How to Install Arched Window Trim

What You'll Need
Arched window trim (stool, 2 side trims and the arch)
Measuring tape
Circular saw
1-inch nails
Nail gun
Construction adhesive
Saw blade

Of the many different types of windows, the arched window is the most sophisticated and elegant design of all. While it may seem difficult to install trim on a rounded window, you can actually do it with ease and precision. Follow the simple steps below to install the basic “half round” design of trim to your arched window.

Step 1: Pre-Installation Considerations

The radius of the semicircle within a half round is exactly half of the diameter of the semi-circle—this measurement makes it different from elliptical arches. When you are purchasing the arched molding, remember to measure the width of your window to find out the desired diameter of the arch. Make sure that the trim is flexible and not rigid. Before installation, paint and stain the wood as desired.

Step 2: Placing the Window Stool In

Start by placing one of the side trims against the side of the window. Use a pencil to mark the point where it ends. Do the same for the other side trim, and mark the measurement. Measure the distance between the two pencil marks, and add 2 more inches to determine how large the stool should be. The stool is the bottom edge of the entire window frame. The manufacturers should have provided you with the wood for the stool, which often has extra length.

Use a circular saw to cut the stool to the length determined earlier. Now place the stool in the stool gap below the window, and use 1-inch nails to fix it in.

Step 3: Fixing the Side Trims

The arch usually comes in 2 identical pieces that are to be joined to form an arch. Place one of the halves of the arch onto the window frame where it is supposed to be attached. Mark the point where the arch is ending and the side trim is to start. Now place one of the side trims onto the stool, perfectly parallel to the side of the window. Using the mark you made, trim the extra length with a circular saw. Do this for both of the side trims. Use a 1-inch long nail (or longer, depending on the trim’s thickness) to tack the side trims in on both sides.

Step 4: Attaching the Arch

Attach one of the halves of the arch by applying construction adhesive between it and the side trim. Follow with the nail gun to firmly tack it in. Repeat the process for the other half. Also use construction adhesive between the 2 arch halves. This process, however, is possible if the arch is less than 4 inches thick as it is flexible and can easily be made to fit the window arch. If the arch is thicker than 5 inches, cut across the length of the arch molding with a saw blade. The thicker it is, the more longitudinal sections you will have to cut out. Eventually, tack the flexible arches in place with the nail gun, one longitudinal section at a time, and stack them onto each other.