How to Build an Interior Window Apron

Measure from the outer edge of the left vertical window trim molding to the outer edge of the right vertical window trim molding, using a retractable measuring tape. This is the apron length measurement. Many window aprons match the width of the framed window for a balanced appearance. Choose a different length if you do not desire a symmetrical look.

Measure one long edge of the apron molding to the apron length measurement and mark it with a pencil. Set one arm of a woodworking square against the side of the molding. Place its corner on the pencil mark, allowing its other arm to lie across the trim’s surface. Trace the edge of the square across the molding with a pencil to make the cutting line.

Place the apron molding on a sturdy work surface. Cut the trim at the line using a fine-tooth handsaw. Sand the rough-cut ends by hand using ultra-fine sandpaper.

Apply wood glue across top edge of the apron. Center it on the wall under the window and butt it against the bottom of the windowsill. Drive one finishing nail through one end of the apron using a hammer, securing it to the wall. Repeat at the other end. Insert more nails across the apron at 16-inch intervals. Let the glue dry according to the manufacturer’s recommendation. Insert more finishing nails through the top of the windowsill and into the apron approximately 10 inches apart.

Place the tip of a nail-set tool on the head of a nail. Tap the end of the tool lightly with a hammer to countersink the nail head into the wood. Countersink each nail in the same manner. Exposed nails show through wood stain, and paint can rust nail heads. If you will paint the apron, fill each nail hole with wood putty, wipe off the excess, let it dry and then paint it. If you prefer stained wood, home improvement expert Tim Carter of Ask the Builder recommends staining the apron before filling the nail holes to prevent wood discoloration. Fill each hole with wood putty that matches the stain and wipe off the excess. Apply a coat of sealer.