Over time you may notice that your floors are starting to have an angle to them. When you place a level on your floor, you may see that your floors are sagging. To level sagging wood floors, you need to find the originating area of the sag, and then add support to it. Sagging floors are the result of ageing floor joists, and you can find them by getting underneath the floor and finding where the joists are bowing the most.
Examine the floor joists from the basement to identify the joist that is the most bowed; this is the joist that is causing the sagging.
Find the centre point of the sagging joist and place a concrete block directly under the joist at that point.
Use the middle concrete block as a starting point and determine the point on the left side of the joist that is equidistant from the wall and the centre block. Place a concrete block on the floor of the basement under this point.
Repeat the measurement for the right side of the joist. This will allow you to evenly distribute the weight among the supports you will be putting under the joist.
Place a floor jack support on each of the concrete blocks and in place up against the joist. Only raise the jacks enough so that they stay in place. Do not completely raise them yet.
Place a level in the middle of the floor and start raising the support jacks slowly. According to Hammerzone, you should only raise the floor 1/8 of an inch per day to avoid damaging the floor boards and your walls.
Continue to raise the floor a little each day until it is level.
This may only be a temporary solution. Have a structural engineer look your home over to rule out a much more serious problem, such as a cracked or sinking foundation, or dry rot in the floor joists.