Expert: Dan Griffin - 2/7/2010

Question
what wrong with a floor when the ceiling shakes down stairs when u walk up stairs . you can see the ceiling shaking badly from down stairs.

Answer
Niva, it sounds like your joists are inadequate for the loads imposed.  This is a bit unusual, as codes have required minimum sizing for joists for many years.  Code does allow L/360 deflection, so that a room that is 16'wide can deflect slightly over 1/2".  A typical upstairs bedroom floor should have been designed for 30#/SF loading.  There are many things that could be marginal - joist size, joist lumber grade, joist spacing, lack of bridging, excessive floor load, remodel work that increased joist span, etc.

You can see by the possible causes that the correction will be difficult.  If your concern is safety or collapse, I don't think you have anything to worry about unless the movement you detect seems to be more than a 1/2" or you know about a huge load upstairs.  There are usually quite a  few warnings before any type of failure.  If your concern is stopping the problem, you will need to add joists, decrease span, or perform some other extensive work.  If this is what you want to do, I can offer a few suggestions.