Foundations settle over time. Even brand new houses experience some movement of the foundation. The question is, how do you level a floor that sinks toward the center of a home? And how much of this can homeowners do? Of course, this probably isn’t something you should attempt on your own with zero experience; however, having an idea of what goes on can help you make the best decisions for yourself and for you home.
Consult a qualified Structural Engineer first. The structure may not be as you expect and you may cause more problems than you solve.
Determine how far the middle of the house needs to be lifted. With a taut string, water or laser level, make a straight line from one side of the foundation to the other. You may find this task easier if you hold the string off the sagging floor joists by attaching blocks of the same thickness at opposite sides of the basement. Nail them to the underside of the joists as near the foundation as you can. Measure the distance from that line to the bottom of the floor joists. Repeat this process in several locations under the house.
When you know where the lowest portion of the center is located, mark that location.
Build a temporary post using concrete blocks or large blocks of wood. Make sure that the post is located on firm, level ground under the house. Alternate the placement of each layer of blocks by 90 degrees to improve stability.
Buy or rent a hydraulic jack. When the space above the temporary post is adequate to place the jack and reach the structural member above, start jacking the house up. As the house moves, add smaller blocks to support the center of the structure at the new elevation.
When the lowest point has been raised to the height of the next lowest points, remove the jack carefully, allowing the house to rest on the temporary post.
Check for problems that may have been created by movement of the house. (See Warnings below.) Sheetrock cracks may appear.
Build another temporary post and repeat the process.
When the floor is level and stable on the temporary posts, it is time to prepare for permanent supports. Existing supports may be usable with minor modifications. Check the bottom of the supports for rot or other signs of trouble. If the bottom of the posts are no longer sinking, they can remain in place. By cutting the center supports shorter, a new beam can be placed on top of the shortened posts to support the center of the house.