We bought a ~50 year old house a few years ago. At the time we knew there was a few humps in the flooring, in particular over the beams. However since we've lived in the house for a while now, we've noticed we have sagging floors in a few spots throughout the house.
It occurs on both the main floor and the second floor (above). On the main floor, we see the biggest sag underneath our eating area and under our fridge. I've measured the sags (as best I can) and they range from 1/2 inch to maybe 3/4 inch sags (over 10-12 feet span). We've also noticed that essentially everywhere in the house when your near the outside frame, you can feel the floors sag right away. It wouldn't be noticeable to 75% of people, but I notice it.
We're debating doing a renovation of our main floor and trying to decide the best ways to spend our money. We have hardwood floors throughout the main floor and while there are areas where we will need to replace some boards, on the whole refinishing the hardwood will be much cheaper than refinishing the entire floor. Our basement is finished so hard to view joists without ripping out the ceiling.
My concern is the structure of the house might be flawed and is something that needs to be fixed, but I've talked to friends who are "handy" and they say they think it's unlikely. If I have to sister every joist in our house, my guess is it will eat up a lot of our renovation budget. Should I simply fix the sloped floors where possible and live with the rest? Or should I rip up the ceiling in the basement and inspect/fix the joists ?