What Causes Floor Joists to Crack?

Joist Failure

• A joist may crack because it is bearing a disproportionate amount of weight. For instance, if it was higher than surrounding joists, a particular joist would be bearing more weight than intended. A deep notch cut in a joist for installation of electrical and plumbing fixtures or a deep notch to make a joist fit on the sill plate both create a weak spot that can split under a heavy load.

Water & Insects

• Mildew, mold, fungus, rot and other water damage can weaken a floor joist and cause it to crack. Wood-boring insects such as carpenter ants, powder post beetles and termites can hollow out a joist and cause it to fail. The joist board itself may contain sappy wood, knots and other defects that create a weak spot that can crack or split under stress. Older homes may have had joists too long or spaced too far apart for the load being carried, causing an overloaded joist to crack.

Joist Repair

• If you have access to a cracked joist through the basement, you can repair it yourself. First, remove any wiring, plumbing and other obstructions. If the old joist was notched for wiring or piping and it broke at the notched point, you should either move the wiring or piping out of the way or consult with a builder about notching the repair joist. Obtain a new joist the same height as the existing joist. Tack a 4-by-4 temporary support beam crosswise under the broken joist and extend it to a sound neighboring joist for stability.

Give Support

• The temporary support beam itself should be held up with a vertical beam on top of a hydraulic jack. Jack up the broken joist about 1/8 inch per day until the broken joist is level again. Set your repair joist alongside the old, making sure it extends at least 3 feet on both sides of the problem area. Apply a generous bead of construction glue to the old joist, fit the repair joist to the broken area and secure it with nails, lag screws or bolts. Use 16d nails or 3/8-by-3-inch lag screws, set in a vertical line of three every 16 inches. Wait 24 hours and remove the temporary supports.