Popcorn ceilings have a textured surface, which makes them popular to builders -- because the texture helps disguise imperfections on the ceiling surface. Popcorn ceilings used to be popular, but they're falling out of favor. If you want flat, smooth ceilings, remove the popcorn texture. You can do this quite easily, but it will be messy. You also need to ensure that the popcorn material does not contain any asbestos before starting any work.
Use a utility knife or a putty knife to scrape off a small sample of the popcorn ceiling and send it to a lab for asbestos testing. In older homes, the ceiling material could contain asbestos, which is toxic. If the test result shows that you have asbestos in your ceiling, don't attempt to remove the popcorn texture yourself. You need to get a professional to do it for you.
Turn off the electricity in the room where you want to remove the popcorn ceiling. You will use water to flatten the ceiling, so electricity can be dangerous.
Move all the furniture out of the room to prevent dirt, debris and water from leaving stains or causing damage. Also, remove light fixtures to flatten the entire ceiling and to not leave textured patches from attached light fixtures.
Cover the walls with large sheets of plastic and lay drop cloths all over the floor to prevent the debris from sticking to and damaging the floor and walls. Use painter's tape to keep these covering materials in place. To make cleanup easier, place a layer of resin paper over the drop cloths on the floor. The resin paper will absorb the moisture and keep the working surface from getting too slippery.
Apply water to the entire ceiling using a garden sprayer. Thoroughly wet the ceiling, in 4-foot-square sections at a time, to loosen the popcorn texture so you can remove it easily later. Go over a spot several times if you have to, but don't worry too much about covering every inch, because you can always add more water later.
Scrape off the popcorn texture using a floor scraper or any other tool with a wide blade. If the popcorn texture feels hard in any one area, apply more water and try scrape it off again. The popcorn texture should come off easily.