Sonotube and Bigfoot Footing Forms

A Sonotube and a Bigfoot footing form do two different jobs. While the Sonotube itself shapes the concrete column, the footing beneath it is what actually transfers the load safely into the ground. The cardboard tube gives you a clean round pier. The Bigfoot bell base spreads that load across a wide footprint of soil. Pour them together and you get a footing and pier in one shot.

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For the deck builder, fence setter, or pole-barn guy in Montana, the buying decision comes down to three things: how much load the post carries, what the soil underneath can bear, and how deep your frost line runs. Get those right and the footing outlasts the structure. Get them wrong and you’re chasing cracks and settling for years.

Here’s the part that matters most around here. Bigfoot’s plastic tube is immune to moisture before and after pouring — ground water, wet soil, or moisture poses no threat. The cardboard Sonotube does the job and has for decades, but it has to be dry before filling it with concrete. In a climate where framing season overlaps with rain and snowmelt, that distinction is real.

What Each Form Is and Why They Work Together

The Sonotube is the old reliable. Sonotube concrete forms, the industry standard for more than 70 years, are respected for their strength-to-weight quality and durability. They are recognized as a practical way to create columns for buildings, and to provide structural support and footings. No assembly. Cut to length. Drop it in the hole. The forms are designed to be left in place.

The Bigfoot is the upgrade for the soil side of the equation. It’s an engineered footing form made from recycled high density polyethylene which replaces the wood formwork. The footing form is cone-shaped with an allowance for standard fiber construction tubes to be attached to the top. Once joined, it’s filled with concrete to create a monolithic pour which forms the footing and pier as one unit. The construction tube and footing form remain in place after the concrete is cured.

The reason to bother with the bell base: the flared bell increases effective bearing area 4-6× without increasing the tube diameter — a smart way to upgrade footing capacity on weak soil. On Montana’s glacial till, lake-bed clay, and moraine soils that change lot to lot, that’s a useful trick.

The cone shape earns its keep two other ways. The sloped footing base eliminates any 90-degree angles and allows any ground water running down the pier to drain away from the footing. And the geometry fights heave: the sloped surface creates a resistance area when backfilled which withstands frost heave and bending forces exerted against the vertical piers. The all-plastic BFT10 tube goes further — it eliminates “friction frost heaving” in colder climates. Across western Montana, where frost runs 36 to 48 inches, that matters.

Sizes, Models, and What Fits What

Start with the tube. Sonotube Builder’s Tube comes in 6”, 8”, 10” and 12” diameters in standard lengths of 4’, 8’ and 12’. Concrete demand climbs fast as diameter grows.

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

Match the Bigfoot model to your tube. The bell models step up in base size, and each one accepts a range of tube diameters.

Bigfoot Model	Base Diameter	Fits Tube	Concrete Volume
BF20	20”	6”–8”	—
BF24	24”	8”–10”	2.50 cu ft
BF28	28”	10”–12”	3.25 cu ft
BF36	36”	12”–18”	6.00 cu ft

Western Building Center carries the BF20 (fits 6”–8” tube), the BF24 (fits 8”–10” tube), and the BF28 (fits 10”–12” tube). Model BF20 is 20 inches in diameter at the base, which accommodates 6” and 8” construction tubes. Model BF24 is 24 inches at the base, accommodating 8” and 10” tubes. Model BF28 is 28 inches at the base, accommodating 10” and 12” construction tubes.

There’s also an all-plastic tube option. The BFT10 measures 10” by 4’ and can be used with either Bigfoot models BF24 or BF28. Its concrete volume is 2.18 cubic feet. It meets or exceeds all local and national building codes when used with a Bigfoot footing form, and ground water, wet soil or moisture poses no threat.

One caution on the biggest bell: the Model BF36 is not recommended for above ground applications.

Choosing the Right Diameter: Load, Soil, and Frost

Size by what the post carries and what the dirt can hold. General rule of thumb on diameter:

Sonotube sizes range from 6 inches to over 60 inches in diameter. The right size depends on your load requirements, soil conditions, and project type.

Diameter	Typical Use
6–10”	Fence posts, mailbox posts, small footings
12–18”	Deck piers, porch columns
20–36”	Structural columns, light pole bases

The math behind it is straightforward. A 12” sonotube provides 0.785 sqft bearing × 2000 psf = 1,570 lb allowable per post. If the tributary load on that post runs higher than the allowable, you upsize — or you switch to a bell base to gain bearing area without going to a fatter tube.

Soil is the variable that catches people. Use a real number from a soil report when you have one. Common soils run 1,500 psf for soft clay, 2,000 psf for the clay and sand mix default presumption, 3,000 psf for dense gravel. If you’ve never had a soil test, 2,000 psf is safe. The decision tree is simple: a straight sonotube works in stable soil ≥ 2,000 psf. Below that, the Bigfoot earns its keep. A Bigfoot bell lets you use a smaller tube diameter — often the better call across 4 or more footings on weak soil.

Don’t undersize to shave the spec. Incorrect Sonotube sizing can cut strength by up to 30%, increase cracking, and lead to repairs down the road. Oversizing goes wrong the other direction — more concrete, more labor, no added value. The sweet spot is the smallest diameter that passes with at least 20% headroom. Smaller leaves you under-spec; larger just adds work and material you don’t need.

Two Montana-specific points. Frost: in cold climates, footings need to extend below the frost line, often 36–48 inches deep, to prevent heaving. Your tube sizes and lengths must account for this depth plus any height above grade. Snow: design load isn’t just the deck and furniture. IRC minimum is 50 psf — 40 live plus 10 dead — and you add 25 to 35 psf in a snow region. That extra load pushes you toward larger diameters or a bell base on most Montana decks.

Installation Basics

The sequence is the same whether you’re working below grade or above. Get the base set right and the rest follows.

Set the base. Place the Bigfoot in the excavation on top of undisturbed soil or 4 to 6 inches of compacted crushed stone or gravel, level, and have it inspected by a building inspector if necessary, then backfill.

Trim and attach the tube. Cut off the rings you don’t need. The rings not used must be cut off at the top of the ring being used and removed before any concrete is poured. Then fasten: attach the required length of construction tube to the Bigfoot with a minimum of four 3/4” to 1” long #8 wood screws. If the tube frays inward when cut with a handsaw, remove the excess frayed material with a utility knife for ease of application.

Anchor it down for above-ground work. Concrete wants to float the form. Drill 3/8” holes on a 45-degree angle at each rib location to accept minimum 12” spikes, then place the Bigfoot on undisturbed soil and drive spikes at a 45 degree angle inwards to prevent it from rising when concrete is being poured. If the soil won’t let the spikes resist that upward pressure, use sandbags or backfill to hold the form in place.

Brace anything tall. The two systems differ slightly on the threshold. Sonotube calls for additional support with a collar or wooden frame and side braces if the tube extends more than 2 feet above the ground line. Bigfoot requires bracing if the height of the construction tube exceeds 3 feet above ground. When in doubt, brace it.

Pour in lifts. Don’t dump the whole pier at once. The concrete must be placed in lifts that are 10 to 16 inches in height, with the concrete being consolidated after each lift. A pouring tip from the manufacturer: pour as one unit using another Bigfoot, upside down, on top of the construction tube as a funnel. The payoff of doing it as a monolithic pour — this method eliminates air pockets, possible frost heaves and effectively sheds water away from the footing.

One hard rule for the cardboard tubes: do not use a mechanical vibration technique when placing concrete in the Sonotube form. To maintain placement, the form must remain stationary until the concrete sets.

Mind the height caps. The total combined height of Bigfoot Systems footing forms must not exceed 13 ft. And tubes that go up high need an engineer: construction tubes exceeding 8 ft in height above grade must be approved by a Professional Engineer and require external vibration.

Frequently Asked Questions

Do I need a footing under a Sonotube? In most cases, yes. While the Sonotube shapes the concrete column, the footing beneath it transfers the load safely into the ground. Without a properly sized footing, even correctly selected tube sizes can sink, tilt, or crack over time — especially in poor soil conditions. A Bigfoot bell base serves as that footing, poured monolithically with the pier.

Straight tube or Bigfoot bell — which do I need? It hinges on soil. A straight sonotube works in stable soil of 2,000 psf or better. On weaker ground, the bell base buys you 4 to 6 times the bearing area without a bigger tube, and across several footings it often makes more sense than upsizing every tube diameter.

How deep does the footing have to go in Montana? Below the local frost line. In cold climates that’s often 36–48 inches deep to prevent heaving, and your tube length must account for that depth plus any height above grade. Confirm the exact number with your local building department before you dig.

How much concrete will I need? For the tube portion, figure by diameter: 0.20 cubic feet per foot of height for a 6” tube, 0.35 for 8”, 0.55 for 10”, and 0.80 for 12”. The bell adds its own volume — 2.50 cubic feet for the 24” base, 3.25 for the 28”, and 6.00 for the 36”.

Can the Bigfoot tube get rained on before I pour? The plastic one, yes. The BFT10 is resistant to moisture before and after pouring, and can be installed and completely backfilled, then filled with concrete at another time. The cardboard Sonotube is the opposite — it must be dry before filling with concrete.

Can I reuse these forms? No. Sonotube Builder’s Tube forms are single-use and designed to be left in place. The Bigfoot likewise stays buried as part of the finished footing.