The center of the floor? If it was a drop from one side of the room to the other I'd say it has to do with the house settling unevenly. But if it's in the center of the room it seems the joists have been under a lot of strain at some time, like something really heavy in the middle of the room, maybe a water bed. Also, if it leaked the joists would have gotten wet, allowing them to flex even more.
It seems to me the joists are bowed, in which case you may have to take up the floor and sister straight joists to the existing ones - not a small task for the average do it yourselfer.
This is just off the top of my head. You'll get replies from others pretty soon, who might have some better ideas.
[COLOR="DarkRed"][B]~ ToolGuy ~[/B][/COLOR]
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I have an idea..lets call it old home character. Old homes are prone to these issues says obvious-man. My suggestion would be to post some pictures or get a professional to look over what could be multiple issues. You could have pier settlement, termites, dry rot, leaks, or just plain undersized joists for what you are supporting.
I would have someone look at it, then tell us what they say....we can then help out with solutions. And also tell if you are being sold an fresh, clean, iceberg from Florida.
Very good point ToolGuy the only way I know to repair is tearing off the ceiling below and going from there. No way to really know whats going on till then but it sure sounds like under sized joists or heavy objects in the room . What is in the bedroom upstairs now? Is there any heavy items in the center of that bedroom and if yes how long has it been there.
this house has been a rental house for years alot of young people have lived here over the years a waterbed is a is a strong possiblity i think tool guy has the right idea but im still open for suggestions