Rebar for Suspended and Elevated Concrete Slabs

A suspended slab is a structural slab: it spans between beams, walls or columns, so its rebar is designed by an engineer, with main bars near the bottom at midspan and top bars over the supports. CSA A23.3 sets the limits the design works inside, including a minimum of 0.002 times the gross slab area in each direction and a maximum spacing for main bars of three times the slab thickness or 500 mm. The only suspended slab the Ontario Building Code lets you build from a recipe is a small cold room slab; everything else is engineered.

This post explains how elevated slabs are reinforced, how one-way, two-way and steel deck slabs differ, and what the drawings need to show before bar can be cut.

Why suspended slabs are always designed

A slab on grade rests on the ground. A suspended slab has nothing under it between supports, so it carries its own weight, the floor finishes and the live load by bending, and the steel takes the tension that bending creates.

The 2024 Ontario Building Code reflects that. Section 9.40 covers just one case prescriptively: reinforced concrete slabs suspended over cold rooms in basements, supported by foundation walls around the perimeter with no interior supports, and with a clear span of not more than 2.5 m in the short direction. For those it requires:

• a slab at least 125 mm thick, cast against formwork to CSA A23.1,
• Grade 400 bars to CSA G30.18,
• 10M bars at not more than 200 mm each way, with 30 mm clear cover to the bottom and the second layer laid directly on the first,
• at least 75 mm of bearing on the walls, anchored with 600 mm x 600 mm 10M bent dowels at not more than 600 mm.

Sentence 9.40.1.1.(2) sends every other slab to Part 4 design, and the section does not apply to slabs meant to carry motor vehicles at all. So a concrete second floor, a roof slab, a slab over a garage or a balcony needs an engineer. That is good news for the reader trying to find "rebar spacing for second floor slab": the answer will be on the structural drawings.

Bottom bars, top bars and why both

A suspended slab bends in a predictable way:

• At midspan it sags, stretching the bottom. That is where the bottom bars go, as close to the underside as the cover allows.
• Over supports that the slab is continuous across, it hogs, stretching the top. That is where the top bars go, extending into each span for a length set by the design.
• At cantilevers such as balconies, the whole projection is in hogging, so the main steel is on top and must anchor well back into the slab.
• At edges where the slab ends on a wall or beam, there's some restraint, so the design usually adds top bars at the edge too.

Bottom bars are typically continuous or lapped across the span; top bars are usually shorter and concentrated over supports. CSA A23.3 has rules for how far bars extend and how they anchor at discontinuous edges, including requirements that positive moment bars embed into spandrel beams, columns or walls, and that both top and bottom bars run to the edge of a cantilever. The bar lengths on the drawing reflect those rules, so don't shorten them on site.

Cover depends on exposure. In CSA A23.1 Table 17, slabs in dry interior conditions (class N) need 20 mm, slabs exposed to freezing and thawing need 40 mm, and slabs exposed to chlorides, like parking decks, need 60 mm. See our concrete cover guide for the full table.

One-way and two-way slabs

One-way slabs span in one direction between parallel beams or walls. The main bars run across the span; lighter distribution bars run the other way for shrinkage and temperature. In CSA A23.3 (the 2004 edition is the one we checked), main bars in one-way slabs can be spaced no farther apart than the smaller of three times the slab thickness or 500 mm (Clause 7.4.1.2), and the minimum steel can be spaced up to the smaller of five times the thickness or 500 mm (Clause 7.8.3).

Two-way slabs span in both directions, either onto beams on all sides or directly onto columns (flat plates and flat slabs). They are designed in column strips and middle strips, with more top steel near the columns. Clause 13.10.4 sets the spacing limits:

Two-way slabs on columns also need structural integrity reinforcement: bottom bars that pass through the column core in each direction, at least two bars each way, so that if a slab punches through at a column it hangs on instead of collapsing (Clause 13.10.6). These are easy to miss on site and important to get right.

The minimum steel in either system is 0.002 times the gross area in each direction (Clauses 7.8.1 and 13.10.1). Our post on maximum rebar spacing under CSA A23.3 puts the limits into a table by slab thickness.

Slabs on steel deck

Many elevated slabs in steel-framed buildings are poured on corrugated steel deck. There are two broad kinds:

• Composite deck, where embossments bond the deck to the concrete and the deck acts as the bottom reinforcement in the span direction.
• Non-composite (form) deck, where the deck is only a permanent form and the slab is reinforced like a conventional slab.

Either way, deck slabs usually carry welded wire mesh or light bars for shrinkage and crack control, and often additional top bars over the supporting beams where the slab is continuous. Openings, cantilevered edges and heavy point loads get extra bars. The deck supplier's load tables and the engineer's drawings set what is required; "rebar spacing for steel deck slab" has no generic answer.

Placing elevated slab steel

The sequence on a formed slab usually runs:

1. Bottom bar supports and bottom bars in the first direction, then the second direction.
2. Bars for beams, drop panels and openings.
3. Top bar supports (high chairs, continuous high chairs or support bars), then the top bars.
4. Tying, inspection and cover checks.

Supports have to keep the bottom mat at its cover and the top mat at its height while crews walk on it and concrete is placed. Top bars that get trampled down to mid-depth lose much of their capacity over the supports. Our post on where rebar should sit in a slab explains how to work out heights.

The RSIC Manual of Standard Practice makes the placing contractor responsible for bar supports, unless reinforcing bar used as a support is shown on the structural drawings.

What the drawings and bar list need to show

Before suspended slab steel is detailed and cut, the drawings should give:

• slab thickness and any drops, beams or thickenings,
• bottom and top bar sizes, spacing and directions, with the extents of top bars,
• cover for top and bottom faces,
• lap locations and lengths, or a lap schedule,
• integrity bars at columns, bars at openings and edges, and cantilever anchorage,
• the edition of CSA A23.3 and the exposure class.

Our post on rebar detailing for slabs covers how those requirements become placing drawings and bar lists.

Getting suspended slab steel detailed and cut

Elevated slabs are where detailing pays for itself: top bar extents, laps, integrity bars and edge bars are all easy to get wrong when a crew works straight from the structural sheets. We prepare bar lists and placing drawings from your structural drawings, cut and bend bar from 10M to 55M, and deliver tagged, bundled bar across the GTA and Ontario. Engineering services are available for projects that need them; tell us about your project when you ask for a quote.

Send your structural drawings for a free take-off and quote, or see our detailing and fabrication services.