Running beneath the floor of your home is a series of pieces of lumber called joists. Joists support the floor, preventing squeaks and cracking. Uneven joists can cause the subfloor to come loose, causing all sorts of problems. Extremely buckled floor joists may indicate an issue with moisture. If all the floor joists are coming loose and sagging, call out a contractor to make sure there isn't major structural damage. Otherwise, you can try to fix uneven joists.
Measure the length of the sagging floor joist using a tape measure.
Rip a piece of lumber that is the same length as the joist. You can use two pieces of lumber if the joist is very long.
Apply wood glue to the piece of lumber and press it against the joist. Butt the top of the lumber against the floor above.
Clamp the lumber in position.
Pound 12d nails into the joists every 16 inches.