How to Fix a Sloping Floor

Lisa Wampler

A sloping floor in your home indicates a structural issue with the support beams called the joists. Joists run the length of your house. Your floorboards attach to the joists, and when the joists start to sink, the floor slopes. This can become a serious issue for your home. Drywall can crack and doors will fail to shut because the door frames start to slope with the floor. Fortunately, you can correct this issue by jacking up the joists and then by installing sister joists to help support the original joists in the floor.

Fix a Sloping Floor

Step 1

Nail a 4-by-6-inch wooden beam across the affected joists at the lowest point of the sag. Cut the beam long enough to extend across all of the affected joists so the ends of the beam rest on structurally sound joists. Use a toenail method to nail the beam to the joists. This means, drive the nails at an angle through the side of the beam and into the joists. The beam is temporary and you will remove it at the end of the process.

Step 2

Place a 4-by-6-inch wooden beam on the floor directly under the beam you nailed to the joists.

Step 3

Place a 1/4-inch steel plate on top of the wooden beam at each end. The plate will help distribute the weight of the house evenly over the beam. Place a 1-ton hydraulic jack on each plate.

Step 4

Measure the distance between the top of the hydraulic jacks and the bottom of the beam you nailed to the joists. Adjust the adjustable ends of two Lalley columns to a length that will allow you to place the columns between the jacks and beam. A Lalley column is a steel tube with a threaded adjustable end. They are commonly used to support joists.

Step 5

Jack up the hydraulic jack to raise the floor no more than 1/4-inch per day. If you go more than 1/4-inch per day, you can cause cracking in the drywall of the home. Continue this daily until the floor is level.

Step 6

Measure the length and width of the joists in your home with a tape measure. Cut hardwood sister joists that measure the same length and half the thickness of the original joists with a table saw. If you cannot source lumber long enough, cut them as long as possible. You need two boards for each sagging joist.

Step 7

Coat one side of each board with construction adhesive and then place the glued side of the boards against each side of the effected joists. The adhesive will hold the sister joists in place.

Step 8

Drill two evenly spaced rows of pilot holes that measure 1/4-inch in diameter with a hand drill. Space the holes in the each row approximately 10 inches apart.

Step 9

Drive lag bolts into each of the holes. The 1/2-inch diameter lag bolts must go half way into the original joist. For example, if the original joist is 4 inches thick and the sister joist is 2 inches thick you need 4-inch long lag bolts. Drive the bolts in with a socket and ratchet.

Step 10

Lower the hydraulic jack and then remove the Lalley columns and support beams you nailed into the joists.