Rebar in Sonotubes and Concrete Piers

Rebar in a sonotube pier isn't always required by the Ontario Building Code for small residential piers, but most designers call for it: a few vertical bars running from the footing to near the top of the pier, held in place by ties, with enough concrete cover all round. The bars help the pier resist frost uplift, sideways loads from tall posts, and cracking at the joint between the pier and its footing. When your drawings show rebar, the drawings set the bar size, number, ties and cover.

This post explains when pier rebar is needed, how a typical sonotube pier is reinforced, how to fit bars inside a small round form, and how the same thinking applies to post footings, pergola footings and small concrete stairs.

Do concrete piers need rebar?

Under Part 9 of the 2024 OBC, Article 9.15.2.3 says pier type foundations must be designed to support the applied loads from the superstructure. It doesn't spell out reinforcement for a small pier under a deck post. That mirrors footings: the Part 9 footing rules don't call for rebar in plain footings either, as covered in do footings need rebar under the Ontario Building Code.

So why do so many pier details show bars? Because plain concrete is weak in tension, and piers see tension in ways that aren't obvious:

• Frost uplift. Freezing soil can grip the sides of a pier and push it upward while the footing below holds it down. That pulls the pier apart along its length, and the weak point is often the joint between the pier and its footing. Vertical bars tie the two together.
• Lateral loads. A tall deck post, a pergola in the wind, or a post carrying a gate or railing pushes sideways on the top of the pier. That bends the pier, putting one face in tension.
• Cold joints. When the footing and the tube are poured at different times, the joint between them is a plane of weakness. Bars crossing it hold it together.
• Anchor pull-out. Post bases and anchor bolts are pulled upward by wind and uplift. Bars around the anchor help keep the top of the pier from cracking.

The short answer: if the drawings or your building department's deck guide show rebar, it's required for your job. If you're designing the deck, talk to the designer before leaving it out.

How a typical sonotube pier is reinforced

A reinforced pier is a small column on a footing. The common pieces are:

• Footing. A pad or bell at the bottom, sized by the designer for the load and soil. It's often cast against the soil.
• Vertical bars. A small group of bars, commonly 10M or 15M, spaced evenly around the inside of the tube. They run from near the bottom of the footing to just below the top of the pier, with a bent leg at the bottom (an L-bar) that sits in the footing.
• Ties. Closed hoops or ties around the verticals at the spacing on the drawing. They hold the verticals in position during the pour and confine the concrete.
• Anchor or post base. A cast-in anchor bolt, J-bolt or post base at the top, set to the layout and height on the drawing.

If the footing and pier are poured separately, dowels from the footing lap with the pier verticals instead of the verticals running straight through. Our post on prefabricated rebar cages for footings and piers covers when it's worth assembling cages before they go in the hole.

Fitting rebar inside a round form

Small tubes leave little room. The bars must sit far enough from the tube wall to get the concrete cover on the drawings, which comes from CSA A23.1 and depends on exposure. Where concrete is cast directly against soil, as the bottom of a footing usually is, the minimum cover is 75 mm. Our guide to concrete cover for rebar under CSA A23.1 covers the other cases.

A worked example of the geometry: in a 250 mm diameter tube, if the drawings call for 40 mm of cover to the ties, the ties have an outside diameter of about 250 minus 2 x 40, or 170 mm. The vertical bars sit just inside the ties, so with 10M ties and verticals the bar centres end up roughly 135 mm apart across the tube. In a 200 mm tube the same cover leaves a tie about 120 mm across, which is tight for 15M verticals and means small, carefully bent hoops.

Practical points that keep the steel where it belongs:

• Centre the cage. Spacers or chairs on the ties, or a cage tied to a brace across the top of the tube, stop the bars drifting to one side during the pour.
• Bend hoops in the shop. Small-diameter hoops are hard to bend accurately by hand. Our post on bending rebar circles and hoops explains why minimum bend diameters limit how small a hoop can be.
• Keep the L-legs off the soil. The bent bottom legs need cover too, so they sit on chairs, not the dirt.
• Leave room for the anchor. The anchor bolt or post base needs to drop into the middle of the cage without hitting the bars.

Rebar for post, pergola and round footings

The same approach applies to round footings for posts and pergolas, with a different emphasis:

• Pergolas and shade structures are light but catch wind, so uplift and sideways load at the anchor often govern. Bars around and below the anchor matter more than bulk compression.
• Fence and gate posts see sideways load at the top. Vertical bars on all sides help resist bending.
• Deck posts on tall decks combine vertical load with lateral load from bracing. The designer checks the pier as a short column.

Whatever the structure, the post base or anchor detail at the top of the pier and the footing size at the bottom come from the drawings, and the rebar follows them.

Rebar in small concrete stairs

Exterior concrete steps are another small pour where rebar questions come up. The 2024 OBC (Article 9.8.9.2) requires exterior concrete stairs with more than 2 risers and 2 treads to be either:

• Supported on unit masonry or concrete walls or piers not less than 150 mm in cross-section, or
• Cantilevered from the main foundation wall. Cantilevered steps must be designed to support their loads (Article 9.8.10.1) and anchored to concrete foundation walls not less than 200 mm thick (Article 9.8.10.2).

Foundations for exterior steps with more than 2 risers follow the minimum depth rules in Section 9.12, while steps with 1 or 2 risers may be laid on the ground (Sentence 9.12.2.2.(4)). Stairs serving a single dwelling unit are designed for a specified load of 1.9 kPa (Clause 9.8.9.1.(1)(a)).

In practice, a stair cast in place usually has bars in its sloped slab and landing, and a cantilevered stair has bars doweled into the foundation wall. That's design work, and doweling into an existing wall has its own rules, covered in how to dowel rebar into existing concrete. Get the detail from your designer before ordering.

Ordering bars for piers and small pours

For a set of piers, the order is usually a short bar list: vertical L-bars, hoops or ties, and any dowels, all repeated for each pier. Cutting and bending them in the shop gives consistent hoops and legs, and tagging them per pier keeps the site simple.

We cut and bend rebar from 10M to 55M, including fabricated dowels, ties and custom bent shapes, tagged and bundled to your bar list, and we deliver across the GTA and Ontario. Send us your drawings or bar list for a quote, or see our rebar products.