What to do about a sagging 1st floor?

troffasky

The 1st floor under a partition wall between two rooms is sagging somewhat. On a run of skirting board about 2400mm, the difference in the gap in the middle and at the ends of the skirting is about 20mm. I only have a metre rule but the skirting board seems pretty straight. The rest of the upstairs doesn't suffer from this problem. I suspect a wall beneath the problem area was taken away about 10 years ago but I can only guess as to the wall's construction.
I started investigating because sat at my desk next door, the lamp wobbles when my 5yo moves about in his bedroom.

So what to do about it? My guess is that I need to cut away the plasterboard from the ceiling and support the joists with a steel beam, which itself would be supported by ...something...?

Aylesbury_Duck

You don't need a longer rule. Pin a length of taut string at each end and use that as a level from which to measure the drop. If you have a spirit level, use it to check the string is horizontal first.

As to the drop and its possible causes, get a professional in to take a look. If there's a chance that someone previously removed a supporting wall without putting in the necessary reinforcement, you don't want to start poking around behind plasterboard.