This story shows how to make a wavy ceiling flat.
Older homes, particularly those with plaster construction, may have cracked and wavy ceilings. If you install drywall directly to this surface, you'll still have a wavy ceiling minus the cracks. That's some improvement, but not enough to justify the effort.
The real solution involves the installation of furring strips -- either wood as shown here or resilient channels. Find the lowest spot on the ceiling and shim all the furring to that point. It's not as difficult as it sounds. The procedure shown here is a systematic method for locating the lowest spot on a ceiling, then installing the furring strips.
You may need a good number of shims to complete the furring strip installation, but you can easily make them at home from scraps of plywood and hardboard.
The time depends on the condition of the existing ceiling. For a room that measures 10x12 feet, you and your helper should allow 4-6 hours to check the ceiling and install the furring.
Stud finder, hammer, ladders, two chalk lines with different colors of chalk, mason's line, level, line level
Measuring and accurate marking, leveling, driving fasteners
Clear the room, and mark the location of all ceiling joists.
Shims, furring strips, nails, screws for installing strips
String a nylon mason's line between the first top nails on the two strips. Hold a scrap piece of strip next to the string at the furring strip centerline marked on the ceiling. If the string is below the furring strip, ignore it for now. If the strip projects past the string, measure the amount, and write it on the ceiling. Move the string 12 inches, and repeat the process until you've checked the entire ceiling surface. The largest measurement indicates the lowest part of the ceiling. If there are several spots with an identical measurement, you can start at any one of them.
Install each furring strip by first shimming its ends flush with the chalk lines on the walls. Using a level or straightedge along the bottom of the strip, secure its midpoint. At the remaining attachment points, gently slide in the shims so that you don't bow the strip, then nail through the shims into the joists. Work carefully, checking that each new strip you install is straight and level with the previous one.