Concrete Form Tubes (Sono Tube and Big Foot)

A concrete form tube is a single-use cylinder that shapes wet concrete into a round column, pier, or footing. Paperboard column forms are a staple for the construction industry — a single-use, lightweight alternative to fiberglass and steel column forms. You set it in a hole, fill it, let it cure, and you’ve got a clean round support for a deck, fence, post, or column.

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

Here’s the part most folks get wrong: the tube shapes the column. It does not carry the load. The footing under the tube does that. And in Montana, where frost runs deep and snow stacks up, undersizing that footing is how a deck ends up heaved out of level by spring.

That’s where the two-part thinking matters. The tube — Sonotube and similar — gives you the column. A Big Foot bell base bolts onto the bottom of that tube and spreads the load across a much wider footprint. The Bigfoot system is a cone-shaped footing form made of lightweight recycled high-density polyethylene plastic, which replaces traditional wooden boxes. It attaches to the construction tube with 3 to 4 screws, then sets into the ground on 4 to 6 inches of crushed rock and is leveled, then poured as one unit using another Bigfoot, upside down, on top of the construction tube as a funnel. The payoff: a bigger bearing area without going to a bigger, harder-to-fill tube.

Straight Tube vs. Bell Base — and Which Grade to Buy

Sonotube comes in a few grades, and picking wrong means either a blowout on a tall pour or overpaying for a backyard mailbox post.

Builder’s Tube is the DIY-grade form. These are built strong to withstand the heavy pressure of wet concrete, designed for below ground-line placement, but can be used up to 3 feet above ground line for extra support. Minimal bracing is required above 2 feet of ground line. One thing to respect: Sonotube Builder’s Tube concrete forms should not be used with mechanical vibrators.

Commercial grade steps up the strength. It offers improved strength and tear resistance, and can withstand full liquid head pressures up to 20 feet. The proprietary outside ply makes it impact resistant, highly water resistant, and it will not rust. That added tear and impact resistance earns its keep on taller pours.

Finish Free rounds out the Sonoco line alongside Commercial and Builder’s Tube.

The Big Foot bell base is the other half of the system. The bell sizes pair with specific tube diameters:

Big Foot model	Base diameter	Fits tubes	Concrete volume
FSBG 24	24 in	8″–10″	2.50 cu ft
FSBG 28	28 in	10″–12″	3.25 cu ft
FSBG 36	36 in	12″–18″	6.00 cu ft

That base spreads load far wider than the tube alone — the whole reason to run one. Pouring as one unit with an inverted Bigfoot funnel eliminates air pockets, possible frost heaves, and effectively sheds water away from the footing. The volume of concrete used is marked on each Bigfoot.

Builder’s Tube sizing — handy for figuring how much mud you’ll need:

Tube diameter	Concrete per foot of height
6″	0.20 cu ft
8″	0.35 cu ft
10″	0.55 cu ft
12″	0.80 cu ft

Builder’s Tube comes in 6″, 8″, 10″, and 12″ diameters in standard lengths of 4′, 8′, and 12′. Western Building Center’s Sonotube listings cover the 6″, 8″, 10″, and 12″ diameters, sold by the lineal foot. One caution worth repeating: diameter sizes are nominal — if critical, you need to measure. Inside diameter can run larger than the number on the label.

Sizing to the Load, the Soil, and Montana Frost

This is where projects live or die. A tube diameter pulled out of thin air isn’t sizing — it’s guessing.

The honest answer on structural loads: construction tubes exceeding 8 ft in height above grade must be approved by a Professional Engineer and require external vibration. For anything carrying real structural weight, confirm sizing with a licensed engineer and your local code. A 12″ pier under a small deck and a 12″ pier under a roofed porch are two different math problems.

A reasonable starting point: match tube diameter to roughly three times the post width, so a 4×4 lands on a 12″ tube. Small fence, mailbox, and lamp posts fall in the 6″–10″ range. Deck piers and porch columns typically run 12″–18″. Bigger structural columns go up from there.

The frost line is the controlling factor here, not the diameter. Across most of western Montana, frost runs 36 to 48 inches deep. The base of every pier has to sit below that line on firm, undisturbed soil — otherwise freeze-thaw will lift it. This is exactly what the bell base earns its money on. Whatever the weather, the Bigfoot tube delivers a dependable way to build footings — ground water, wet soil, or moisture poses no threat. The HDPE tubes also eliminate “friction frost heaving” in colder climates — the grip frozen soil gets on a rough column and ratchets upward each cycle.

Soil matters too. Where bearing is weak — clay, lakebed deposits, saturated ground — a bell base spreads the same load across far more square footage than the column alone, which is the whole point of running a Big Foot instead of just dropping in a bigger tube.

Pouring It Without a Mess

Get below frost, on solid ground. The pier base sits below the frost line on undisturbed soil. With a bell base, place the Bigfoot in the excavation on top of undisturbed soil or 4 to 6 inches of compacted crushed stone or gravel, level it, and have it inspected if necessary, then backfill.

Brace it plumb. If the construction tube height exceeds 3 ft above ground, the tube must be braced. On a straight Builder’s Tube, if it extends more than 2 feet above the ground line, provide additional support with a collar or wooden frame and side braces. A tube that drifts out of plumb during the pour stays crooked forever.

Backfill by hand, in lifts. When backfilling around a bell base, avoid dropping or placing heavy rocks or heavy wet clay on the footing form. Easy does it — mechanical tampers and backhoe buckets can crush the form.

Keep it dry before you fill. Be sure the Builder’s Tube form is dry before filling it with concrete. This is a real concern on wet Montana sites. Standard fiber tubes don’t belong in standing water. That’s where the grades and the HDPE base separate themselves — Commercial is highly water resistant, and the Bigfoot HDPE tube is unaffected by ground water, wet soil, or moisture, before and after pouring. For a saturated hole, that’s the move.

No vibrators on Builder’s Tube. Do not use a mechanical vibration technique when placing concrete in the Builder’s Tube form — it must remain stationary until the concrete sets.

Strip it — or leave it. Builder’s Tube forms are designed to be left in place. If you want to strip them, use a reliable release agent before pouring, remove as soon as possible after a 24-hour curing period, and cut the forms vertically with a knife or power saw.

A time-saver worth using: erect all the tubes for your job in advance so you can pour them all at once.

Frequently Asked Questions

Does the tube carry the load, or the footing? The footing does. The tube only shapes the concrete column. The bearing comes from the base area in contact with the soil, which is why a bell base matters — the Bigfoot is a cone-shaped footing form that replaces traditional wooden boxes and spreads load across a far wider footprint than a straight tube of the same diameter.

What size tube do I need for a 4x4 post? A reasonable rule is a tube diameter about three times the post width, which puts a 4×4 on a 12″ tube. For anything structural, sizing should be confirmed against your local code and, where loads are significant, a licensed engineer.

How deep does the footing have to go? The base has to sit below the frost line on firm, undisturbed soil. Across most of western Montana that’s 36 to 48 inches. Going below frost is what keeps freeze-thaw from heaving the pier out of level.

Can I set a form tube in a wet hole? Not a standard fiber tube — keep it dry before pouring. For wet ground, Commercial grade is highly water resistant, and the Bigfoot HDPE tube is built for it. Ground water, wet soil, or moisture poses no threat to the Bigfoot tubes, and they eliminate friction frost heaving in colder climates.

When can I strip the form? Remove the forms as soon as possible after a 24-hour curing period if you’re stripping at all. Many builders simply leave Builder’s Tube in place, since it’s designed to stay.

What’s the difference between Builder’s Tube and Commercial? Builder’s Tube is the economical DIY-grade form for decks, fence posts, and the like. Commercial is heavier-duty: improved strength and tear resistance, impact resistant, and rated for full liquid head pressures up to 20 feet. Tall pours and demanding jobs want Commercial.

How much concrete will a tube hold? For Builder’s Tube, figure 0.20 cu ft per foot at 6″, 0.35 at 8″, 0.55 at 10″, and 0.80 at 12″. Big Foot bases add their own marked volume — 2.50 cu ft for the FSBG 24, 3.25 for the FSBG 28, and 6.00 for the FSBG 36.