There are guidelines in interior decorating that deal with what to do with an unwanted or unattractive architectural element. These guidelines suggests that you can ignore the element, remove it or camouflage it. A crooked ceiling presents a real challenge in that it can't be removed without substantial cost, and it may be very difficult to ignore. The remaining solution -- camouflage -- is actually the simplest solution and is within the skill set of an advanced-level home decor renovator and decorator.
Choose a wall color that is as dark as possible for the decor scheme, and paint the ceiling the same color. The camouflage trick with this solution is that the same color on both surfaces creates a lack of contrast between the ceiling and wall. This blurs the ceiling line, which is the reference line for the perception of the ceiling being straight or crooked. A dark color on both surfaces also will increase the perceived height of the ceiling and make the walls appear further away, making the room appear larger, and the crooked lines of the ceiling less noticeable.
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Use an eggshell or flat paint finish. These finishes reflect less light than glossier surfaces, thereby reducing the shadows created by an uneven ceiling surface. Walls made for stage plays are most often painted with a flat paint because the "flats", (the actual removable walls), are re-used and have many uneven layers of paint, resulting in light reflection that amplifies the uneven surface.
Remove any in-ceiling light sources, such as pot lamps, hanging lamps or flat fixtures. Keep any suggestion of straight lines away from the visual plane of the ceiling. If there are no definite horizontal lines in close proximity to the crooked ceiling, there will be fewer straight lines for comparison, and the ceiling will appear less crooked.
Direct any light sources to shine away from the ceiling. Camouflage can mean drawing the eyes elsewhere. Therefore, direct lighting to important wall art, a tablescape or a reading chair.
Choose window coverings the same color as the walls to reduce visual contrast, and do not mount the drapes' top edge less than 12 to 14 inches from the ceiling. Paint exposed hardware the same color as the wall. The strong horizontal line of the drapery rod will only enhance the perception of a crooked ceiling if it is not color coordinated and part of the camouflage scheme.
Remove crown moulding, if applicable. Crown moulding also can create a strong horizontal line that should be avoided.