Rebar lying on the dirt does almost nothing for a slab. Reinforcing steel only works when it’s held at the correct height inside the concrete — and the little supports that do that job are called chairs. Concrete chairs for rebar are among the cheapest items on any pour, a few cents to a dollar apiece, yet skipping them or spacing them wrong is one of the most common reasons residential slabs crack early: the steel ends up at the bottom of the pour, or walked down into the mud, where it can’t intercept tension. Getting chairs right takes ten minutes of planning. Here’s the full picture — types, heights, spacing, and the cover rules that drive everything.
Why Rebar Position Is Everything
Concrete is strong in compression and weak in tension; steel carries the tension. In a typical slab-on-grade, cracking tension develops in specific zones, and the bar must sit where that tension occurs — for most residential flatwork, at or slightly above the slab’s mid-depth. Equally important is cover: the minimum concrete thickness between steel and any surface. ACI 318 requires 3 inches of cover for concrete cast against earth, 1-1/2 to 2 inches for formed surfaces exposed to weather, and 3/4 inch for interior faces — because insufficient cover lets moisture reach the steel, and rusting rebar expands up to four times its volume, spalling the concrete from within. A chair’s whole purpose is to guarantee both numbers: bar high enough to work, buried deep enough to survive. In a 4-inch slab over grade, a 2-inch chair centers the steel; supported on nothing, that same bar delivers roughly zero structural benefit and a corrosion path from the soil.
Chair Types and When to Use Each
The market breaks into a few families:
- Plastic chairs: the residential default. Individual snap-on or set-under supports in fixed heights (3/4 inch to 6 inches), typically $10–$25 per bag of 50–100. Non-corroding, fine on compacted grade or vapor barrier. Bar chairs grip the rebar; slab bolsters spread load on soft ground.
- Wire chairs (steel): individual high chairs and continuous bolsters used widely in commercial work; plastic-tipped or epoxy-coated versions prevent rust telegraphing to exposed surfaces. Continuous bolsters at $1–$3 per 5-foot length excel under mats and mesh.
- Concrete blocks (dobies): small precast blocks, often 2 or 3 inches, sometimes with embedded tie wire — about $0.30–$0.75 each. The go-to on grade because they won’t sink into soil, won’t blow around, and match the surrounding concrete perfectly. The traditional choice under footings and thickened edges.
- Mesh chairs/castles: tall plastic supports made for welded wire mesh, which otherwise gets walked to the bottom of every pour — the fate of most un-chaired mesh.
Match the base to the substrate: sharp-footed wire chairs punch through vapor barriers and sink in sand, so use plate-based plastic chairs or dobies over poly sheeting; on hard formwork (elevated decks, footings with forms), standard wire or plastic all work.
Choosing Chair Height
Work backward from slab thickness and target bar position. For slab-on-grade with a single mat: bar center at mid-depth or slightly above means chair height equals roughly half the slab thickness minus half the bar diameter — practically, 2-inch chairs for a 4-inch slab, 3-inch for a 6-inch slab. For footings, chairs (usually 3-inch dobies) establish the 3-inch earth cover under the steel. For driveways and exterior flatwork, some engineers spec steel at one-third depth from the top to fight surface shrinkage cracking — follow the plan if you have one. When in doubt on unengineered residential work, mid-depth is the defensible default. One caution: don’t “fix” height by hanging rebar high on tall chairs in a thin slab and violating top cover; in a 4-inch pour there is only about an inch of legal territory for a #4 bar, which is why the right chair matters more than field improvisation.
Spacing: How Many Chairs You Need
The governing principle is that the steel must not deflect to the ground when stepped on during the pour — because it will be stepped on. Standard practice: support #3 and #4 rebar every 3 to 4 feet along each bar, and place a chair within 6 to 12 inches of bar ends and at intersections of a grid (intersections are efficient — one chair supports two bars). Heavier #5+ bar spans farther and can stretch toward 4 to 5 feet. Welded wire mesh is far floppier: chairs every 2 to 3 feet in both directions, no exceptions, or it ends up on grade. Continuous bolsters simplify all of this at roughly one row per 3–4 feet. Quick estimate for a 20 ft × 20 ft slab with #4 bar on 18-inch centers each way: supporting the grid at alternating intersections on a 3-foot pattern needs roughly 50 chairs — call it a $20–$40 line item on the material list. Buy 20 percent extra; they’re consumed by mud, breakage, and re-spacing.
Placement Technique and Pour-Day Rules
Set chairs after the base is compacted and the vapor barrier (if any) is down, snapping or resting bar into them and tying the grid at intersections with 16-gauge tie wire so nothing shifts. Walk the assembled mat before ordering concrete — every spot that flexes to grade underfoot needs another chair now, not during the pour. On pour day, crews should place from mats or planks where possible and lift bar back onto displaced chairs as they go; the “hook it up with a rake mid-pour” method beloved of driveway crews reliably leaves steel low, which is precisely why inspectors in permitted work check chairing before allowing a pour. If an inspection applies to your project, chairs and cover are on the checklist — dirt-supported steel is a correction notice everywhere.
Chairs vs. the Alternatives People Actually Use
Every concrete forum eventually hosts the same debate, so it’s worth addressing the improvised methods head-on. Bricks and rock chunks are the most common substitute, and they half-work: they hold height, but odd shapes tip underfoot, clay brick wicks moisture against the steel, and random rubble creates uneven support that lets the mat flex and walk off. Purpose-made dobies cost pennies more and eliminate all three problems. The “hook and pull” method — laying steel on grade and lifting it with a rake as concrete is placed — deserves its bad reputation: studies and core samples consistently show hooked steel settling back to the bottom third of the slab, and no inspector accepts it on permitted work. Plastic chair skeptics sometimes cite shadowing — faint surface marks or a weak plane where the chair meets an exposed soffit — which is a legitimate concern on architectural elevated slabs and the reason commercial decks often spec steel bolsters with protected tips; for slab-on-grade flatwork it’s irrelevant, since the chair sits on dirt or plastic nobody ever sees. The one context where supports genuinely become optional is fiber-reinforced slabs with no bar or mesh at all, an increasingly common residential spec — but that’s a different reinforcement strategy, not a cheaper way to support steel.
Buying Guide: What to Get for Typical Projects
For a standard 4-inch residential slab — shed base, patio, sidewalk — with #3 or #4 bar: 2-inch plastic bar chairs with wide bases, or 2-inch dobies if pouring over bare compacted soil; a $15–$30 bag covers most projects. For a 5- or 6-inch garage or driveway slab: 3-inch supports, and consider continuous slab bolsters if you’re using mats of mesh, at $1–$3 per 5-foot stick. For footings: 3-inch dobies with tie wires, sold by the dozen at masonry suppliers cheaper than big-box singles. Add a 3.5-pound roll of 16-gauge tie wire ($10–$15) and a $15 tie-wire twister tool, which saves your wrists on anything bigger than a doghouse pad. Home Depot and Lowe’s stock the basics; concrete supply houses beat them on price and stock the full height range, heavy-duty chairs for #5+ bar, and mesh-specific supports the big boxes often lack. One last sizing note: chairs are sold by the height they hold the bottom of the bar, so a “2-inch chair” puts the center of a #4 bar at about 2-1/4 inches — close enough for flatwork, but worth remembering when an engineer’s drawing calls out bar centerline elevations on structural pours.
Cheap Insurance, Summarized
Chairs cost less than one percent of a slab and control a huge share of its performance. The working rules: pick chair height from slab thickness to put bar at mid-depth with ACI cover respected (3 inches against earth, 1-1/2 to 2 exposed, 3/4 interior); support every bar every 3 to 4 feet and mesh every 2 to 3; match chair base to substrate — dobies or plate-footed plastic on soil and poly, anything on forms; tie the grid and proof-walk it before the truck arrives. Do that, and the steel you paid for will actually be where the cracks want to happen — which is the entire point of putting it there.
