DIY: How to Un-Sink a Sunken Floor

Concrete Floor

• 1

  Bore a row of evenly spaced 1-inch-wide holes over the sunken part of the concrete floor slab, 1 inch inward from the edge. Depending on the size of the sunken slab, space these holes 4 to 8 feet apart. Drill a 12-inch-deep hole through each bore using a rock hammer.

• 2

  Mix Portland cement, sand and fly ash in a wheelbarrow until the grout mix achieves a mortar-like consistency. Scoop the mix into a concrete pump. Insert the pump’s hose into a concrete floor slab hole.

• 3

  Pump the mix into the hole at a rate of 50 to 100 pounds per square inch. The sunken concrete slab slowly begins to rise as the grout mix is pumped under it.

• 4

  Remove the hose from the hole and insert it in another hole. Continue the process of pumping the grout mix until the concrete rises to the required level. Check the top of the concrete slab with a level to ensure it is even, or pump more grout over the side that is slightly lower.

• 5

  Insert quick-dry mortar into the holes and level with a trowel. Allow the mortar to cure for the time specified on the label directions.

Wood Floor

• 6

  Examine the floor joists from the basement to identify the bowed one that is causing the floor to sink or sag. Select a sister joist that is identical to the sagging joist so it helps un-sink or raise the floor and brings it back to its original level.

• 7

  Cut a length of 4-by-4 lumber with a saw, long enough to run perpendicularly under three joists, with the middle one under the joist that sags. Place two hydraulic jacks under the post.

• 8

  Raise the hydraulic jacks a little at a time, each in turn, until the post rests snugly against the lower sides of the joists. Jack the joist 1/8-inch every day until level.

• 9

  Apply construction adhesive to the side of the sunken joist in a zig-zag pattern. Hold the sister joist against the glued section of the sunken joist, with its upper edge flush against the subfloor.

• 10

  Hammer a series of three 16d nails every 16 inches along the span to secure the sister joist to the sagging joist.