How to Lay Block Under Floor Joists

Determine the amount of sag in your floor. Tape a piece of string from one side of the room to the opposite side to determine where the floor dips. Measure the distance from the string to the floor where it sags. The lowest point that you find in the floor will serve as a general guide to how high you should lift the floor to even it out.

Examine the crawl space under your floor. Determine how you will get to the spots in the floor where the floor is sagging the most. If necessary, map out your floor and its dips on a piece of paper to serve as a guide once you are underneath the floor.

Crawl under the house with all of your supplies, including a flashlight, hydraulic jack, bag of gravel, several solid concrete blocks, some roofing shingles, at least one 2-by-8 board and a small hand shovel.

Find the floor joist located nearest the largest sag in your floor. Place the jack underneath one or two of the blocks stacked upon one another. Slowly raise the jack underneath of the joist until it rises by the distance of your string measurement.

Dig out a small amount of dirt right next to the jack and block assembly. This should create a hole about 1 to 1 1/2 inches deep and large enough in which to set a stone block. Poor some of the loose gravel into the hole to cover the bottom.

Place the stone blocks in a stack until you reach the joist above. If necessary, raise the joist a little more with the jack until you can fit the last block up under it securely. Leave just enough space to slide a piece of roofing shingle between the block and the joist. The shingle's rough surface will help prevent slippage if the house shifts slightly. You can also insert a 2-by-8-inch board on top of the block and shingle stack if you are unable to raise the joist high enough to insert a full block under the joist to fill the remaining space.

Lower the jack assembly slowly, allowing the joist to lower onto the block assembly slowly.

Reexamine your floor using your string and a level. Do this after each set of blocks that you put in place. Doing so will give you an idea of exactly how many blocks you will need and how many times you will have to crawl under the house. Avoid adding multiple block assemblies at one time because you may shift the floor too far and have to go back under the house to undo some of your work.

Repeat the process as necessary until the floor sag has disappeared and the floor sits close to being level.