How to Relevel a Sunken Floor

Determine where the floors supports are. Is there a crawl space or basement below the floor? If there is a crawl space, then you will have to work with the jacks while laying down. If it's the basement, then jacks while standing up.

Put a level on the floor in several different positions in the affected room to determine which way the floor is leaning.

Check the perimeter of the room to see if there are any additional areas that are out of level. Draw on some paper a floor diagram and indicate what direction the floor is leaning. This will help a lot when the jacking begins.

Get the jacks under the floor. Generally there are posts and beams under the floor joists. If a pier that the post is standing on has sunk into the ground you will need to lift the beam off the post and replace the post with a taller one.

Place a couple of two foot long pieces of 2X6 lumber side by side on the ground under the beam and place a 3/4" plywood 1'X1' on top of the 2X6's. Place the jack (some people prefer at least 25 ton hydraulic or a house screw jack) on the plywood and then use a piece of thick hardwood between the jack's ram and the beam to prevent damage to the beam. If two beams meet on the post that is sunk then a jack on both sides of the post is required.

Begin jacking up the beam or beams and make sure you jack equally with two beams. Check the floor regularly to make sure you do not over level it.

Look for low areas. Once the floor is brought to level you check all around to see if there is any other areas that are still low. This is done by placing the level on the floor in several different positions and areas as before.

Remove the old post and measure the distance between the beam and the pier. Cut a replacement post of the same dimension lumber that was removed. You can go larger lumber but never smaller. But the best is to go with the same dimension. Also it is not a good idea to try to raise the pier block up by placing more dirt under it and then putting the old post back in. You aren't going to be able to compact the soil enough to keep it from sinking back down under the weight of the house.

Place a piece of tar paper on top of the concrete pier and the new post on top of the paper. This will ensure moisture doesn't come up through the concrete and get to the wood. If the reason the floor sank is due to a rotten post and there was no tar paper you can now see what happens. Now put the weight of the house on the post. Keep it lined up with the sides of the beam. Check the floor for level again to make sure you're still good.