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Magnetic torpedo level
Both the ceiling panels and the grid work that supports a suspended ceiling can sag over time. Ceiling panels often sag after coming into contact with moisture, which can weakens the bond between the fibers. The design of a suspended ceiling allows a sagging panel to be removed and replaced to fix the problem. On the other hand, sagging grid work is often a case of loose supports and isn't repaired as easily.
Erect a stepladder directly below the lowest part of the sag in the suspended ceiling.
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Push up on a ceiling panel located next to the sagging section of grid work. Tilt the panel on a slight angle. Slide it through ceiling grid. Prop it against a wall that sits away from the work area. Remove all panels from the sagging section of grid work.
Move the ladder to allow access above the ceiling grid. Locate the support located nearest the low-point of the ceiling sag. Place the magnetic side of torpedo level against the flat surface of the ceiling grid that sits parallel to the floor.
Untwist the lower portion of the support wire by turning it counterclockwise with a pair of lineman's pliers. Push up on the ceiling grid until the torpedo level shows the grid is level. Pull the shorter end of the support wire through the grid until no slag remains on the long length of wire connected to the support joists of the house. Pull the short section of the wire over the longer section to lock it in place.
Read the level. Continue adjusting the level until the sag is removed. Twist the shorter section of support wire over the longer section in a clockwise direction with the lineman's pliers until secure.
Set the ceiling panels back in the ceiling grid.