A sagging or bouncy floor is can be the source of much aggravation for homeowners. It can also pose a roadblock to new floor installations such as laminate, hardwood, or tile. If the sagging floor is in the middle of the room, it can be repaired using the following steps.
Step 1: Inspect the Floor
Before the floor can be repaired, it's important to know what is causing it to sag or bounce. In some cases the problem may be obvious from the surface (loose nails, cracked boards, and so on). But most likely, the problem lies below. This means you will need access to the underside of the floor. Use a ladder and flashlight to look at the sagging area. Examine any joists or beams running underneath the floor. It is not uncommon for these to sink over time from supporting the weight above. Remember, most floor joists and timbers not only hold the floor, but also share in supporting some of the weight above it.
Step 2: Level the Floor
If the problem is a support issue from underneath, you will need to reinforce the existing joists and other supports. To do this, you must first level the floor. Using a hydraulic jack, place it under the sagging joist(s). Lift the jack up until the floor is level. Place a level on the joist to determine where level is. You may need more than one jack if the floor joist is really long. Placing floor supports under the joist will help hold the floor and add safety.
Step 3: Floor Joist Reinforcement
With the floor level, you must now reinforce the joists. To do this, cut a 3/8-inch (or greater if added support is needed) plywood the width of the floor joist. You may need to cut several small pieces to run the length of the joist. Attach them using a hammer and nails. Now cut the 2x12 board. As with the plywood, run the board the length of the floor joist. Hammer it to the plywood. Now using longer nails, hammer into the 2x12 board from the joist end. This will fasten the joist, plywood, and 2x12 board together. By running the boards together along the same length, they share in the above weight load.
Step 4: Checking Strength
After the joist(s) have been reinforced, it's time to check your work. Remove any floor supports used. Slowly release the pressure on hydraulic jack and let it down. Now check to see if the floor joist remains level. Using the level, run it along the length of the joist. If it looks level, you're done. If not, find those areas where the joist is still sinking or uneven. Check to see if there is any noticeable change. If so, the supports are helping, but may need added strength. You can attach more plywood and board or a metal plate may need to be installed.