Popcorn ceilings were once popular because the spray-on texture hides imperfections in the ceiling. Removing the texture is time consuming, messy and possibly dangerous since some popcorn coatings contain asbestos. Lightweight expanded polystyrene ceiling tiles can be glued right to the ceiling over the popcorn texture, as long as the texture is hard and stable. If your popcorn ceiling is soft or crumbly, you'll have to scrape it off or install ceiling tile over wood furring strips.
Remove lighting fixtures, fans, vent covers and smoke detectors from the ceiling before you begin working. Turn the power off at the breaker box before removing any electrical fixtures.
Measure the ceiling and mark the middle of all four sides of the room. Snap a chalk line between opposite walls at the marks to find the center of the room.
Apply adhesive to the back of the first tile with a putty knife. Put dots of putty on the corners, sides and middle of the tile in a grid pattern. For a popcorn ceiling, use a generous amount of adhesive, about 1/2-inch -thick per dot, so it will sink into the texture, creating a sturdy bond, but not so much that it oozes out the sides. Don't spread the adhesive flat on the back of the tile.
Press the tile onto the ceiling at the intersection of your chalk lines, in one of the four quadrants. This ensures your tiles are being installed square and not askew. Press the tile firmly onto the popcorn so the adhesive squishes deep into the texture, but don't press hard enough to warp the tile.
Continue applying adhesive and sticking tiles to the ceiling until you have covered the ceiling with all the full tiles that will fit. Install the tiles right up against one another. When you apply a tile over a hole for a fixture, use a utility knife to cut through the tile and reopen the hole.
Measure the border between the full tiles and the edge of the ceiling to find the size for the partial tiles. Measure a tile to that size and mark the back. Cut it to size with a straight edge and utility knife. Dry fit the tile to make sure it fits and adjust if necessary before gluing it to the ceiling. Measure and fit each tile individually to ensure a perfect fit. Repeat the process until the borders are filled in with partial tiles.
Replace the light fixtures, fans, vent covers and smoke detectors and turn the power back on.