Concrete Form Tubes: Sizes, Setting & Pouring Footings

Most failed deck piers don’t crack because the concrete was weak. They fail because the form bulged, tipped out of plumb, or floated up during the pour. Concrete form tubes are the simplest way to cast round piers and columns, but they only work well when you size them correctly, brace them firmly and fill them in the right sequence. Get those three things right and you end up with clean, round footings that carry deck, porch and pergola loads for decades.

A form tube is a hollow cylinder, usually spiral-wound waxed cardboard or rigid plastic, that holds wet concrete in shape until it cures. You set it in a dug hole, plumb it, brace it, drop in reinforcement and pour. Once the concrete hardens, you strip the exposed portion of the tube and leave the rest buried. Diameters typically run from 6 to 48 inches, with 8 to 24 inches covering almost every residential job.

What Concrete Form Tubes Are Used For

Homeowners and builders reach for round forms whenever they need a round, isolated pier rather than a continuous footing. Typical uses include deck footings, porch and stair landings, pergola and carport posts, light pole bases, sign posts and decorative porch columns. Cardboard tubes for concrete are by far the most common on residential sites because they’re cheap, light and easy to cut with a handsaw.

Tubes are not a substitute for a proper footing pad. Many designs call for a wider spread footing at the bottom of the hole, with the tube sitting on top to form the pier that rises to grade. Some builders use a plastic bell-shaped base that attaches to the bottom of the tube, creating a wide foot and a narrow column in one pour. Whichever approach you take, the footing size and depth come from your local code and your deck or structure plans, not from the tube itself.

Choosing the Right Tube Diameter

Diameter controls how much load the pier can spread into the soil and how much room you have for reinforcement and anchor hardware. Bigger isn’t automatically better: a 24 inch tube holds more than four times the concrete of a 12 inch tube at the same height, which adds cost and mixing labor fast.

A 24 inch tube shows up most on taller porch columns, carport supports and decks built on soft clay where the plans call for a wide bearing area. For a typical backyard deck with 6×6 posts, 10 to 12 inch tubes over a wider footing pad are common. Your building department’s deck guide or an engineer’s plan will list the required pier diameter based on beam spans, joist spans and soil bearing capacity, so check that first.

Estimating Concrete

Multiply the cross-sectional area by the height. For a 12 inch tube that’s 0.785 square feet times the depth in feet. A 48 inch deep, 12 inch pier needs about 3.1 cubic feet, or roughly five 60 lb bags of premixed concrete. Add 10 percent for spillage and the flared bottom of the hole.

Cardboard vs Plastic Form Tubes

Spiral-wound cardboard tubes are coated with wax or a moisture barrier on the inside. They’re inexpensive, usually sold in 4 and 12 foot lengths, and they cut cleanly. The downside is that rain and soggy soil soften them. A tube that sits in a wet hole for a week before the pour can collapse or bulge under the weight of fresh concrete.

Plastic round cement forms cost more but resist moisture, stay rigid and can sometimes be reused for above-grade columns. Some plastic forms have a ribbed or textured interior that leaves a decorative finish. For buried deck piers, cardboard is usually plenty. For exposed columns where appearance matters, or for jobs that might sit through rainy weather before the pour, plastic is worth the premium.

Whatever material you choose, buy tubes rated for the height you’re pouring. Tall columns put real hydrostatic pressure on the bottom of the form, and thin-walled tubes can split at the base on pours over 8 feet.

Tools and Materials

• Form tubes in the diameter your plans require
• Post hole digger or power auger, plus a digging bar
• Handsaw or reciprocating saw to trim tubes
• 4 ft level and a string line with stakes
• 2×4 lumber and deck screws for bracing
• Rebar (usually #4) plus tie wire and a tie tool, or prefabricated rebar cages
• Premixed bagged concrete or ready-mix, a wheelbarrow and a mixing hoe
• Gravel for the hole bottom if your plans call for it
• Anchor bolts or post bases set in wet concrete
• Waterproof gloves, safety glasses, long sleeves and a dust mask

How to Set and Pour Concrete Form Tubes

1. Call 811 and get permits. Call 811 at least a few business days before digging so buried utilities are marked. Most decks attached to a house, or decks above a set height, need a permit and a footing inspection before you pour.
2. Lay out the piers. Use stakes and string lines to mark pier centers, checking diagonals so the layout is square. Mark each center with spray paint.
3. Dig the holes. Dig to the depth your local frost line and plans require, plus any gravel base. Make the hole a few inches wider than the tube, or wider still at the bottom if you’re forming a spread footing. Keep the bottom flat and on undisturbed soil.
4. Cut and set the tube. Trim the tube so it rises 2 to 4 inches above finished grade to shed water away from posts. Set it in the hole, centered on your string line.
5. Plumb and brace. Check plumb on two sides with a level. Screw 2×4 cleats to the top of the tube and stake them to the ground in two directions so the form can’t lean or float. Backfill around the outside of the tube in lifts, tamping lightly, leaving the inside empty.
6. Install reinforcement. Drop in the rebar cage, centered with at least 3 inches of concrete cover on all sides. Use small concrete blocks or plastic chairs at the bottom so steel doesn’t touch soil.
7. Mix and pour. Mix concrete to a stiff, oatmeal-like consistency. Pour in lifts of 12 to 18 inches, rodding each lift with a length of rebar or tapping the outside of the tube to release air pockets.
8. Set hardware. Strike the top flat, then set the anchor bolt or post base on your string line while the concrete is still workable. Check height and alignment twice.
9. Cure and strip. Keep the tops damp and covered for several days. After the concrete has hardened, usually 24 to 48 hours, peel or cut away the exposed portion of the tube. Wait for adequate strength, often about a week, before loading posts.

Rebar Cages for Tube Forms

Many deck piers in moderate climates are poured without reinforcement, but plans for tall piers, columns and anything carrying lateral loads usually call for steel. A typical tube rebar cage uses three or four vertical #4 bars tied to horizontal hoops spaced about 12 inches apart. Prefabricated cages sized for 8, 10 and 12 inch tubes save a lot of tying time when you have more than a few piers.

Keep the cage away from the tube walls and the soil. Steel that sits too close to the surface rusts, expands and spalls the concrete over time. If an anchor bolt or J-bolt drops into the center of the cage, make sure there’s room for it before you pour so you aren’t fighting the steel at the last minute.

Safety When Working With Concrete Footings

Wet concrete is caustic. Prolonged skin contact causes chemical burns that often don’t hurt until hours later, so wear waterproof gloves, long sleeves and safety glasses, and rinse any splashes off skin with clean water right away. Mix bagged concrete outdoors with the wind at your back and wear an N95 dust mask, because cement dust irritates lungs and contains crystalline silica. Never mix concrete additives or cleaners together, and keep acid-based cleaners away from fresh concrete.

For structural safety, set footings on undisturbed soil below the frost line, anchor posts with code-approved connectors rather than burying wood in concrete, and follow any HOA rules or permit conditions. Wind and snow loads matter on pergolas and covered porches, so let the plans dictate pier size and spacing rather than guessing.

Troubleshooting Common Tube Form Problems

The tube bulged or split

This usually happens when concrete is poured too fast or the tube was waterlogged. Pour in lifts, backfill the outside of the tube before pouring, and replace any tube that feels soft. A small bulge below grade is cosmetic; a split that lets concrete blow out should be dug out and re-formed.

The form floated up

Wet, soupy concrete can lift a light tube, especially if backfill hasn’t been placed. Stiffer mixes, good bracing and partial backfill solve it.

The pier is out of plumb

Check plumb after every lift. Once the concrete starts to stiffen, corrections become difficult. Minor lean can sometimes be handled with adjustable post bases, but a significantly tilted pier should be replaced.

Voids and honeycombing

Rough, gravelly surfaces show that air was trapped. Rod each lift and tap the tube with a rubber mallet. Small surface voids can be patched with a repair mortar once the form is stripped.

Cost of Concrete Form Tubes and Footings

Cardboard tubes are among the cheapest parts of a footing job. Small diameters often run a modest amount per 4 foot section, while 24 inch tubes and plastic forms cost considerably more. Bagged concrete, rebar and hardware usually add more than the tube itself. A DIY deck with eight 12 inch piers typically lands in the low hundreds for materials. Hiring a contractor to dig, form and pour piers often costs several hundred dollars per footing depending on depth, soil, access and whether a power auger can reach the site.

When to Call a Pro

Bring in a contractor or engineer if your soil is soft clay, fill or rocky ledge, if the structure is two stories high or supports a roof, or if your frost line requires very deep holes that are hard to dig by hand. An engineer is also a smart call for retaining wall areas, slopes and columns that carry significant lateral loads. Many deck builders own powered augers and can set a full layout of concrete footing tubes in a day, which can be worth the cost when you have a dozen piers to dig.