Crawl Space Jacks: Complete Guide for Homeowners

A floor that dips toward the middle of a room, a dishwasher that no longer lines up with the counter, or a hallway that bounces when someone walks by usually points to the same culprit: undersupported framing below. Crawl space jacks are adjustable steel posts that lift and hold beams and girders from underneath, and they are the most common fix for sagging floors in homes built over a crawl space. Installed correctly, they stop the sag, stiffen the floor, and protect the finish flooring above from cracked tile and gapping hardwood. Installed poorly, they can punch through a thin slab, crack drywall upstairs, or simply sink back into the dirt within a year.

What Crawl Space Jacks Are and How They Work

Most crawl space jacks are screw-type steel posts. A threaded rod sits inside a heavy-gauge tube, and turning a nut or the top plate raises the head a fraction of an inch at a time. The top plate bears against the underside of a beam or girder, while the base plate rests on a footing. Once the jack is adjusted to the right height, it becomes a permanent support column rather than a temporary lifting tool.

That distinction matters. A hydraulic bottle jack lifts a beam quickly, but it is not meant to hold load for years. Permanent crawl space support posts are designed to carry a rated load indefinitely, typically somewhere between 12,000 and 60,000 pounds depending on the model and height. The screw threads lock the height in place, so there is no seal to leak and no pressure to lose.

You will also hear these posts called adjustable steel columns, lally-style posts, floor jacks, or pier jacks. The names overlap, but the function is the same: carry the weight of the house from the beam down to solid bearing.

Signs Your Home Needs Crawl Space Support

Not every squeaky floor needs a jack. Look for patterns that point to structural movement rather than loose subfloor fasteners:

• A visible dip of more than 1/4 inch over 10 feet when you stretch a string or level across the floor
• Interior doors that stick at the top corner or swing open on their own
• Diagonal drywall cracks radiating from door and window corners
• Tile grout cracking in a line across the room, or hardwood boards separating along one axis
• Kitchen cabinets pulling away from the wall or counters that are no longer level
• Bouncy floors under heavy furniture, a piano, a fish tank, or a full bookcase

Go into the crawl space with a flashlight and look at the beams. Common findings include original wood posts that have rotted at the base, masonry piers that have shifted, beams spanning too far between supports, and damp soil that has softened under old footings. Moisture is often the root cause, so note any standing water, white efflorescence on block walls, or musty odors while you are down there.

Types of Crawl Space Jacks Compared

Several styles are sold at home centers and through foundation contractors. The right choice depends on crawl space height, the load, and whether you want a DIY or professional installation.

Screw jack posts

These are short, stout posts with a threaded top, usually available in heights from about 8 inches to 3 feet. They cost roughly $30 to $90 each at retail. Because crawl spaces are often only 18 to 36 inches tall, this is the style most homeowners end up using.

Telescoping columns

A telescoping column has an inner tube that slides inside an outer tube and pins at set intervals, with a screw for fine adjustment. They are common in basements but also useful in tall crawl spaces. Expect $50 to $120 each.

Professional galvanized systems

Foundation repair companies usually install proprietary galvanized steel posts that sit on engineered precast or poured footings. These carry higher ratings, resist corrosion in humid crawl spaces, and often come with transferable warranties.

Footings: The Part Most DIY Jobs Get Wrong

A jack is only as strong as what it stands on. Setting a 20,000-pound-rated post on bare dirt, a single paver, or a 2-inch concrete rat slab is the number one reason repairs fail. The post will simply push down into the soil as the house settles onto it.

Most local codes and manufacturers call for a concrete footing at least 24 inches by 24 inches and 8 to 12 inches thick, bearing on undisturbed soil below the frost line where applicable. In soft clay or fill soil, an engineer may specify a larger pad, up to 30 by 30 inches. Precast concrete pier pads rated for structural use are available, but check that the size and thickness meet your local requirement before relying on one.

Pour footings and let them cure for at least 7 days, ideally closer to 28 days for full strength, before you put significant load on the jacks. Placing a rigid plastic or rubber isolation pad between the steel base and the concrete helps reduce corrosion where moisture wicks up.

How to Install Crawl Space Jacks Step by Step

If you are comfortable working in tight spaces and the sag is modest, installing support posts yourself is a reasonable weekend project. Here is the general sequence:

1. Measure the sag. Run a string line or laser along the beam and mark the low point. Record how much it needs to rise.
2. Plan the spacing. Beams are commonly supported every 5 to 8 feet. A doubled or tripled 2×10 girder typically needs closer spacing than a solid 6×8 timber.
3. Dig and pour footings at each post location, sized as described above.
4. Set the jacks plumb, centered under the beam, with a steel or pressure-treated bearing plate on top if the beam is narrow.
5. Snug each jack until it just contacts the beam. Do not lift yet.
6. Raise gradually. Turn each jack no more than about 1/8 inch per week. Some pros allow up to 1/4 inch per week on newer homes, but slow is safer.
7. Fasten the posts to the beam and footing with lag screws or concrete anchors once final height is reached.

The slow lift is what separates a good repair from an expensive one. Wood framing, plaster, drywall, tile, and plumbing all adjusted to the sag over decades. Jacking a beam up an inch in an afternoon can crack walls, pop tile, bind doors, and stress supply lines. Patience here is cheap insurance.

Crawl Space Jacks Cost: DIY vs. Professional

For a DIY job, budget $30 to $120 per post plus $40 to $100 per footing in concrete, forms, and rebar. Supporting a single sagging girder with four new posts often runs $300 to $700 in materials.

Professional installation is significantly more because it includes engineering, excavation, footings, and warranty. Typical pricing is $300 to $1,000 per jack installed, with many whole-house jobs landing between $2,000 and $6,000. If the beam itself is cracked or undersized, sistering or replacing it adds $1,500 to $5,000 or more. Pairing the repair with crawl space encapsulation or a dehumidifier to address moisture can add $3,000 to $15,000, but that work often determines whether the fix lasts.

Get at least three quotes. Ask each contractor to specify the post model, load rating, footing dimensions, spacing, and lifting schedule in writing.

When to Call a Structural Engineer

Some situations are beyond a weekend repair. Bring in a licensed structural engineer, usually $300 to $800 for an inspection and letter, if you see any of the following:

• Sag greater than 1 inch, or floors that are still actively moving
• Cracked, split, or insect-damaged beams and joists
• Perimeter foundation walls that are bowing, cracking in a stair-step pattern, or separating
• Soft, wet, or expansive soils
• Plans to add heavy loads like a stone countertop island, a soaking tub, or a wall removal above

An engineer’s stamped plan also helps if your jurisdiction requires a permit for structural repair, and it gives future buyers documentation that the work was done correctly.

Protecting Your Floors After the Repair

Once the framing is level and stable, give the house a few weeks to settle before addressing cosmetic damage above. Re-grout cracked tile lines, reset or replace any cracked tiles, and let hardwood acclimate before filling gaps. If you plan to install new flooring, confirm the subfloor is within the manufacturer’s flatness tolerance, which is usually 3/16 inch over 10 feet for luxury vinyl plank and 1/8 inch over 6 feet for large-format tile.

Check the posts at least once a year, ideally at the end of the wet season. Look for rust at the base, gaps between the head and beam, or signs of footing movement. A quarter turn to re-snug a post that has lost contact is normal in the first year as the structure finds its new equilibrium.

Bottom Line on Crawl Space Jacks

Used as permanent supports on proper footings and raised slowly, crawl space jacks are an affordable way to correct sagging floors and protect everything installed above them. Deal with the moisture that caused the problem, choose posts with a load rating well above what the span requires, and do not skip the concrete. If the sag is large or the framing is damaged, a structural engineer’s plan costs little compared with redoing the work, and it ensures your floors stay flat for decades.