Jack Posts and Floor Support Jacks for Basements, Crawl Spaces and Decks

A jack post is an adjustable floor support that holds beneath a beam or floor joists while damaged supports are repaired or replaced. You buy one to carry the weight while you get the repair done. Get the height and load right together: a post that reaches the beam still has to carry its share of the building.

Short Floor Jacks for Crawl Spaces

The floor jack with a height range of 1 foot 7 inches to 3 feet is the starting point for a low crawl-space repair. That is 19 to 36 inches of reach, useful where a basement-height post simply takes up too much room. Buy the short jack for the short space; extra height is no advantage underneath a low beam.

Measure from the supporting surface to the beam, and allow for the adjustment the repair needs. A jack that reaches the beam at the end of its travel leaves you nowhere to go when the job calls for lifting.

In Montana, spring snowmelt can leave the ground wet for weeks. The footing matters as much as the post: a jack on inadequate bearing can sink instead of supporting the floor. Address drainage along with moisture-damaged framing so the repair tackles the cause of the trouble.

Mid-Height and Tall Jack Posts for Basements

The mid-height floor jack runs from 2 feet 10 inches to 4 feet 7 inches. Buy this size for a space above the short jack’s reach but below a full-height basement. Its 34-to-55-inch range makes it the useful middle choice under lower beams.

Tall floor jacks cover spaces roughly 4½ to 8 feet high. Start with this group for a jack post for basement work where you have standing room beneath the beam. Choose the individual height range around your actual opening and the movement the repair requires.

Floor jack size	Height range	Where to start
Short	1 foot 7 inches to 3 feet	Low crawl spaces and low beams
Mid-height	2 feet 10 inches to 4 feet 7 inches	Deeper crawl spaces and lower basement beams
Tall	Approximately 4½ to 8 feet	Taller basement openings

Winter is interior renovation season in Montana. Before new flooring goes over a sagging floor, settle the support problem underneath it. In Great Falls, expansive clay makes the foundation part of that diagnosis. Raising a beam and correcting moving ground are different jobs.

How to Choose an Adjustable Post Jack

Choose a floor joist support jack by working height, required adjustment and load. Supporting weight and lifting it are different demands, so check both capacities before buying. Have a structural engineer establish the load each post must carry and the footing and beam arrangement for the repair. The biggest pound figure alone does not choose the right support.

A steel jack post uses steel for the support itself. A screw jack post can also mean a screw-jack assembly used with lumber. Lumber size alone does not establish the capacity of that complete assembly. For a telescoping floor-support job, start with the short, mid-height or tall floor jack that matches the opening.

Can You Leave a Jack Post Permanently?

Only a support intended for permanent use belongs in a permanent repair. Temporary support, permanent secondary support and permanent primary support are different assignments. Settle that choice, including applicable code approval, before buying a post you intend to leave in place.

For the lally column vs jack post question, the original Lally column is a concrete-filled steel column. A telescoping jack post has sections fitted inside one another and secured with pins or bolts. They are different support types, and reaching the same height does not make them interchangeable.

How to Use a Jack Post Under a Floor, Porch or Deck

Installing a jack post in a crawl space starts with a solid, flat foundation and a sound load path through the beam. Keep the post plumb in both directions and secure it to the framing. A post merely standing under a beam is not a finished support arrangement.

To jack up a deck or porch to replace a post, establish temporary support for the load before removing the old post. Choose the jack for the available height and both the holding and lifting demands. Holding the structure during replacement and raising a sag are separate tasks; make any leveling movements slowly and in small increments while watching the structure’s response. Additional cracking or unevenness calls for a structural engineer.

Montana’s spring thaw matters on this job. A porch repair needs dependable bearing beneath the temporary support, even when the surrounding ground is wet. The jack’s adjustment cannot make up for a base that settles under it.

The Pick

Buy the 1-foot-7-inch to 3-foot floor jack for a low crawl-space opening. Buy the 2-foot-10-inch to 4-foot-7-inch jack for a lower beam that needs more reach. Buy a tall floor jack for a basement or porch opening in the roughly 4½-to-8-foot group. Match the load as carefully as the height, and choose a support intended for permanent service when it will remain part of the building.

Frequently Asked Questions

What is a jack post?

A jack post is an adjustable support used beneath floor joists or beams to carry weight during repairs or help correct sagging. You can also use a suitable support beneath kitchen areas with granite counters and heavy appliances. Height alone isn’t enough: the support has to suit the load and whether the job is temporary or permanent.

How do you jack up a deck to replace a post?

You need temporary support carrying the load before the old post comes out, and the same principle applies when replacing a porch post. A floor support jack needs dependable bearing underneath it and a secure connection above before the load transfers. Holding the structure in place and lifting it require different capabilities, so a listed pound figure alone isn’t enough to plan the lift.

How do you install a jack post in a crawl space?

Measure from the supporting surface to the beam, allowing room for the adjustment your repair needs. Set the jack post on a solid, flat foundation, check that it’s plumb in both directions, and fasten it according to its installation instructions. Check the wood where the support bears against it, too: pressure from a jack can compress and damage a wooden beam. In a damp crawl space, pay particular attention to rust around the base.

What’s the difference between a Lally column and a jack post?

The original Lally column is a concrete-filled steel column; a telescoping jack post has sections fitted inside one another and secured with pins or bolts. They aren’t interchangeable just because they fit the same opening. If you’re planning a permanent repair, distinguish permanent primary support from permanent secondary support, since permission for one doesn’t establish suitability for the other.