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30-Minutes with Workroom Tech / Episode 26: Basic Cornice Board Construction

On Episode 26 of The Sew Much More Podcast: 30-Minutes with Workroom Tech, Ceil DiGuglielmo and Susan Woodcock talk about planning and making cornice boards. You can listen to this episode of the podcast here:

The Basics

A cornice board is a style of top treatment made with a wooden base that is upholstered with padding and fabric. Before you begin construction, sewing, and stapling, you must measure the window and plan the design.


A cornice board will have legs, or vertical supports made from boards on each side. Be sure to allow enough clearance on each side, to clear the window frame, and enough projection to clear blinds or other hardware that might be at the window. The inside width will be 1.5-inches less than the outside width measurement. Susan suggest planning for at least 2.5 inches or greater than the window trim for the finished outside width.

sketch shows the inside and outside measurements

Look at magazines, books and online albums for design inspiration. This video from Stevenson Vestal is filled with great ideas for shapes, and embellishments.

Your client may have shown you the design or style, or you may be creating this on your own. Sometimes the fabric will inspire a design. "I like to draw it out on graph paper", Susan said. "I will then enlarge the small drawing onto gridded pattern paper, which is marked in one-inch squares (source: Rowley Company). Or, you can use craft paper, or any other paper that you have on hand.

"A style with a scalloped bottom design will need to be adjusted if there are different size windows in the room" Ceil shared. "Some designs work out better than others".

Susan added, "That’s a really important thing to think about. If you have any influence on the design, having a long run in the center will make it much easier to adjust. You can point that out to the client." Use pattern paper to experiment with shapes and styles before cutting the wood. You can tape the paper pattern up on the window.

sketching cornice board designs to scale

Construction Materials

The face of the cornice can be made from OSB (oriented strand board) or plywood (1/2” in thickness). Use 1" x 4" or 1" x 6" lumber for the top piece (dust board), and side pieces (legs). The size lumber used for the top and sides depends on the projection needed, and can be customized.

Another product used in cornice construction is FirmaFlex, a dense, polyester fiber board sold by Rowley Company. FirmaFlex is lightweight, and easy to cut and join together. "I will often use FirmaFlex for the face, and wood for the frame, or on arched cornices, wood for the face, and Firma-Flex for making the curved frame", Susan shared.

Building the Cornice

Basic construction of the cornice "box":

  1. Cut a top board the finished width

  2. Cut the end boards the finished length minus the top board thickness (usually ¾ inches).

  3. Trace the design onto the wood and cut using a jigsaw.

  4. Build the frame by nailing or screwing together the top and side boards, and then attach the plywood face.

cornice board pieces
putting together the cornice board
attaching the face piece to the cornice board

Ceil asked, "do you pre-line the boards"?. "It depends on the project", Susan said. "If it’s a straight cornice, where I will not need to make relief cuts then pre-lining is a great idea! But for a shaped cornice, I will still have to cover all those edges, so I might just pre-line the boards, and not the plywood."

Susan recommends using blackout or dimout lining because it will hide stains or discolorations on the wood, and it doesn’t fray.

Cornice Padding

After the wooden cornice is constructed, it is covered with padding. The most common padding to use is polyester batting. Susan prefers a low-loft, flat batting. "I have used bump interlining for padding. But that’s my personal preference. For a more rounded, softer look use a thicker, or fluffier batting", she said.

One drawback of a highly padded surface is that it can show dimples and pulls in the fabric. To create a smooth surface, pre-line by stretching and stapling a cover of interlining or lining before adding the face fabric. Covering the padding with lining is also helpful when using lightweight, thin or delicate fabrics.

Apply the padding to the wood with spray adhesive. The padding is on the front of the face and side boards, and does not wrap around the edges. Trim away excess padding even with the edges.

trim the padding even with the wooden face of the cornice board

Adding Fabric

Join fabric together for larger cornices, matching patterns and pressing seams open. It's best not to have a seam in the center of the cornice board. Starting in the center, place the fabric over the cornice and secure the top and bottom to the edges with pushpins. Smooth the fabric, pulling gently and securing on the center of each side. Make sure the pattern motif is centered on the front and running even across the top.

Continue with push pins along the top and sides. Pull the fabric taut and even but not too tight. Staple the sides and top first, leaving room at the ends for finishing the corners. Staples can go into the edge that is against the wall.

staple to the side and top boards first along the back edge that will be against the wall

Flip the cornice over and staple the bottom edge from the reverse side, removing pushpins as you go. You can work in sections starting at the center, pinning, clipping and stapling then continue across to the left and right. You will need to make relief cuts for curves and shapes.

make relief cuts to fit around curves and staple to the back of the cornice board

Finishing Details

Cover enough welt cord to go across the bottom edge and returns. Cut the fabric for the welt cord on the bias; especially for cornice boards with curved designs. Sew the welt cord to an additional 4-inch wide bias strip. This creates a finished welt cord with enough lip for stapling with no visible stitching.

sew welt cord to an additional strip of fabric

glue the cording to the bottom edge of the cornice board

To apply, Susan uses hot melt glue (which sets immediately), or fabric glue and then pin in place until it is dry. After the welt cord is applied, staple the cut edges on the back.

cover the back with blackout or dimout lining

Cover the back with lining. Use a sharp pencil to draw along the edge of the shape. Cut above the line and glue gimp braid to the bottom, cut edge of the lining.

draw around the bottom edge and then cut above the line
trim the lining above the line

Applying Trim

Trim is most often applied with glue, or an adhesive tape. "Sometimes you will want to hand sew tapes and trims so you can ease them around curves, so it might be a combination of stitching and gluing", Susan said.

Learn more about applying trims in episode 8 of the podcast, and see the

Workroom Tech blog for detailed information. Click here: Applying Trims

finished cornice board with fringe


To install cornice boards, mount L-brackets to the wall, set the board on top and screw the brackets to the board. On inside mounts you can screw right up through the board. A cleat may be needed to install cornice boards if brackets can't be used.

Best tool for cornice board installation: an extra long, magnetic bit holder for your drill.

Be sure to consider the installation before construction of the cornice box. A shallow return may be difficult to install if you can't reach under the cornice with the drill. If you would like to learn more about sewing window treatments and making cornice boards, instructions can be found in Susan's book

Singer(R) Sewing Custom Curtains, Shades and Top Treatments.

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