Do Sonotubes Need a Footing at the Bottom?

Do sonotubes need a footing at the bottom, or is the tube enough? Sonoco and the 2021 IRC speak to this in different words, and the code text never mentions tubes. The sonotube volume calculator counts only the concrete inside the tubes, so the footing question sits with your building official.

Do Sonotubes Need a Footing? What the Manufacturer Says

Sonoco treats the tube as a form: its FAQ says Sonotubes “are not designed to carry structural loads.” The same FAQ says pier depth is set by local building codes, with the base generally deeper than the frost line and in contact with firm, undisturbed soil.

It names the building codes inspector or a licensed structural engineer as the people to check with. For tube size, tube base and rebar, Sonoco says the answer depends on the full structure design, the soil and local codes, and that a licensed engineer or architect must provide it.

Sonoco also sells a separate Tube Base footing form. Its 2016 products brochure says the form is used with Sonotube forms for added strength, increased stability and greater bearing capacity, and lists decks, porches and posts as uses. No source read for this article compares a tube pier on undisturbed soil with one on a poured pad, so that comparison is not stated.

What Does the 2021 IRC Say About Footings?

The 2021 IRC sizes footings by load and soil, and the text read for this article never uses the word tube. Section R403.1.1 says the size of footings supporting piers and columns is based on the tributary load and the allowable soil pressure in Table R401.4.1.

Section R403.1.4 puts exterior footings at least 12 inches below the undisturbed ground surface and sends deck footings to Section R507.3. Section R403.1.4.1 says piers and other permanent supports must be protected from frost by listed methods, including extending below the frost line in Table R301.2. It also says footings may not bear on frozen soil unless the frozen condition is permanent.

The text does not say whether a poured tube alone counts as a footing. That question belongs to your building official. For footing depth in general, see how deep footings should be.

How Big Is a Deck Footing Under the 2021 IRC?

Table R507.3.1 sizes a deck footing by tributary area and allowable soil bearing, so the footing grows as the load grows. Section R507.3 says decks sit on concrete footings, or other approved structural systems, designed for all loads, with footings sized to carry the imposed loads to the ground.

Three cells from the table for 1,500 psf soil and a 40 psf live or ground snow load: 20 sq ft of deck needs a 12 in round footing 6 in thick; 40 sq ft needs 16 in and 6 in; 160 sq ft needs 31 in and 11 in. The table allows interpolation but not extrapolation. Check the table in your adopted code before relying on any figure.

As arithmetic from those cells, a tube with a 12 in nominal diameter matches only the 20 sq ft case. Sonoco says nominal means the inside diameter is at least the stated size and may be up to 1 in larger. Section R507.3.2 sets deck footings at least 12 inches below undisturbed ground, and R507.3.3 adds frost protection where a deck attaches to a frost-protected structure.

What Does DCA 6 Add, and Which Edition Is It?

DCA 6, the American Wood Council deck guide, shows a narrower concrete stem on a larger footing as an accepted option, but the copy read here is based on the 2015 IRC. Its commentary says Figure 12 includes an option with a 12 in diameter concrete stem to reduce the concrete required.

The guide says footings bear on undisturbed soil at least 12 inches down or below the frost line, whichever is deeper. It adds that the building official verifies bearing conditions in the field before concrete is placed. Its Table 4 lists round footings from 18 in to 42 in across its rows, under its own 2015 assumptions of 40 psf live load and 1,500 psf soil.

Do not read those sizes against the 2021 table. A newer DCA 6 edition may exist: not verified.

Should the Tube Stay in the Ground?

Sonoco gives two different statements, one for each product, so follow the instructions for the tube you buy. Its Builder’s Tube sheet says the form is “designed to be left in place” and is not designed to be stripped; a release agent is needed if stripping is wanted.

The Sonotube FAQ says below-grade forms need no stripping unless local code requires it, and that removal is general practice for exposed columns. The same FAQ also calls the forms temporary, single-use concrete forms to be removed before project completion. Your local code and inspector settle which applies.

Does the Sonotube Calculator Count a Footing?

No, the calculator counts the concrete inside round tubes only, and its FAQ says it adds no footing or flare. The tool page points to the concrete bell pier calculator for a flared footing and the concrete footing calculator for footings under piers.

Here is the tool’s default case, as arithmetic only: 4 tubes, 12 in across, 4 ft long, 10% waste.

1. One tube. 3.1416 x 0.5 ft x 0.5 ft x 4 ft = 3.14 cu ft.
2. Four tubes. 3.1416 x 4 = 12.57 cu ft.
3. Add 10% waste. 12.57 x 1.1 = 13.82 cu ft.
4. Cubic yards. 13.82 / 27 = 0.51 cu yd.
5. 80 lb bags. At the tool’s 0.60 cu ft per bag, 13.82 / 0.60 = 23.04, rounded up to 24 bags.

None of those numbers includes a footing under any tube. Estimate that separately.

Who Decides the Footing at Your Site?

Your building official decides, using the code edition your area has adopted. The 2021 IRC is a model code, so local amendments and the local frost depth apply, and the inspector approves the footing before concrete is placed.

Sonoco and DCA 6 both point to the same person: Sonoco says to check with the building codes inspector, and DCA 6 says the building official verifies bearing before the pour. Ask the department these questions:

• Which code edition and local amendments apply to my deck?
• What is the local frost depth?
• What footing size does the table give for my tributary area and soil bearing?
• Is a tube-formed pier with no wider base accepted? The IRC text is silent.
• Is a footing form such as Tube Base accepted? The IRC does not mention it.

FAQs About Sonotube Footings

Sources and Further Reading

This article reports what Sonoco, the 2021 IRC and a 2015-based DCA 6 edition say, not a design for your pier. Your building department decides the footing for your pier, so ask before you pour. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-20.