clamp or screw the joint together. Wipe off any excess right away with a damp rag; unlike PVC pipe cement, it's water soluble and won't mar finished surfaces if you remove it immediately.

Step 4Preassemble window and door trim

Prebuild PVC trim assemblies and then install them as a unit rather than a piece at a time as you would with wood. This will take a little longer but will result in perfectly tight joints that won't need caulk. Cement and screw end joints like miters and butt joints. Longer joints like corner boards can be simply cemented and clamped.

Pocket screws are the best method of joining corners when you're cementing window and door trim assemblies (Photos 1 and 2). Use the coarse-threaded version of pocket screws. Cut miters to fit and dry-fit your cuts to check the joints and lengths. Measure carefully when applying trim around vinyl or aluminum-clad windows to leave a 1/8-in. expansion gap between the trim and frame for caulk.

Step 5Preassemble corner PVC boards

Some manufacturers offer pre-made trim for outside corners, but it's somewhat pricey. It's easy to make your own. Start by ripping 45-degree bevels on one edge of each board on a table saw. Then push the bevel tips together tightly and tape the boards together with packaging or duct tape. Apply cement to both bevels, then fold the boards together and clamp the joint for about 15 minutes with more strips of tape.

Step 6Leave expansion gaps on long pieces

PVC trim boards will expand and contract with wide variations in temperature. If you're installing trim in temps higher than 80 degrees F, cut pieces to fit tightly. If it's between 60 and 80 degrees, leave a 1/16-in. gap for every 18 ft. of length. Below 60 degrees, leave a 1/8-in. gap. After installation, cover the gap with acrylic or urethane caulk. Avoid silicone-based caulks; they don't adhere well to vinyl.

Step 7Fill fastener holes before you paint

Fill small holes with an exterior filler and lightly sand it smooth after it dries. Or use a paintable caulk and smooth it with your finger. Fill larger holes or damaged areas with auto body filler, and again, sand it after it cures.

Step 8Splice long trim pieces with scarf joints

Splice long pieces of PVC trim with “scarf joints,” that is, overlapping 45-degree joints. Cut the first piece of trim to fall just short of a stud so the second overlapping trim piece can be fastened to the stud an inch or so from the end to prevent splitting.

Step 9Paint vinyl trim with 100 percent acrylic exterior paint

Although painting PVC trim boards isn't necessary, we recommend it. Otherwise your trim will look stark white and you may see filler over holes that's not an exact color match. And the edges and ends (especially ones that you have cut or routed) will collect dirt and begin to look gray over time.

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