How to Reinforce Floor Joists. Floor joists are the slats the run underneath your flooring to offer support. Whether you are looking to reinforce a floor that has begun to sag, or you are simply trying to prepare a space to hold a heavy item, reinforcing the floor joists is a necessary step to keep things on the level.
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Gain access to the floor joists. In unfinished basements, floor joists are not difficult to access, but if you have a drop ceiling or something similar hiding the floor joists, you need to remove the paneling, remove the existing ceiling, or gain access from the floor up above. If the floor is sagging, you must gain access to the floor joists from below.
Assess the damage. Once you have a clear view of the problem area of flooring, figure out what needs to be repaired. If the floor is sagging, determine where the main problem areas are. Figure out which joists are sagging, and if there's any rot that is contributing to the problem.
Measure the length of the floor joists, their width, and their distance from each other. Draw out a diagram of the existing floor joist structure as a reference. Write down the measurements on the diagram to give you an accurate record.
Reinforce the floor joists with sturdy wood or steel plating. Do this by screwing a steel plate or a piece of wood of the same length into to the original joist as well as to the cross beam.