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Arch Trim: How to Install Molding on an Archway

Arch Trim

Installing Arch Trim

You are likely reading this article because you have a window or doorway that you would love to trim with molding but you aren't quite sure how it can be done. There are several ways to get your project done and it all depends on the shape of the arch and the tools you have available.

If the arch you are wanting to trim is a true semi-circle (like on the picture to the right) then you can look at our Arched Molding department. If you can't find pieces that fit the size of your window then return to this page and continue reading. For our smaller polyurethane moldings you can use a heat gun to warm the material. For larger polyurethane moldings, or if you don't have a heat gun, you can cut the molding to give it the flexibility to fit around your arch.

Using a Heat Gun to Shape Arch Trim

An advantage of polyurethane trim is that it becomes more flexible when it is heated to a high temperature. When you are working with a small piece of trim (about 3.5" or smaller) you can use a heat gun to make it take on the shape of the archway. Once the molding cools, it will harden in that shape.
Trim Heat Gun
When using a heat gun, be sure you don't paint the trim until after it is installed. If you paint the trim before the installation the heat gun will make the paint blister. Also, if the heat gun is too close to the trim piece the coating over the polyurethane can also slightly blister. If this occurs, using a piece of sandpaper can easily remove any effects of the blistering.

This process works best on windows that have a slight arch and with smaller trim; if you are using a larger arch trim or have windows that are a semi-circle you will want to cut the molding as described below.

Cutting Trim to Fit the Arch

When you have a larger piece of polyurethane trim that you want to fit around your arched windows and doorways the best method is to cut the molding lengthwise to increase its flexibility. When a piece of trim is 4-5" thick it will have almost no flexibility, but when it is cut into pieces that are 2" or less it becomes much easier to flex it around an archway.

Arch Trim Profile When making your cut(s) it is important to cut where there is a visible line already in the trim (an area where there is a change in elevation of the profile). By doing this, you can easily caulk the line when you reconstruct the trim and it will look like one piece when you finish the arch. If you cut along a smooth area of the molding it will be difficult to hide the cut. The piece on the right is a 5" trim and the three dashed lines represent a good area for making the cuts.

Small Arch Trim
For pieces that are less than 4" a single cut is typically sufficient to make it flex around your window. Saw blades are typically 1/8" thick so when you make a single cut on the molding you are going to lose 1/8" of the material. The 1/8" difference can easily be caulked when you install the trim.

Large Arch Trim
When working with a piece larger than 4" you will likely need to make more than 1 cut through the molding to make it flexible enough to trim your arch. In the installation below a 5" piece of molding was cut 3 times to flex around the archway; with the saw removing 1/8" with each cut the final piece would be 3/8" less than the original molding. In that situation you can either caulk each 1/8" line or if you want a better finish you can use two pieces of trim to create the arch. Learn how to cut two pieces of molding to create an Arch Molding.
Arch Molding Cut
This picture is of the ET8751 trim (whose profile is shown in blue above) after making the cuts through the saw. The trim pieces are now thin enough to easily flex around an arch.

Arch Window Project: Installing a 5" Trim

For more details on how to install arch trim around your windows read about this Arch Molding Installation.
Arch Molding Project

In this installation the ET-8751 polyurethane molding was used to surround the window with 5" of trim. Two pieces were cut and reconstructed to create the arch above the "eyebrow window". Read step by step instructions on how the inflexible ET8751 became an Arch Molding.

Read more How to Articles.
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