My advice is to hire a competent carpenter and fix the stair before someone gets hurt.

Hi Angie1010,

I know it's not the answer that you want but I agree with Renovator8. You're going to have to hire a professional to fix this for you. Trust me, I had the same problem and tried to fix it with screws. Needless to say, the stairs gave out on me and I fell hard and got bruises several times.

It cost me about $100 per stair to fix but this was many years ago.

How old are the stairs? The first thing you should probably do is identify the cause of the failure.

A stringer is what supports the stairs. There are usually 2 attachment points.One is at the top and the other is at the bottom, check there first, the bottom could be rotted. Another problem could be how the stringer was constructed.Is it a 2 x 4 with pieces scabbed on (triangular pieces nailed to a 2x4) or is it a 2 x 10 cut properly. More info

There are a number of ways to construct stairs, and that likely to determine what needs to be done.

Are the treads bedded into the stringer in grooves (ofte3n then held with wedges, or are the stingers cut to profile on the top and the treads sitting on the 'steps' cut into the stringer?