Jacks and Lumber Supports

The first question isn’t which jack. It’s whether you’re holding a load up while you work, or holding it up forever. That single decision splits the whole category in two.

For broader project planning, explore WBC’s building materials resources.

Adjustable steel jacks — screw jacks, jack posts, lifting shores — are built for leveling and temporary shoring during a job. Steel screw jacks are adjustable support posts made from steel tubing with a threaded screw mechanism that lets you fine-tune the height under beams or joists, with the base plate resting on a concrete footing and the top plate pressing against the structure. Lally columns are the permanent answer. A lally column is a 3” or greater diameter concrete-filled non-adjustable steel column typically used for permanent structural support, while a jack post is a two-section adjustable steel pipe post often used as temporary support — though it can be permanent provided it meets code requirements.

For Montana basements and crawl spaces, the moisture angle matters as much as the load rating. Steel rusts where it stays wet, and that’s most crawl spaces on the valley floor. More on that below.

Jack Post vs. Lally Column: Pick the Right Tool for the Job

These two terms get used interchangeably at the counter. They’re not the same thing.

A jack post adjusts. A lally column doesn’t. Lally columns come in fixed lengths and must be cut to size on-site, and once installed they cannot be adjusted without removing and replacing the column — which makes them less flexible but very stable once in place. A true lally column isn’t adjustable, so it’s often cut to length on site.

Material spec separates them too. Lally columns are 3” to 4” diameter columns of 16- to 11-gauge tubular carbon steel with a rust-resistant coating, filled with high-strength concrete. Jack posts are 11- and 12-gauge rust-resistant coated 3” to 4” tubular carbon steel with 3-gauge top and bottom plates.

If you want a jack post to stay permanent, it has to earn it. A jack post is usually temporary, but can serve as permanent support — it must be at least 3” in diameter to be used for permanent support. Lally columns are code compliant out of the gate; a permanent jack post must meet set code requirements.

Capacity numbers tell the real story for a fixed-length lally:

Lally column (72” tall)	Safe load
3” diameter	~18,000 lbs
4” diameter	~29,100 lbs
4” Schedule 40	~41,500 lbs

A 72” tall 3” diameter lally supports 18,000 lbs, a 72” × 4” diameter supports 29,100 lbs, and a 72” × 4” Schedule 40 steel lally supports 41,500 lbs.

Here’s the bottom line. Need to adjust height after installation? Choose steel screw jacks. Is the support permanent and heavy-duty? Lally columns are better. For a sagging beam where you want to dial in the height, the jack wins. For a permanent column carrying a heavy load in a crawl space, a lally column offers solid, long-lasting support.

Capacity Drops as the Support Gets Taller — Don’t Size by the Headline Number

This is where people get burned. A jack rated at 15,000 lbs is rated at 15,000 lbs short. Run it tall and the safe load falls off a cliff.

Look at the 4x4 screw jack built on dimensional lumber:

Total shore height	Max safe load
0–4 ft	15,000 lbs
6 ft	12,000 lbs
8 ft	7,200 lbs
10 ft	5,000 lbs
12 ft	3,000 lbs
14 ft	2,600 lbs
15 ft	0 lbs

The 4x4 screw jack carries 15,000 lbs at 0–4 ft, drops to 7,200 lbs at 8 ft, 5,000 lbs at 10 ft, and reaches 0 lbs at 15 ft. A 6x6 holds up better at height but still tapers. The 6x6 carries 30,000 lbs from 0–8 ft, drops to 20,000 lbs at 12 ft, and 6,000 lbs at 20 ft.

The two-pipe jack post tells the same tale. An adjustable 60” to 96” two-pipe jack post supports 8,090 lbs at 60” but only 3,720 lbs at 96”.

A word on those ratings — they already build in a cushion. Load capacity is based on actual load test results using Douglas Fir #1 4x4 dressed lumber (3.5” square), and capacities include a standard 2.5/1 safety factor. Steel shore products follow the same pattern at extension. The Light Duty Steel Shore offers up to 20,000 lbs at short heights but drops to 3,000 lbs in the 109” to 133” range.

Building a Custom-Height Shore from a Steel Head and a Stick of Lumber

You don’t always need a full steel post. Pair a steel jack head with standard dimensional lumber and you’ve got a shore at whatever height the job demands. It’s the cost-effective route, and the lumber is the structural member.

The screw jacks are sized to the lumber. The 4x4 screw jack uses standard nominal dressed 4x4 lumber (3.5” square), cut square to any length — do not use true cut lumber. The 6x6 screw jack uses standard nominal dressed 6x6 lumber (5.5” square), cut square to any length, and again, do not use true cut lumber.

For low-profile work, the Timber Jack family does the same job in a short package:

Model	Lumber	Load capacity	Adjustment
TJ-4	4x4	15,000 lbs / 7.5 tons	5”
TJ-6	6x6	30,000 lbs / 15 tons	5”

The TJ-4 carries 15,000 lbs with a 9-1/8” overall height and 5” adjustment range; the TJ-6 carries 30,000 lbs with a 10” overall height and the same 5” adjustment.

The lumber grade isn’t optional if you want the rated number. Load capacities are based on Douglas Fir #1 4x4/6x6 dressed lumber, and capacities include a standard 2.5/1 safety factor.

To tie the vertical post into the horizontal beam, you use a head. 4x4 heads are available for securing a vertical 4x4 to a horizontal beam: Purlin Splicer, Joist Holders, Pivoting, T-Head, and U-Head. The assembly is straightforward — drill the center hole, mount through the top plate with lag screws, attach the Purlin Splicer to the beam, slip the lumber into the square tubing of the splicer and secure with nails, then adjust by turning the “U” shaped handle at the bottom of the screw, which rides on greased Acme threads.

Installation Basics: Footing, Plumb, and Slow Lifting

A jack is only as good as what it stands on. Get the footing wrong and nothing above it matters.

Permanent supports need a real pad under them. Stable footing is required — typically a 24-inch square concrete pad for crawl space jacks. Same goes for a jack post you intend to leave in place: it needs a proper 2’x2’ footing pad underneath.

Three rules show up on every manufacturer’s caution list, and they’re worth repeating. Screw jacks should be attached to a flat solid foundation, should not be subject to any lateral force, and should be used to support straight vertical loads only. No racking. No side load. Plumb and vertical.

When you’re raising a sagging floor, patience saves your drywall. If using two or more posts, turn each screw a quarter turn at a time. Take your time lifting a floor or ceiling a 1/2” or more — it may take a week to raise it an inch, so as to minimize cracking and nail or screw pops.

Lock it before you load it. The screw jack is equipped with a safety lock — tighten it with an Allen wrench once final adjustment is made to prevent accidental height change. And for anything beyond a simple level-up, get a professional eye on it. Check with local authorities or a structural engineer to ensure the chosen support meets load requirements and installation standards.

Finish and Rust: The Montana Crawl-Space Factor

Steel in a wet crawl space is a rust problem waiting to happen. This is where finish choice stops being cosmetic.

Screw jacks come two ways. They’re offered in galvanized or powder coated finish. The threads are the vulnerable part. Screw threads can corrode over time if exposed to moisture, so they may require occasional inspection and lubrication, and paint or coatings help prevent rust. Good news is the factory greases them — the screw is greased for easy tuning and to prevent rusting.

Lally columns aren’t immune either. They’re generally low maintenance but can rust if the outer steel is exposed to moisture and not properly coated.

For exterior and treated-wood connections, the galvanization spec steps up. ZMAX galvanization offers extra corrosion resistance for exterior and treated-wood applications. The Simpson LSSJ jack hanger, for example, provides support underneath the joist, rafter, or beam, is made from 18-gauge steel, and connects jack rafters to hip members with a hinged seat that’s field adjustable to rafter slopes from 0:12 to 12:12.

One note on sizing the smallest gear: a low-range adjustable hand jack covers roughly the 16.5” to 23.5” window, handy for tight crawl-space leveling where a full-height post won’t fit.

For load math beyond a straightforward level-up — removing a bearing wall, setting a new beam, correcting a settled foundation — bring in a certified structural engineer before you load anything. Consult a certified engineer for approval of these products within your specific application before construction begins.

Frequently Asked Questions

Can a jack post be a permanent support, or only temporary? It can be either. Jack posts are temporary or permanent adjustable posts, but they must be at least 3” in diameter to be used for permanent support. If installed on a permanent basis, they should be rated for it, and they must meet local code. A temporary jack post typically stays in place 1 to 3 months; a permanent one lasts the life of the building barring corrosion.

Why does my jack’s load rating depend on how tall I run it? Because a taller column buckles more easily under the same load. A 4x4 screw jack rated at 15,000 lbs from 0–4 ft drops to 5,000 lbs at 10 ft and to zero at 15 ft. Always size off the rating at your actual shore height, not the headline number.

Do I really need a concrete footing under a basement support? Yes for anything permanent. A jack post used as a permanent support needs a proper 2’x2’ footing pad underneath, and crawl space jacks typically need a 24-inch square concrete pad. A jack standing on bare dirt or a thin slab will push through over time.

What lumber do I use with a 4x4 or 6x6 screw jack? Dressed dimensional lumber, cut square. The 4x4 jack uses standard dressed 4x4 lumber (3.5” square) cut square to length — not true cut lumber. For the rated capacity, the lumber needs to be Douglas Fir #1 or comparable graded stock, since the published loads were tested on that grade.

How fast should I lift a sagging floor? Slowly. Lifting a floor or ceiling a half inch or more can take a week to raise it a full inch, which minimizes cracking and nail pops. With two or more posts, turn each screw a quarter turn at a time so the load comes up evenly.

Which finish should I pick for a damp crawl space? Go galvanized where moisture is a constant. Screw jacks come in galvanized or powder coated finish, and the galvanized option resists rust better in wet conditions. Either way, the threads can corrode if exposed to moisture and need occasional inspection and lubrication.