A sagging floor in a basement or crawl space usually triggers fears of a costly foundation repair. In many cases the fix is far simpler: a lally column, a steel post that supports an unsupported joist or beam and transfers its load to a footing below. Contractors have used these columns for decades because they install quickly, adjust to the actual floor height, and carry thousands of pounds in a small footprint. Before you frame around one or hide it behind drywall, it pays to understand the different types, how the load is calculated, and what a safe installation involves. Homeowners who want to dress the post up rather than hide it can start with the column wrap and PVC cladding guide, which covers cosmetic treatments that leave the steel untouched.
The oldest form of a lally column is a non-adjustable steel tube cut to length and filled with concrete to improve its strength and stability. The name has since grown to cover several families of mobile steel columns: adjustable steel columns, teleposts, and jack posts. Modern units consist of two telescoping steel tubes, one inside the other, that adjust upward and downward and lock in place.
What Is a Lally Column?
A lally column is a steel column used to temporarily or permanently support an unsupported joist or beam. It transfers the weight of the floor system down through the column shaft to a concrete footing or slab. In a crawl space or unfinished basement, a lally column can correct minor sagging without excavating or replacing the foundation, which is why it shows up in nearly every residential repair estimate for floor deflection.
Parts of a Lally Column
- Outer tube: the larger steel section that carries the bulk of the load.
- Inner tube: telescopes inside the outer tube to adjust the height.
- Locking pin or collar: holds the two tubes at the set height.
- Base plate: spreads the load across the footing or slab.
- Screw jack or threaded rod: fine-tunes the final height adjustment.
Why the Concrete Core Matters
Older columns were filled with concrete because the fill stops the thin steel wall from buckling locally and adds mass that resists crushing. Modern adjustable columns reach similar strength with heavier steel walls and precise tube tolerances, so they ship hollow and weigh less. That trade-off matters in a damp basement: concrete-filled units are heavier to move but keep the interior of the tube protected. Columns that are short relative to their width behave differently from slender ones under load, and the short column effect explains why a squat steel post carries far more than a tall, skinny one of the same diameter.
Types of Support Columns
Not every steel post under a beam is a lally column, and the names get used loosely on job sites. The table below compares the common families so you can match the product to the job and read the label correctly.
| Type | Adjustment | Typical capacity | Best use |
|---|---|---|---|
| Lally column | None or screw jack | 20,000+ lb | Permanent support in basements and crawl spaces |
| Jack post | Threaded screw | 8,000-12,000 lb | Temporary shoring during repairs |
| Telepost | Telescoping tubes and pin | 10,000-20,000 lb | Permanent support with height adjustability |
| Wood post | Cut to fit | Varies with size | Permanent support in dry, ventilated spaces |
Adjustable vs. Fixed Columns
Fixed columns are cut or ordered to a precise height, set in place, and left alone. Adjustable columns let you raise the post into contact with the beam using a screw or telescoping action, which makes them forgiving when floors are out of level. The trade-off is that adjustable units have moving parts that must be locked properly, and the load rating depends on how far the inner tube extends. The difference between short columns and long columns decides which failure mode a support can experience: crushing for short sections, buckling for long ones, so the extension height on the label is the number that matters.
Load Transfer and Column Design
The load a column must carry is not a guess; it is the weight of everything above it that drains to that point. Floor joists rest on a beam, the beam rests on the column, and the column sits on a footing sized for the total. Engineers call the floor area that feeds a single column its tributary area, and the tributary area in column load transfer explains how to divide a floor plan into those zones and sum their weights.
Estimating the Load by Hand
- Measure the tributary area: the rectangle of floor the column supports, typically half the joist span on each side times the beam spacing.
- Add the dead load: framing, subfloor, and finishes run about 10 to 15 pounds per square foot.
- Add the live load: building codes use 40 pounds per square foot for residential floors.
- Multiply the total by the area, then add the beam weight to get the column load.
- Compare the result against the manufacturer’s rating and the footing size.
Footing Size Rules
A lally column is only as good as what it sits on. A 6,000-pound column load on soft soil needs roughly 4 to 5 square feet of bearing area to stay under typical soil capacities of 1,500 to 2,000 pounds per square foot. In an existing basement that usually means pouring a new concrete pad before the column goes in, and the pad must reach below frost depth in cold climates.
Reinforcement and Installation Details
The steel column does the heavy lifting, but the details around it decide how long it lasts: the footing, the base plate connection, the contact patch at the beam, and corrosion protection in a damp basement. In concrete construction the same logic applies to cast columns, where lateral stirrup reinforcement keeps the vertical bars from buckling and holds the core together under load.
Step-by-Step Installation
- Mark the column location under the beam and check for plumbing, electrical, and duct conflicts.
- Pour a footing pad at least 12 inches square and 4 inches thick, or verify the existing slab can take the load.
- Set the base plate and column, then extend the inner tube until the top plate contacts the beam.
- Use the screw jack to snug the column; do not over-jack a sagging beam in one pass.
- Lock the pin or collar, then verify the column is plumb with a level in two directions.
- Leave the column accessible for inspection, or box it in with removable access panels.
Over-jacking is the most common installation error. Raising a beam about a quarter of an inch per day lets the structure settle gradually; forcing it up in one afternoon can crack finishes, jam doors, and overload the column connection.
Corrosion protection extends the life of any column. In a basement with high humidity, paint the tube with a rust-inhibiting primer, keep the base plate off standing water, and check the locking mechanism every year. A column that looks fine at a glance can hide rust at the base plate where moisture collects.
Sizing Checks and Code Requirements
Before buying a column, confirm three numbers: the load, the height, and the allowable capacity at that height. Manufacturers derate columns as extension increases, so a column rated for 20,000 pounds at 7 feet may carry half that at 10 feet. Most residential checks compare the short versus long column behavior using the effective length ratio, which tells you whether the section will crush or buckle first.
Reading the Label
A proper column label lists the tube diameter, wall thickness, extension range, and a load table. If the label is missing or the column shows rust pitting, treat the rating as unknown and replace the unit. Local building codes often require a stamped engineer’s calculation when a column supports a beam over a certain span, and the permit office will tell you the threshold.
Code officials look for three things at inspection: the footing size, the column rating, and the connection at the beam. Photograph the label and the base plate before you box the column in, because the inspector may need to confirm the details after the finish work is done.
Safety, Permits, and Knowing Your Limits
A lally column is a structural element, not a shelving bracket. Work on load-bearing beams should be inspected, and many municipalities require a permit even for a straightforward basement column because the repair changes how the house carries its weight. On new concrete construction, temporary support starts with the formwork: a column kicker locks the form in position so the concrete cures plumb and true.
When to Hire a Structural Engineer
- The floor sags more than about 1/2 inch over a 10-foot span.
- Beams show cracks, splits, or signs of rot.
- The footing area is unknown or the soil is questionable.
- You plan to remove an existing post or wall.
- The column will support a new second-story load.
Once the engineering is settled, the concrete work is straightforward math. Before ordering ready-mix for new footings or a basement pad, run the volumes through the concrete calculator for slabs, beams, columns, and footings so the pour matches the plan and you are not short at the end of the day.