Reinforcing floor joists with plywood is a straightforward solution for preserving structural integrity. Consider sistering: attaching 3/4-inch plywood pieces to both sides of the joist, full-depth, and at least two feet beyond either side.

There are many methods for sistering a joist, and not all involve dimensional lumber – but using the same sized lumber is the most common method. Plywood, OSB, and even metal flitch plates can also be used as a “sister” to an existing wood joist.