Floor Jacks and Lumber Support Jacks

The first thing to get straight: are you lifting, or are you holding? That question decides everything. Floor jacks and bottle jacks are lifting tools, while adjustable screw jacks or Lally columns are intended for structural support. Mix those up and you either buy the wrong tool or set up something dangerous under a sagging floor.

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

Most of the heavy-duty support gear in this category works the same basic way. A steel jack head clamps onto standard dimensional lumber — usually a 4x4 or 6x6 — and turns that wood post into an adjustable column. The screw mechanism lets you dial in the height and load the beam above. Done right, it re-levels a sagging joist. Done wrong, it cracks the structure or kicks out sideways.

One thing worth understanding before you buy: contractors refer to adjustable jacks as temporary supports sometimes and permanent solutions other times. That causes understandable confusion. Technically, adjustable jacks are both temporary and permanent, because the answer depends on how you’re using them. Steel floor jacks can be used as permanent supports when they are secondary supports. Meaning they share the load alongside other structural elements — they’re not a standalone fix for whatever caused the problem in the first place.

That distinction matters in Montana crawl spaces and basements. In one repair, a floor was sagging because the beam was decaying due to moisture damage, and the columns supporting the beam were insufficient. Adding jacks held the floor — but the smart move was also addressing the water. As part of a whole-home solution, addressing the water drainage issue matters, since moisture was the root cause of the decay. Under a damp crawl space, propping up a rotted beam without fixing the source just buys you time before the next failure.

Matching the Jack to the Load

There are a few distinct jack styles, and they don’t do the same work. Bottle jacks are compact for tight vertical lifting. Hydraulic floor jacks give you reach and lateral positioning. Screw jacks are the long-term play for holding up sagging joists and beams.

A critical point most buyers miss: a jack’s rated capacity is only good at low heights. It drops hard as the shore gets taller. Take the Ellis 4x4 screw jack, which is built for constructing adjustable structural supports using standard 4x4 lumber, suitable for both permanent and temporary use in residential, commercial, and emergency shoring. Its safe load looks impressive at floor level and then falls off a cliff as you go up.

Total shore height (incl. jack)	Max safe load — Ellis 4x4	Max safe load — Ellis Timber Jack TJ-6 (6x6)
0–4 ft	15,000 lbs	30,000 lbs
6 ft	12,000 lbs	30,000 lbs
8 ft	7,200 lbs	30,000 lbs
10 ft	5,000 lbs	27,000 lbs
12 ft	3,000 lbs	20,000 lbs
14 ft	2,600 lbs	15,000 lbs
15 ft	0 lbs	—
16 ft	—	11,500 lbs
20 ft	—	6,000 lbs

Those numbers come from actual load tests on Douglas Fir #1 dressed lumber (3.5” square), and capacities include a standard 2.5/1 safety factor. Lumber must be cut square on both ends. A taller crawl space shore carries a fraction of what the same jack handles down low. Read the chart for your actual height — don’t assume the headline rating applies.

The lumber rule is non-negotiable. Use standard, nominal, dressed 4x4 lumber (3.5” square), cut square to any length. Do not use true cut lumber. Rough-sawn full-dimension lumber throws off the fit and the rating.

A few of the jack types and what they’re built for:

Jack type	Lumber	Safe support load	Adjustment
Ellis 4x4 Screw Jack	Nominal 4x4 (3.5” sq.)	15,000 lbs (0–4 ft)	6” acme screw
Ellis Timber Jack TJ-4	4x4	15,000 lbs	5”
Ellis Timber Jack TJ-6	6x6	30,000 lbs	5”
Shore Jack (S301944PC)	Nominal 4x4	up to ~24,700 lbs*	4”

*The Shore Jack along with a 4” x 4” wood post cut to custom length becomes a versatile adjustable column, and load capacity depends on the 4x4 used, up to 24,700 lbs — consult your wood supplier for allowable load.

The Timber Jack earns its keep on value. It can support up to 30,000 lbs. Fasten it to the end of a 4x4 or 6x6 to create a shore with 5” of adjustment. The low cost and high load capacity rank it as the best value in the Ellis Screw Jack family. Note the 6x6 jacks like the TJ-6 hold serious load even at taller heights, which is where the 4x4 versions give out.

One more capacity warning, this one about lifting versus holding. Support capacity and lifting capacity are not the same number. On the Ellis 4x4, you can lift up to 1,800 lbs by hand or 4,000 lbs with a 2x4x6’ piece of lumber placed between screw and handle for extra leverage. Beyond that, you need hydraulics — if loads require more than 4,000 lbs of lifting capacity, use the Ellis Hydraulic Jack for extra lifting power.

Western Building Center carries telescoping floor jacks in several height ranges — the C-3 (1’7”–3’0”), the C-2 (2’10”–4’7”), the C-79 (4’6”–7’9”), and the C-84 (4’8”–8’4”) — covering everything from low crawl spaces to taller basement spans.

When you’re sizing for a job, build in margin. Buy gear rated for 25 to 50 percent more than the anticipated load to protect the equipment. Running a jack at its max rating leaves you no room for error.

Setting Up a Jack the Right Way

Foundation first. The base plate has holes provided to secure it to a solid foundation — always secure to a solid foundation. On a dirt-floor basement, that means something to spread the load. Set the jack straight on dirt and the narrow flanges punch into the soil. A solid concrete block or pier under the jack gives a wide, level area to disperse the weight so the post won’t kick out.

That footing point matters double in Montana. Dirt-floor basements, high spring water tables, and frost-affected ground all work against a jack set straight on soil. A concrete pier or solid block under each one keeps it from sinking.

For the lifting and leveling itself, the order goes like this. Measure the total shoring height floor to beam, then subtract the height of whatever you’ll set the jack on top of — like a pier block — plus 9.5” for a 4x4 or 11” for a 6x6 Ellis Screw Jack. Cut your lumber, attach it to the jack, and secure your purlin splicer or beam holder to the joist or beam where you want to raise the floor.

Distribute the force across multiple joists. Don’t try to lift on a single point. For proper leveling, use Ellis Screw Jacks every 2 feet. When you run a beam across several joists, jack placement on the ends is critical — put the outermost jacks directly under the very first end joist. Set them one joist in and the unsupported end creates a cantilever load that flexes the beam, which is exactly what you’re trying to avoid.

Then go slow. This is where impatient DIYers crack things. Lift roughly 1/8 inch per day to keep from cracking the structure above. Screw jacks should be attached to a flat solid foundation and should not be subject to any lateral force. Screw jacks should be used to support straight vertical loads only. Keep every jack plumb — check it on two faces 90 degrees apart — and brace against any sideways push.

Lock it down when you’re set. The Ellis jacks come with a safety feature: tighten the safety lock with an Allen wrench once final adjustment has been made to prevent accidental height change — use a 6mm Allen wrench.

On the rust question — a galvanized finish is the call for damp crawl spaces and basements. The Ellis 4x4 comes in galvanized or powder coated finish, and galvanized holds up better against the moisture those spaces throw at steel.

For anything beyond a straightforward joist support, get a pro to sign off. Before construction begins, consult with a certified engineer for approval of the products within your application. Structural work isn’t the place to guess.

Frequently Asked Questions

Are floor jacks temporary or permanent? Both, depending on use. Adjustable jacks are technically both temporary and permanent, because the answer depends on how you’re using them. As a temporary setup, they hold a structure during repairs to relieve pressure while you work. They can also be used as permanent supports when they are secondary supports. Even as a permanent install, they share the load with other structural elements rather than acting as the sole fix.

Why does a jack’s rated capacity drop at greater heights? A taller column is less stable under load, so the safe rating falls as shore height climbs. The Ellis 4x4 holds 15,000 lbs at 0–4 feet but only 7,200 lbs at 8 feet, 5,000 lbs at 10 feet, and effectively nothing at 15 feet. Always read the load chart for your actual shore height instead of assuming the top number applies.

Can I use rough-cut lumber in a screw jack? No. Use standard, nominal, dressed 4x4 lumber at 3.5” square, cut square — do not use true cut lumber. For maximum safe load capacity, the lumber should be cut to at least 5.5” long and support straight loads only. Rough lumber doesn’t fit the head correctly and voids the tested rating.

Is one jack enough under a sagging floor? Usually not. Using a single jack concentrates stress and can cause damage while attempting to repair it. Whenever floor jacks are used, you need to consider the amount of stress being applied to each one. Spreading the load across multiple jacks — and a beam across several joists — keeps you from overloading any single point.

My floor is sagging in a damp crawl space — will a jack fix it? A jack supports the floor, but it doesn’t address why the floor sagged. Moisture is a common culprit. In one case the beam was decaying due to moisture damage, and the recommendation included addressing the water drainage issue since moisture was the root cause of the decay. Fix the water source along with adding support, or you’ll be back under there.

How fast should I lift? Slowly. Raise about 1/8 inch per day to avoid cracking the structure above. Rushing the lift puts sudden stress on framing, drywall, and finishes that were never loaded that way.

What if my load is more than 4,000 pounds to lift? Switch to hydraulics. The screw jacks top out around 1,800 lbs by hand or 4,000 lbs with a 2x4 for added leverage. If loads require more than 4,000 lbs of lifting capacity, use a hydraulic jack for the extra lifting power.