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How to Install Polyurethane Crown Moulding

Polyurethane and PVC Molding Take the Difficulty out of Upgrading your Home

© 2012 by all rights reserved; content may not be copied, rewritten, or republished without author’s written permission. Author’s Google profile

Polyurethane crown molding; photo courtesy Asifthebes

Polyurethane crown molding and a skylight; photo courtesy Asifthebes

This article was updated on 08/03/20

This innovation is molded plastic-based architectural trim. It has notable advantages over traditional wood. First, it’s flexible without the risk of cracking. Next, it is more affordable. There is less of a chance of splitting when you nail or screw it up.

By its very nature plastic does not expand nor contract due to humidity. Does it have a downside? Yes, you can’t finish it with wood stain or Danish oil intending to highlight the natural beauty of the wood grain. On the other hand, it can be painted.

The Challenge of Wood Molding

Arguably the hardest part of installing traditional wood moulding is cutting you inside and outside corner miters accurately. Unfortunately, this results in waste as well as frustration on a DIY project, meaning you have to buy an average of 10% more material.

This issue is done away with with polyurethane. How? Both the inside and outside corners are engineered and sold as pre-fab units. They are available as one-piece units or as right and left-hand components.

Installation Tools and Material List

  • Corners and lengths of moulding. It is always prudent on any remodel project to purchase 8% or so of extra material. This is to allow for waste, except for obviously the corners in this case; just count those up so you know how many you need.
  • Either nails or screws, which ever your manufacturer recommends.
  • A stepladder or small rolling scaffold.
  • A saw to cut the material. A compound miter saw will give the best results.
  • A dripless caulking gun along with a sufficient number of paintable silicone latex caulk tubes for the job.
  • The recommended adhesive if your manufacturer recommends one.
  • An electronic stud finder.
  • A tape measure.

Trim Installation Steps

To start with, choose a corner in which to start. Your ceiling corners and straight runs at the ceiling line need to be as level as possible. Usually, you will have to chip off some of your ceiling texture.

Squeeze out a bead of adhesive on the top and bottom of your corner piece(s) where it touches the wall Now nail or screw it to your wall as recommended.

You’ll find that on inside corners it’s easy to locate an inside wood stud; they are right in the corner. On the other hand, with outside corners the stud will start a little over 1/2” from the corner (due to the drywall, the corner bead and the drywall mud.

The job will be easier if you attach all your corner units before filling in any of the wall between them. Next, fill in the runs between your corner components with your straight lengths of moulding.

Scarf Joints Can be a Bit Tricky

It is key to ensure that all your field joints center on a stud. To be more specific, these joints in the field should be scarf joints. This is where the first stick of molding ends in a 45° angle sloping from the outside to the wall.

The next stick is also has a 45° as well, but angled in the opposite direction such that it overlays the miter on the first stick. The overlap should split the wall stud on center so you can nail through both angles into the stud.

A crown molding scarf joint is angled in two directions; photo courtesy Kelly Smith

Unlike scarf joints on baseboards, crown needs another 45° angle from upper right to lower left. This is because the trim is angled out with respect to your vertical wall. See the photo above to see what I mean.

On the other hand, when starting out from a corner, just butt it up flush, unless the manufacturer has scarfed it. In the field, many people recommend butt joints, assuming the joints are going to be caulked and painted anyway so visuals are irrelevant.

This is simply wrong. Using a scarf joint (45 degree angle) no offsets because of surface irregularities will be visible. Finish up installing your polyurethane crown moulding by caulking all your joints and nail or screw holes and paint.

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About the Author:

Photo of Kelly R. Smith and Frankie, Southern Black Mouth CurKelly R. Smith was a commercial carpenter for 20 years before returning to night school at the University of Houston where he earned a Bachelor’s Degree in Computer Science. After working at NASA for a few years, he went on to develop software for the transportation and financial and energy trading industries. He has been writing, in one capacity or another, since he could hold a pencil. As a freelance writer now, he specializes in producing articles and blog content for a variety of clients. His personal blog is at I Can Fix Up My Home Blog where he muses on many different topics.

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