Q. How do I repair a sagging floor?
A. Sagging or sloping floors are often due to support beams that are deflecting or failing. Rotted sill plates or a weak foundation can also cause floors to sag. You can sometimes correct minor sags on the first floor using a pair of screw-type jack posts and a wood or steel beam to bridge the jacks. To do so, the posts need a very solid foundation to support them, such as a concrete pier for the jack that’s about 24 inches square and at least 12 inches deep poured over an existing concrete foundation. Once the concrete has cured and the jacks are in position, slowly raise the floor, as little as a quarter turn per week. Raising the jack in such gradual increments allows the structure to settle with minimal stress or damage to the building. However, keep in mind that lifting the floor—even slowly—might create cracks in the walls and alter the margins around windows and doors, requiring additional repair. Sagging beams caused by rot, water or insect damage should be inspected by a professional contractor.