Sonotube vs. Auger Hole: Which Pier Footing Method to Use
Both methods produce a concrete pier footing, but they solve different problems. Picking the wrong one means fighting your soil, wasting concrete, or building a footing that doesn’t meet code.
On this page13 sections
- Sonotube vs. Auger Hole at a Glance
- Why the Choice Matters
- Sonotube vs. Auger Hole vs. Belled Footing: Key Terms
- How to Decide
- How to Test Your Soil Before Choosing
- Concrete Usage: Formed vs. Unformed
- Alternative: Leaving the Tube In vs. Stripping It
- Common Mistakes
- Which Should You Use: Decision Table
- Other Factors to Weigh
- Choosing Your Method: Step by Step
- Frequently Asked Questions
- Related Calculators
Whichever method you choose, the concrete pier calculator gives you the exact concrete volume for your diameter and depth. This guide breaks down when each method actually makes sense.
Sonotube vs. Auger Hole at a Glance
| Sonotube (Formed) | Auger Hole (Unformed) | |
|---|---|---|
| Best soil type | Loose, sandy, or unstable soil | Firm clay or hardpan that holds its shape |
| Concrete usage | Precise — matches the tube’s inside diameter exactly | Variable — the hole is rarely a perfect cylinder |
| Cost of forms | Added material cost per pier ($5–$15 depending on diameter/length) | None |
| Above-grade footing | Yes — tube extends above grade cleanly | No — hard to finish an unformed footing above ground level |
| Labor and speed | Extra step to set, plumb, and brace the tube | Faster — dig and pour directly |
| Inspection appearance | Clean, uniform, easy to verify diameter | Harder to confirm exact diameter after backfill |
Why the Choice Matters
An auger hole poured without a form relies entirely on the surrounding soil to hold its shape while the concrete cures. In firm clay or hardpan, that works fine and saves both the cost of a tube and the labor of setting one. In loose, sandy, or previously disturbed soil, the hole walls can slump or cave before the pour finishes, wasting concrete into voids and producing a footing with an unpredictable, undersized diameter exactly where you need strength most.
A sonotube solves that by giving the concrete a rigid wall to push against regardless of soil condition. It also lets the footing extend cleanly above grade, which an unformed hole generally can’t do without extra formwork anyway.
Sonotube vs. Auger Hole vs. Belled Footing: Key Terms
| Term | What It Means |
|---|---|
| Sonotube | A round, waxed cardboard concrete form, sold in fixed diameters, left in place or stripped after the pour. |
| Auger hole | A hole cut directly into soil with a power or hand auger, poured without a form, relying on the soil itself as the mold. |
| Belled footing | A footing with a wider base than shaft, formed by hand-flaring the bottom of the hole — increases bearing area without a larger overall pour. |
| Form stripping | Removing the sonotube after the concrete cures, versus leaving it in place as permanent formwork below grade. |
How to Decide
If soil holds a clean vertical wall when test-dug → auger hole is viable.
If soil crumbles, slumps, or is sandy/loose → use a sonotube.
If any portion of the footing must sit above grade → use a sonotube regardless of soil type.
How to Test Your Soil Before Choosing
- Dig a test hole to your planned depth. Leave it open for 30–60 minutes and check whether the walls hold their shape or start sloughing in.
- Check for groundwater. A test hole that fills with water needs a sonotube (or dewatering) regardless of how firm the soil looked when dry.
- Ask your local building department. Some jurisdictions require a form regardless of soil type, especially for any footing extending above grade.
- Check soil consistency at your actual footing depth, not just at the surface. Topsoil and subsoil often behave very differently once you’re 18–36 inches down.
Example 1: Firm Clay Soil, Below-Grade Footing
- Soil: Firm clay, test hole held its shape after an hour
- Footing depth: 30 in., fully below grade
- Method chosen: Auger hole, no form
The soil does the job a sonotube would otherwise do, so skipping the form saves material cost and a setup step with no downside here.
Example 2: Sandy Soil, Below-Grade Footing
- Soil: Loose sand, test hole walls began sloughing within 15 minutes
- Footing depth: 36 in.
- Method chosen: 10-in. sonotube
Without the form, the hole would keep widening as it’s dug, wasting concrete filling the extra void and producing an oversized, inconsistent footing shape.
Example 3: Firm Soil, Footing Extends Above Grade
- Soil: Firm loam, would hold an unformed hole fine
- Requirement: Pier top needs to sit 8 in. above finished grade for post anchor placement
- Method chosen: Sonotube, despite good soil
Soil condition doesn’t matter once any part of the footing needs to rise above grade — an unformed pour can’t hold a clean shape once it’s above the soil that would otherwise contain it.
Whichever method fits your soil, run your diameter and depth through the concrete pier calculator to get exact concrete volume and bag count before you order.
Concrete Usage: Formed vs. Unformed
| Method | Typical Concrete Overage | Why |
|---|---|---|
| Sonotube | 0–5% | Tube diameter is fixed and known exactly; overage is mostly rounding to the nearest bag |
| Auger hole, firm soil | 5–15% | Auger bits cut slightly oversized, and hole walls are rarely perfectly smooth or vertical |
| Auger hole, loose/uneven soil | 15–30%+ | Sloughing and widening during digging can significantly inflate the effective hole volume |
Always calculate concrete based on your form or auger diameter, then add a buffer matching your soil condition rather than assuming a formed-hole precision on an unformed pour.
Alternative: Leaving the Tube In vs. Stripping It
Sonotube doesn’t have to be a one-way choice against auger holes — you can also decide whether to leave the cardboard form in place below grade or strip it off after the pour. Leaving it in place below grade is common practice and doesn’t affect strength, since the soil around it provides lateral support either way. Stripping is more common on the above-grade portion, both for appearance and because exposed cardboard degrades with weather exposure over time. Either way, the choice doesn’t change your concrete volume calculation — it’s a finishing decision, not a structural one.
Common Mistakes
- Choosing based on cost alone. Skipping a sonotube in loose soil to save $10 in materials often costs more in wasted concrete filling an oversized, sloughed hole.
- Not testing soil at actual footing depth. Firm topsoil can sit over loose subsoil, or vice versa — test at the depth you’ll actually be pouring.
- Ignoring groundwater. A hole that fills with water needs a form (and possibly dewatering) regardless of how the soil looked when dry.
- Trying to pour an above-grade footing without a form. An unformed pour can’t hold a clean shape above the surrounding soil line.
- Assuming auger hole diameter matches the auger bit exactly. Actual hole diameter often runs slightly larger, especially in looser soil, which understates concrete needs if not accounted for.
- Skipping a test hole entirely. A quick test dig takes minutes and prevents discovering soil problems mid-pour, when it’s much harder to switch methods.
Which Should You Use: Decision Table
| Condition | Recommended Method |
|---|---|
| Firm clay or hardpan, fully below grade | Auger hole |
| Sandy, loose, or previously disturbed soil | Sonotube |
| Groundwater present at footing depth | Sonotube (with dewatering as needed) |
| Footing must extend above grade | Sonotube, regardless of soil type |
| Local code requires a form | Sonotube, regardless of soil type |
| Uncertain or mixed soil conditions | Sonotube — the safer default when in doubt |
Other Factors to Weigh
- Diameter available. Sonotubes come in fixed standard diameters (8, 10, 12 in., etc.), while an auger hole can match any bit size you have access to.
- Number of piers on the job. Sonotube cost per pier adds up fast on a large deck with many footings, making soil-appropriate auger holes more attractive where soil allows.
- Access and equipment. A power auger with a form attachment can set diameter and shape simultaneously on some rental units, narrowing the practical difference between methods.
- Inspection requirements. Some inspectors want to visually verify footing diameter before backfill, which a sonotube makes straightforward and an unformed hole makes harder to confirm.
- Reuse potential. Sonotubes are single-use per pier, adding a small ongoing material cost that a reusable auger bit doesn’t carry.
Choosing Your Method: Step by Step
- Dig a test hole to your planned footing depth in a representative spot.
- Leave it open 30–60 minutes and check whether the walls hold shape.
- Check for groundwater intrusion at depth.
- Confirm whether any part of the footing needs to sit above grade.
- Check local code for any form requirement regardless of soil.
- Choose auger hole only if soil holds firm, there’s no groundwater, and the footing stays fully below grade.
- Run your chosen diameter and depth through the concrete pier calculator to get concrete volume and bags.
Frequently Asked Questions
Is a sonotube stronger than an auger hole footing?
Not inherently — strength comes from concrete quality, diameter, depth, and reinforcement, not from whether a form was used. A sonotube’s advantage is consistency and shape control, especially in soil that can’t reliably hold an unformed shape on its own.
Can I mix methods on the same deck?
Yes, and it’s common on sites with mixed soil conditions — use sonotubes where soil is loose and auger holes where it’s firm, as long as each footing meets the same depth and diameter requirements.
Do I need to remove the sonotube after pouring?
No, it’s optional below grade — leaving it in place doesn’t affect strength. Many builders strip the above-grade portion for appearance and weather resistance while leaving the buried section in place.
How much more concrete does an auger hole use versus a sonotube?
It depends heavily on soil condition — firm soil might add only 5–10% over the auger bit’s nominal volume, while loose or sloughing soil can add 20% or more. A sonotube’s usage stays close to its stated diameter regardless of soil.
Can I use a sonotube in firm soil just to be safe?
Yes — a sonotube works in any soil type, it’s just an added cost and step that firm soil doesn’t strictly require. It’s a reasonable choice if you’re unsure or want consistent, easy-to-verify footing dimensions across the whole job.
What diameter sonotube do I need for a deck pier?
Most residential deck piers use 8–12 inch diameter tubes, sized to the post anchor and load requirements for your specific beam and pier spacing — check your local code or an engineer’s recommendation for your exact load case.
More deck pier & footing guides
Locked in your method? Run your final diameter and depth through the concrete pier calculator to get exact concrete volume and cost before you order.