Tools, Materials, and Methods
At least two different methods should be considered for leveling your floors. If your floors are moderately out of level, then you can use floor compound--white powder that is mixed with water and spread out onto the floor to make it level-- to even out those sagging spots. However, if your floors are sagging so much that doors won't close and walls are cracking, then raise the floor from beneath a floor jack and block of wood will do.
You will need floor leveling compound, a 4 foot level, nylon string,a hand trowel, a 5- gallon bucket, a drill and mixing bucket, and sandpaper for leveling your floors using floor leveling compound.
You will need nylon string, a 4 foot level, a tape measure, floor jack, 8 inch concrete blocks, short scraps of 2-by-8 block that are at least 16 inches long, a hammer, and wooden wedges for jacking and leveling the floor from beneath.
Find the low spots in the floor, and stretch a nylon string across those spots; attach the string to the high points of the floor on both sides of the low areas. Use the string as a reference when spreading and smoothing the floor leveler. Make sure that the floor is clean before you begin applying the floor leveler. Add water and leveling powder to a 5-gallon bucket and mix using the drill and mixing bit. Spread the compound until the floor is level with the reference string. Allow the floor to dry and sand away any imperfections with course sandpaper.
Jacking the Floor
Raise your floors until they are level using a floor jack. Stretch a nylon string across the floor and attach it at the high points on each side of the floor at the walls. Have a helper stay inside while you go beneath and jack the floor from beneath. Place the floor jack on a supporting beam beneath the floor and jack the floor until it levels above. Place 8 inch blocks on level ground, followed by wooden 2-by-8s, and then wedged.