how to build cornice window treatments

How to Build Cornice Window Treatments

By Amanda WilsonMarch 9, 2026

Elevate your window decor with the timeless elegance of cornices. These architectural elements not only add a touch of sophistication but also serve functional purposes like hiding hardware and providing a smooth transition between walls and windows. Here's a comprehensive guide on how to build cornices for your window treatments.

Before we dive into the process, let's understand the types of cornices. The most common are flat cornices, which are simple and easy to install, and box cornices, which offer a more intricate, three-dimensional look. We'll focus on the flat cornice, perfect for beginners and offering a clean, classic aesthetic.

Gathering Materials and Tools

To create your flat cornice, you'll need a few basic materials and tools:

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MDF or plywood (1/4" or 1/2" thick)
Crown molding (optional, for added detail)
Wood glue
Nails or brad nails
Miter saw or miter box
Jigsaw or circular saw
Tape measure
Pencil
Sandpaper (120-grit)
Paint and primer (optional, for finishing)

Measuring and Cutting the Cornice

Start by measuring the length and width of your window. For a flat cornice, you'll need two pieces: one for the horizontal top piece and two for the vertical side pieces. The top piece should be the width of your window, and the side pieces should be the length, minus the thickness of the top piece (if using 1/2" MDF, subtract 1").

Using your miter saw or miter box, cut the pieces to size. If you're adding crown molding, cut it to fit along the top edge of the top piece and the side edges of the side pieces.

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Assembling the Cornice

Lay out your pieces and apply wood glue along the edges where they'll meet. Align the side pieces with the top piece, ensuring they're flush with the top and bottom edges. Nail the pieces together, using a nail set to sink the nails below the surface if desired. If using crown molding, attach it now, nailing it into place.

Allow the glue to dry according to the package instructions. Once dry, use sandpaper to smooth any rough edges or surfaces. If desired, apply primer and paint to finish your cornice. Allow the paint to dry completely before installing.

Installing the Cornice

Before installation, ensure your window frame is clean and free of debris. Measure and mark the installation points, usually every 12-16 inches along the top and sides. Use a level to ensure your marks are straight.

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Attaching the Cornice

Hold the cornice in place, aligning it with your marks. Use a nail or screw through the top piece into the window frame or wall studs. Ensure it's level and secure. Repeat this process along the top piece, then do the same for the side pieces. Use a level to ensure the cornice is straight as you go.

Once the cornice is securely attached, fill any nail holes with wood filler, sand smooth, and paint if necessary. Your window is now adorned with a beautiful, functional cornice that adds a touch of class to your space.

Embrace the satisfaction of a DIY project well done. Your new cornice not only enhances your window treatments but also reflects your creativity and craftsmanship. Happy decorating!