Crawl space jacks: steel posts for a sagging floor
A crawl space jack is an adjustable steel post that holds up the beam under your floor. It is a common fix for bouncy or sagging floors over a crawl space.
In a house with a crawl space, the floor sits on wood joists. The joists rest on beams, and the beams rest on posts or piers spaced across the crawl space.
When the spaces between posts are too wide, or an old post sinks or rots, the beam bends. The floor above sags or bounces. A crawl space jack, also called a support jack or adjustable steel post, is set under the beam to hold it up again.
Signs you may need them
Check for these
- A floor that sags, dips or bounces This is the most common sign of a beam with too little support.
- Gaps under baseboards or doors that stick The floor has moved and the walls above have not.
- Cracked, leaning or sunken posts under the house Look with a flashlight from the crawl space door.
- Wet or soft wood Rot is a moisture problem first. New posts alone will not fix it.
Temporary vs permanent
| Type | What it is for |
|---|---|
| Temporary shoring jack | Holds a load during a repair. Many are labeled not for permanent use. Read the label. |
| Permanent adjustable steel post | Made to stay in place. It sits on a proper footing, not on bare dirt. |
| Post plus a new footing | The pad under the post spreads the weight so the post does not sink into the soil. |
A jack that sits on a loose block or bare soil can sink, and the sag comes back.
What the code says
The International Residential Code covers posts, which it calls columns, in section R407.3. A steel column must be at least 3-inch Schedule 40 steel pipe or an approved equal. A wood column must be at least a 4x4. Columns must be held in place at the bottom so they cannot slide. Your local building department decides which code edition applies and if you need a permit.
Do it yourself, or hire a pro?
Crawl space jacks are sold at home stores, and some owners put them in. The work is in a tight, dirty space under a heavy load. You need a footing, the right post, and a plan for how far to lift.
Hire a foundation contractor if the floor has dropped a lot, if beams or joists are rotten or cracked, if there is standing water, or if you see cracks in the foundation walls too. Those are bigger problems than one post.
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International Code Council, 2021 International Residential Code, Chapter 4, section R407.3 (structural requirements for columns): codes.iccsafe.org.
Questions homeowners actually ask
What is a crawl space jack?
An adjustable steel post that goes under a beam in a crawl space to hold up a sagging floor. It is also called a support jack or adjustable steel column.
Can I leave a temporary jack in place for good?
Only if it is made and rated for permanent use. Many shoring jacks are labeled temporary. A permanent post also needs a proper footing under it.
How far apart should crawl space jacks be?
It depends on the beam size, the span and the load. An engineer or contractor should set the spacing. Too few posts is a common reason floors sag.
Will jacks fix a foundation that is sinking?
No. Jacks support the floor inside. If the outer foundation walls are settling, that is a pier job, such as helical or push piers.