Floors of older homes, and some that are not so old, occasionally sag. The sag is not caused by failure of the floor material itself but instead by the shifting of the floor joists. Joists are the framework below the floor that supports the floorboards or plywood flooring. Raising a sagging wood floor requires some specialised tools and equipment, and it may be beyond the capabilities of some novice do-it-yourselfers.
Determine the slope of the sagging wood floor. Floors that sag to the middle of the house lack support under the centre line of the home and are good candidates for repair. A floor that sags or slopes toward one exterior wall of the home may indicate a failing foundation. In these cases, repair is more difficult and is probably beyond the capabilities of the standard do-it-yourselfer.
Locate the low point of the floor joists, the wood cross-members that support the floor materials, in the basement or crawlspace under your home.
Place a beam perpendicular to the floor joists along the lowest part of the sagging joists.
Place jacks (either hydraulic bottle jacks or floor jacks) under the beam. Use the jacks to create enough upward pressure on the joists to hold the beam in place. Place a jack about every 6 to 10 feet along the beam, depending on the amount of the sag in the floor.
Slowly add additional upward pressure with the jacks. The experts at This Old House recommend that you do this over the course of several weeks. Watch the area where the floor joists rest on the foundation near the ends of the beam. If the joists are beginning to lift from the foundation, too much upward pressure is being applied too quickly.
Monitor the level of the floor. Once the floor is level, replace the jacks with wood or steel posts in order to support the floor and prevent further sagging.
The slow upward pressure is critical to the success of the floor levelling project. The sag occurs when the floor joists bend or bow under their weight. The upward pressure forces the joists to return to their original straight configuration. Bending wood takes time, and too much pressure can damage the home's foundation or floor joists.
Working in a crawl space under a home can be difficult, time-consuming and uncomfortable. All the work is done while crawling through dirt and may involve exposure to insects and vermin.