A dip or slope in the floor could indicate one or more saggy joists beneath the subfloor. The method used to fix the problem depends on the reason the joist is sagging and the number of joists affected. Typically, joist sag is the result of the floor joists spanning too great a distance, but occasionally, an individual joist may sag due to rot, breakage or a structural problem in a slab foundation.
Fixing Span Problems
Joists, positioned on-edge, span the distance from one side of the home to the other, resting on the foundation’s sill plate. When the distance is too great for the joists to remain firm and straight, one or more joists begin to sag in the middle. The standard remedy is to install a cross beam in the crawl space that runs perpendicular to the floor joists. The process involves pouring concrete pads beneath the location of the beam, and then using a series of house jacks to raise and support the cross beam. This is a job for a foundation contractor. An engineer must specify the dimension of the beam and the number of jacks required to support the beam.
Proceed With Caution
Once a support beam is in place and snug against the bottom of the floor joists, it’s tempting to crank the house jacks and push the sag out of the floor. That’s not a good idea. The joist or joists sagged over time and they are still rigid boards. When pushed too far too fast, the ends of the joists, which are higher than the sagging centers, push upward, creating gaps at the edges of the floor and cracking the drywall in the rooms above. The safest way to remove the sag is to raise the jacks no more than ¼ inch per week.
Fixing a Damaged Joist
In the case of a single saggy joist, which could be the result of insect damage, a crack in the joist or rot, the simplest way to fix the problem is to install one or two new joists on either side of the sagging one. Called “sistering,” this involves cutting a new joist to the same dimensions as the affected joist and installing it right beside the original joist. A knowledgeable homeowner can do this, since the saggy joist remains in place. It can be difficult, however, to maneuver in a restricted crawl space. If the space does not have window access, you may have to cut out a section of the foundation to get the new joist(s) into the crawl space.
If a floor joist is sagging and your home sits on a slab foundation, there could be a major structural problem. In a slab-on-grade foundation, the floor joists sit on the slab itself, so sagging indicates a crack or settling of the foundation. Repairs could be extensive and expensive, involving stabilizing or lifting the existing house frame, breaking out the settled concrete and re-pouring part of the slab. If the problem is limited to a small area and no drainpipes are affected, a slab jacking contractor may be able to drill holes and inject pressurized concrete beneath the slab to raise or stabilize it. New joists can be installed only after fixing the slab.