How to Install Plastic Crown Molding

Add architectural interest to your home's ceiling with plastic crown molding. We'll show you how to install it.

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If you want the latest in crown molding, go with plastics. You'll find a wide range of styles, from relatively plain to incredibly intricate. Plastic crown moldings resemble the best work crafted in plaster and found in stately vintage homes, but plastic's low density slashes weight and cost.

Another appealing aspect of plastic moldings is the fact that the skill level required for successful installation is quite modest. With nothing more complicated than square cuts when you utilize corner and connector blocks, it's a project that's accessible to many people. You'll need just a few basic tools; you can even get by without a miter saw. The material has about the same density as pine, so even a hand-powered cut through the widest molding will barely cause you to break a sweat.

As you shop around, you'll discover several styles and sizes of molding and corner blocks. Be certain that the blocks you select coordinate with both the proportion and character of the molding.

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Step 1: Drill Clearance Holes

In each block, drill a 5/32-inch shank clearance hole through the tabs so that the threads of a #8x2-inch roundhead screw don't hang up on the plastic. Note that there are three styles of block: an inside corner block, an outside corner one, and a connector block. If you need to use a connector block, position it in the middle of a run or evenly space a series of connectors along a wall.

Step 2: Screw to Wall

Screw the blocks to the wall, making certain that they are square to the ceiling. Don't overdrive the screws or you'll risk breaking the plastic. To end a run, simply cut a tab off a block with a handsaw. Referring to the depth dimension supplied with the molding (or by measuring it), make a reference mark near each block and about every 4 feet along the run of the molding. The depth dimension is the height of the installed molding on the wall.

Step 3: Mark Stud Locations

With your stud finder and strips of masking tape, mark the location of each stud on the wall. Measure between a pair of blocks and add 1/4 inch to ensure a snug fit; mark this dimension on a length of molding.

Step 4: Cut to Size

Cut the molding to length. A miter saw will do the job quickly, but a handsaw is just as effective. If you choose the manual route, make sure you cut straight across the molding and squarely through it.

Step 5: Apply Adhesive

To ensure a snug fit, apply the adhesive recommended by the manufacturer to the bedding edges of the molding. (The bedding edges are the molding surfaces that contact the wall and ceiling.) Don't overdo the adhesive or you'll have a mess when you install the molding.

Step 6: Secure Molding

Snap the molding into place between the blocks. Having a helper makes this part considerably easier because you need to ensure that the bottom edge of the molding meets the depth dimension marks you made earlier.

Step 7: Drill Screws

Secure the molding with a 2-inch trim-head screw driven into each stud. The slim square-drive trim screws usually don't require a pilot hole. You could drive finishing nails, but be aware that a misplaced hammer blow could smash details of the molding. Screws are far less risky.

Step 8: Fill Holes

Remove excess adhesive with a putty knife and mineral spirits. Fill the screw holes with a paintable caulk that's compatible with the molding. Also caulk any gaps along the wall and ceiling. Follow the manufacturer's paint recommendations.

How to Use Corner Blocks for Wood Crown

Your home center may stock wood corner blocks that you can use in conjunction with wood crown molding. Be sure to check that the entire end of the molding you choose will land on the block with no overhang.

You also can mix media by utilizing plastic blocks with wood molding. As long as the completed installation is painted, no one will be able to detect the difference in the materials.

How to Install Plastic Molding Without Corner Blocks

You don't absolutely need corner blocks when using plastic moldings. If you choose not to use them, note that the installation of plastic molding differs from that of wood molding. Here is how to handle plastic molding.

If you need to join two pieces of molding along a wall, use a butt joint, not a scarf joint, and apply adhesive at the seam. If necessary, touch up the joint with vinyl spackle and sand it smooth.

Utilize butt joints at the ends of the molding. But eliminating the midrun block means that you need to create a temporary anchor point so you can spring the slightly overlong molding into place. This anchor point is nothing more complicated than a wood block temporarily screwed to the wall. For an extremely long molding run, leapfrog the position of the wood block along the wall to install each piece.

Cut miters instead of copes for inside corners, and apply adhesive to the joint.

If you sand through the factory-applied primer, touch it up with a compatible primer before painting.

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