How to Install Floor Jacks in a Crawl Space

How to Install Floor Jacks in a Crawl Space

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There are at least two essential factors to consider when you are educating yourself on how to install floor jacks in a crawl space. The first involves getting the right kind of equipment, which could be heavy-duty screw jacks or the strongest hydraulic bottle jacks you can find.

This choice will depend on how much actual lifting you’re going to do. If the project/house is massive, it might be best to go with the mechanical version of floor jack – the screw jack. If you use hydraulic jacks, use 12-ton units as a starting spot.

Assume for the moment you have the “right” jacks, even up to 20-ton capability. This means you have the mechanical/hydraulic power to get those floor level. The key after this step is to do the job carefully and safely. In fact, it’s best to do this a bit at a time, so you have the jacks in the correct positions and there is a strong foundation under each jack. This will not only make sure the job proceeds smoothly, but you won’t risk cracking the walls or causing damage to the structure itself.

the screw jack

Spreading the Load

The proper floor jacks will handle the weight and pressure necessary to level those floors, but you should make sure you spread the load properly. For example, you can’t lift and level the floor with one jack, and you will damage the frame/lumber if you put far too much pressure in one or two spots.

Spread the load with heavy gauge steel plates. Another general comment: As you take each pressure point up a fraction of an inch each time, use your ears! Listen for audible clues of breaking timber or plaster.

This is not the same as hearing a wood-frame building making noise. There will be some sounds of wood rubbing on wood or other materials. Think of this as groaning and creaking. If you hear something more alarming, stop! You might also consider attaching beams of a length and weight you can handle to the underside of the joists.

This semi-permanent step can be important to keep the lifting beam in place. Because you’ll be working in a crawl space, you won’t be using long post columns, which can be used in a basement.

You should also plan to use a small piece of steel (1/4 inch is best) under each jack, to provide the solid foundation you need. This will keep the jack from pushing through the earth or into wood that you would otherwise use as a base.

It’s also best to have a steel plate above the jack to spread the load across the lifting beam. This top plate should be a bit thicker to avoid bending. Try to use beams that cover several floor joists at one time. Three or four should be possible if you have one or two good helpers to place the beam.

Be Prepared

This is not just about the Boy Scout motto. You should be ready with your supports before the beam is lifting even 1/8 inch. You should have your support columns, beams, or blocks at hand, with sufficient support under them. This means also having steel plate on which to set your blocks or beams. Again, you probably won’t use adjustable metal posts/columns because of the limited space. But, you should have wood shims ready each time you place the concrete block, heavy oak column, etc.

Assuming you have made fractional lifts across your beam (and three or four joists) it might be a good time to check the status of the floor and walls in the room above. If everything seems to be in order and you have blocks and shims holding the beam tightly in place, you might want to give the jack one additional short pump. But don’t force it! One successful floor-leveling job used three screw jacks with a 9-ton rating each, and concrete blocks.

Tip: Always place the blocks so the open spaces point to the side. Placing them with the openings down will invite settling, though you should have a plate under each area. If you can, use two layers of blocks, placed with the ends pointing in opposite directions for each layer.

Stronger Jack


This general guide should give you what you need to level a floor that has not sagged too far. Don’t make the mistake some property owners make by using shims that make the job quick and cheap. This should be only a temporary method, because you’ll probably have to do it all again as the wood deteriorates and squeezes due to the weight.


In addition, don’t trust jacking posts that are too light to get the job done. The key to understanding this is the term “light duty.” They won’t hold the weight over the long term, as the floor above is used and the house settles. You’re always best served by using sturdy beams, concrete blocks, metal plates of no less than ¼-inch thickness. Stronger is always better.

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