How to Measure & Make a Window Treatment Board for a Bay Window

Measure the three sections of the bay window. Measure the outer sections from the angle where the center and outer sections meet, to the position of the outside edge of the window treatment. Measure the center section, from one side to the other, angle to angle.

Cut three pieces of heavy construction paper equal in length to these measurements and 5 inches wide. This is used to make the paper pattern for the wood.

Place the three pieces of paper on the floor in front of the appropriate window section. The paper will overlap at the angles where the center and outer sections meet.

Separate the overlapping paper at the left angle by moving the left paper to the left and the center paper to the right, keeping them snug against the baseboard, until only the outside tips of the papers meets. Looking down at the papers in this position, a “V” will have formed. Each arm of the V will be on one side of the wall angle; the bottom of the V is the position where the two papers meet.

Measure from the wall angle to the edge of the center paper. Remove this much paper from the other end of the center paper. Measure from the wall angle to the edge of the left paper and remove this amount from the outside ends of both outside papers. Position the right section paper, creating the V with it and the center section paper. Check that the outside ends of the papers are in the correct position relative to the outside edge of the windows and add or remove paper as necessary.

Label the papers left, center and right to correspond to the windows.

Cut a 1-by-6 board equal to each of these measurements and label each board.

Install angle brackets, or corner braces, above the window. The bottom of the board will rest on the top of the brackets. Each section of the bay will require a minimum of three brackets; one for the outside edge, one for the middle and one for the angle. Ensure there is at least one bracket for each 24 inches of board. Install the brackets that are nearest to the wall angle at least 6 inches toward the center of the section.

Lay the three boards on the work surface end to end. Tape across the joins to temporarily hold them in place while the window treatment is attached.

Attach the window treatment to one long edge of the joined boards. Typically, stationary valances are stapled to the top of the board, which in this example is a 6-inch surface, and hang down over the 1-inch edge.

Lift the “board” up and lay it in position on top of the brackets. Remove the tape. The boards will split apart at the angles, forming the V. This is what allows the treatment to accommodate the angles without mitering or special fittings. Insert screws up through the angle brackets into the bottom side of the board.