Things You Will Need
Reinforce your stairs without damaging the visible parts by working from underneath.
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Look at the underside of the stairway using a flashlight to find obvious problems. If the underside is finished with drywall or boards, you have to remove them to reach the stairway's framework. Cut drywall along the edges using a utility knife, and pry it away. If there are boards in the way, pry them off with a small crowbar.
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Examine the framework of the stairs. The stringers are the diagonally positioned boards that support the treads and risers. Treads are the horizontal pieces that you step on when you use the stairs. Risers are the pieces that cover the vertical spaces between the treads. All of these elements need to be firmly connected to create a sturdy stairway.
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Move a stud finder across the walls underneath the stairs to find the studs. Mark the location of the studs with a pencil. If you don't have a stud finder, locate the studs by tapping a nail through the drywall, making a horizontal series of holes until you hit a stud.
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Drive 3 1/2-inch-long screws through the stringers that are located against a wall at each point where a stud is. If the screw spins after it has gone all the way into the stringer, you've missed the stud.
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Spread glue along two adjoining sides of a piece of wood, and place it into a corner where the underside of the tread meets the top of the riser. Repeat at each step.
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Test the fit of any shims that you find between the risers, treads and stringers by trying to wiggle them sideways. Many staircases have shims in them to tighten and adjust components. When you find loose shims, use a hammer to tap them in until they are tight.