Q: What's the best way to fix a floor that sags? – Billy, McDonough, Georgia

A: Tom Silva replies: A sagging floor almost always means the structure below it is sagging. In that case, it's important to know whether the sag is in the center of the room or around the edge of the room. It could either be that your foundation is drooping and your whole house is sagging, or it could mean — especially in an old house — that the floor joists have been there for a long time and gravity is pulling them own.

I have to warn you that fixing a sagging floor can be expensive and difficult. If you can put a beam up with some posts from below across the center of the room, you may be able to lift the floor into place. But your floor didn't sag overnight, it sagged over many years, so you have to fix it gradually, too. If you push it up too quickly, you can cause problems on each end of the floor. It might take a few weeks, but if the floor is built into solid structure on each end, you can push it up. Using hydraulic jacks — we typically use 20- to 40-ton jacks — push the floor up some in the morning, and wait a little while. Later on in the day, push it up a little more. A little later in the day, push it up a little more. Any time you jack or lift or try to push something back into position, you run the risk of cracking something, somewhere, so go slow. When you get the floor back into position, you just reinforce it with beams underneath. You should probably consider seeking some professional help for this project if you decide to undertake it.

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