Although some people consider plywood inferior to standard wood, it is generally the stronger of the two types of wood. Plywood is a laminate that is formed using several thin layers of compressed wood glued together. If a joist beam cracks or sags, create a "sister joist" to strengthen it. Use a hydraulic jack to lift the beam and then secure pieces of plywood to the sides of the beam.
Measure from the bottom of the floor to the bottom of the joist beam with a tape measure. In many cases, the measurement is between 6 and 8 inches. Measure the length of the beam with the tape measure. This measurement will vary.
Cut four strips of 1-inch thick plywood with a table saw to the width of the first measurement. Cut plywood as long as possible.
Drill two evenly spaced rows of ¼-inch pilot holes every 6 inches down face of all four plywood boards.
Place a hydraulic jack onto a 6-by-6-inch block of wood directly under the sag or crack in the beam. Measure from the bottom of the beam to the top of the hydraulic jack. Cut a piece of 4-by-4 lumber long enough to match this measurement using the table saw.
Place the 4-by-4-inch board between the beam and the jack. Jack up the joist beam, with the 4-by-4 between the beam and the jack. Jack up the joist by no more than ¼ inch per day until the beam is straight. Jacking up more per day can damage walls or drywall in the house.
Apply construction adhesive on the face of one plywood board and place the face of the board against the side of the joist beam. Place construction adhesive onto the face of the next piece of plywood and press the plywood onto the first piece of plywood that is glued to the joist. This creates a 2-inch stack of plywood glued to the side of the joist. Secure the plywood pieces to the joist by inserting lag bolts into the pilot holes. Tighten the bolts into the plywood and joist with a ratchet and socket.
Duplicate the plywood reinforcement process on the other side of the joist beam.