If you go down to your basement and look up you will see the floor beams, called joists. The joists hold up the floor of your house and are one of the most important supporting structures. If you see that one of the floor joists is cracked, your home's structural integrity is compromised. Repair the cracked joist by applying sister joists to each side of the cracked joist. This will strengthen the joist and prevent further damage.
Place a 4-by-4-inch board that extends across the cracked joist and the two joists on either side of the cracked joist. Nail the board to the joists that are not cracked.
Place a hydraulic jack on the floor of your basement and wedge another 4-by-4-inch board between the jack and the board you placed under the cracked joist. Center the jack and board under the crack.
Jack up the joist approximately 1/8-inch a day until the cracked joist is straight and no longer sags. Do not lift the joist more than 1/8-inch a day or you could cause drywall cracking in your walls.
Measure the width length and thickness of the floor joist with a tape measure and cut two identical boards with a mitre saw or skill saw. For example, if the floor joist is a 3-by-6-inch board that runs 12-feet long, cut two boards to match. Use a hardwood like hard maple, oak or mahogany. If you cannot source lumber long enough, make sure each end of the board extends 3 feet past the crack.
Drill pilot holes in each board you cut with a 1/8-inch drill bit. Drill three rows across the face of the board and space each hole about 10 inches apart.
Place a liberal amount of construction adhesive on one side of the cracked joist and place one of the boards you cut and drilled over the adhesive. Nail the board in place using nails long enough to sink halfway into the joist. Repeat this step on the other side of the joist.
Lower the hydraulic jack from the joists and remove the 4-by-4-inch board you tacked to the bottom of the floor joists.