That single dimension controls corrosion resistance, fire rating, and the structural capacity of the finished slab or beam. At Leeter, we have manufactured wire and plastic rebar supports in Dingzhou, Hebei since 2006, and we ship them to jobsites across North America and Europe. This guide explains the types, the height rules, spacing, and how to choose.
Key takeaways
- Support height must equal the specified concrete cover — the single most important spec on any chair.
- ACI 318 sets minimum cover from 3/4 in for interior slabs up to 3 in for cast-against-earth surfaces.
- Bar chairs suit individual bars; slab bolsters and continuous high chairs carry whole mats of steel.
- Plastic-tipped and plastic chairs stop the rust bleed that stains exposed and architectural concrete.
- Wire diameters run 3.0 to 8.0 mm and heights from about 3/4 in (20 mm) to 15 in (380 mm).
What do rebar supports actually do?
Rebar supports do one job with high stakes: they position reinforcing steel at a precise, repeatable height so the concrete cover meets the structural drawings. Concrete cover is the clear distance between the outermost steel and the concrete surface. That layer is what protects the reinforcement, and it is set entirely by the chair.
Cover matters for three reasons. First, corrosion. The alkaline concrete around the steel forms a passive film that resists rust, and adequate cover keeps chlorides and carbonation from reaching the bar. Second, fire. Cover acts as thermal insulation that delays the steel from losing strength in a fire. Third, structure. The effective depth of a beam or slab — the lever arm the engineer designed around — depends on the bars sitting where they belong.
When chairs are missing, too short, or spaced too far apart, bars sag or float. We have walked decks where a mat drooped an inch between supports, and that inch quietly removed capacity the designer counted on. Chairs are cheap. Repairing a spalled, rust-stained slab is not.
Rebar supports hold reinforcing steel at the height that produces the specified concrete cover, the clear concrete distance protecting the bar. ACI 318 treats cover as a durability and structural requirement, since corrosion, fire resistance, and effective depth all depend on the steel sitting exactly where the chair places it.
Why does concrete cover matter so much?
Concrete cover is the front line of durability, and codes treat it as non-negotiable. Under ACI 318, minimum cover ranges from 3/4 in (20 mm) for interior slabs and joists, to 1-1/2 in (40 mm) for exposed beams and columns, up to 2 in for surfaces exposed to weather and 3 in (75 mm) for concrete cast permanently against earth.
Those numbers are minimums, not targets. Exposure class drives them upward. A parking deck, a coastal balcony, or a bridge sees chlorides and freeze-thaw cycling that an interior floor never will, so specifications push cover to 2 in or more and often call for galvanized or epoxy-coated steel. The chair height must track those requirements exactly.
Here is the honest trade-off. More cover means better corrosion protection but a smaller effective depth if the section stays the same thickness. That is why you follow the drawings rather than adding cover for safety. The engineer already balanced durability against capacity. Your job on site is to reproduce that number with the right chair, every time.
The most common cover failure we see is not too little steel — it is inconsistent chair height across a single pour. A mat set on a mix of 1-1/2 in and 2 in supports gives you a wavy steel plane and localized low-cover spots that corrode first. Standardize the height, then standardize the spacing.
Cover requirements by exposure
| Exposure condition | Typical minimum cover | Common support choice |
|---|---|---|
| Interior slab or joist | 3/4 in (20 mm) | Wire bar chair, plastic chair |
| Beams and columns | 1-1/2 in (40 mm) | High chair, wire chair |
| Exposed to weather | 2 in (50 mm) | Slab bolster, plastic-tipped chair |
| Cast against earth | 3 in (75 mm) | Precast block, slab bolster |
What are the main types of rebar chairs and supports?
Rebar supports split into individual chairs and continuous supports, and the choice depends on whether you are holding single bars or whole mats. CRSI, the Concrete Reinforcing Steel Institute, classifies bar supports into standard classes covering wire, plastic-protected, and precast types, which is the language most North American specs use.
Here are the workhorses you will specify most often.
Individual bar chair. The basic wire chair that holds a single bar or the intersection of a light mat. Heights typically run from 3/4 in to about 3 in. Ideal for thin slabs and residential work. See our rebar chairs range for standard and plastic-tipped versions.
High chair. A taller individual support, usually 2 in up to around 15 in (380 mm), used to hold the top mat in a two-mat slab or deck. High chairs come with legs spread for stability on formwork.
Continuous high chair (CHC). A long, upside-down U made from heavy wire that spans across form joists and carries a line of bars in the top mat. CHCs distribute load and speed placement on elevated decks. They are commonly furnished with a bottom runner wire for use on removable forms.
Slab bolster. A continuous wire support with a corrugated top that cradles the bottom mat of a slab and rides on the form. Standard heights run 3/4 in to 3 in, and the running length lets one piece support many bars at once. Our slab bolsters ship in standard and upper-class finishes.
Beam bolster. Similar to a slab bolster but built heavier to carry beam reinforcement, available in higher load ratings and taller heights.
Rod chair. A simple, economical wire chair for lighter reinforcement and mesh, often used in slab-on-grade. Browse rod chairs for standard sizes.
Bar chair vs slab bolster vs high chair
| Support type | Best use | Typical height range |
|---|---|---|
| Individual bar chair | Single bars, light mats, thin slabs | 3/4 in - 3 in (20-75 mm) |
| Slab bolster | Bottom mat of slabs, continuous runs | 3/4 in - 3 in (20-75 mm) |
| Continuous high chair | Top mat on elevated decks | 2 in - 15 in (50-380 mm) |
| High chair | Top steel, deeper sections | 2 in - 15 in (50-380 mm) |
CRSI classifies bar supports into standard classes spanning bright wire, plastic-protected, and precast types. Slab bolsters and continuous high chairs carry entire mats along a running length, while individual bar chairs support single bars, with wire heights running roughly 3/4 in to 15 in depending on top or bottom mat use.
Wire, plastic, or precast: which support material wins?
No single material wins every job, so match the support to the exposure and the load. Wire supports carry the most load and suit heavy mats. Plastic supports resist corrosion and protect finished surfaces. Precast concrete blocks, sometimes called dobies, shine where crews walk heavily and where cover must never move.
Wire chairs are the strongest and most versatile. They handle the weight of stacked mats and foot traffic during placement. The catch is that bare wire can rust where a leg touches the form face, leaving a stain. That is where the finish and the tip matter.
Plastic chairs and clip-on wheel spacers never rust and never bleed. They are the default for architectural, exposed, and water-retaining concrete. Their limit is load and heat: heavy top mats can overload light plastic, and some plastics soften under direct sun before the pour.
Precast concrete supports match the cover material itself, bond well, and shrug off point loads. They are heavy to handle and offer fixed heights, so crews carry fewer size options.
On coastal and water-treatment jobs our customers almost always move to plastic or plastic-tipped supports, not because the wire fails structurally, but because inspectors reject any rust bleed on a surface that has to last 75 years.
Why choose plastic-tipped chairs?
Plastic-tipped chairs solve one specific, visible problem: rust stains on the finished surface. The chair is a normal steel wire support, but each foot that contacts the formwork is capped with a plastic tip. That tip breaks the path between the steel leg and the concrete face, so no rust bleeds through to stain the slab.
They give you the load capacity of wire with the surface protection of plastic. For architectural soffits, exposed slab edges, and any surface that will be seen or sealed, plastic-tipped chairs are the practical middle ground. They cost slightly more than plain wire and less than fully plastic or galvanized supports.
Would you accept a row of orange dots on a polished floor? Neither would your client. On exposed concrete, the tip earns its keep.
How far apart should rebar chairs be spaced?
Spacing keeps the steel from sagging between supports, and the rule of thumb scales with bar size and mat stiffness. A common field guideline places supports on a grid of roughly 3 to 4 ft (about 1.0 to 1.2 m) for typical slab mats, tightening for smaller, more flexible bars and loosening slightly for larger, stiffer bars.
Lighter bars bend more, so they need closer support. As a starting point many crews use around 3 ft (0.9 m) spacing for #3 and #4 bars, 4 ft (1.2 m) for #5 and #6, and up to about 5 ft (1.5 m) for #7 and larger, then adjust for construction loads and walking. Always defer to the project specification and CRSI placing guidance where they are stricter.
Across the deck and slab-on-grade orders we supply, bottom-mat layouts most often settle near a 4 ft grid, which for a 200 m² slab works out to roughly one chair per 3 to 3.5 m². Use that as a planning figure, then confirm against your bar schedule.
How many chairs per square metre?
Estimate support count from spacing, then add for edges and traffic. On a 4 ft (1.2 m) grid you place about one support per 1.4 m² of grid area for a single mat, but real layouts, mat overlaps, and walking paths push practical density toward one chair per 2 to 3.5 m². For continuous supports like slab bolsters, count linear metres of runner instead of points.
Which supports suit slabs, decks, and bridges?
Match the support to the structure, because a slab-on-grade, an elevated deck, and a bridge deck each load the steel differently. Slab-on-grade rides on soil or vapor barrier and takes light foot traffic. Elevated decks sit on removable or steel forms and carry two mats. Bridge decks demand the highest durability and cover.
Slab-on-grade. Bottom mat only in most cases. Slab bolsters and rod chairs on the sub-base do the work, often with plastic feet or precast blocks over a vapor barrier so the point does not puncture it. Heights typically match 3/4 in to 2 in cover.
Elevated decks. Two mats. Slab bolsters or bar chairs carry the bottom steel on the form, while continuous high chairs or high chairs hold the top mat at the correct depth. Stability on the form is critical, so wide-base supports win.
Bridge decks. High cover, high exposure, high scrutiny. Specifications routinely call for 2-1/2 to 3 in cover, galvanized or epoxy-coated bars, and plastic-protected or coated supports so nothing compromises the corrosion system. This is where cutting a corner on chairs is most expensive.
Galvanized or plain wire supports?
Galvanized supports resist corrosion far longer than bright wire, so the choice comes down to exposure and finish. Plain (bright) wire is economical and fine where it will be fully buried in sound, dry concrete. Galvanized wire adds a zinc coating that delays rust, making it the right call for exposed, humid, or chloride-rich environments.
The trade-off is cost against service life. Plain wire is cheaper per piece and structurally identical when new. Galvanized costs more but protects the surface and the support itself where moisture will reach it. For coated-bar systems, specifications often require the supports to match, so the whole assembly resists corrosion as one.
Our standard rebar supports ship in plain, galvanized, and plastic-tipped forms, so you can match the finish to the specification rather than compromising.
How do you select the right rebar support?
Work through a short checklist and the selection almost makes itself. Start with the required cover, then the mat position, then the exposure, then the load and surface finish. Each answer narrows the field.
- Cover height: Pick the support height that equals the specified cover. This is the first and firmest decision.
- Mat position: Bottom mat means bolsters or bar chairs; top mat means high chairs or continuous high chairs.
- Exposure class: Interior and dry favors plain wire; weather, water, or chlorides favor galvanized, plastic, or plastic-tipped.
- Load and traffic: Heavy mats and busy crews call for wire or precast; light mesh accepts plastic or rod chairs.
- Surface finish: Exposed or architectural surfaces need plastic-tipped or plastic to prevent rust bleed.
- Spacing: Set the grid from bar size, then confirm against the specification.
Nail those six and you have specified correctly. When you are ready, request a quote and we will match heights, wire gauge, and finish to your bar schedule.
Frequently asked questions
How do I choose the right rebar chair height?
Set the chair height equal to the concrete cover on your structural drawings, measured from the form face to the outermost bar. For a 2 in cover on the bottom mat, use a 2 in support. Never guess or add cover for safety, since the engineer already fixed that number for durability and effective depth.
What is the difference between a bar chair and a slab bolster?
A bar chair is an individual support that holds a single bar or a light intersection, ideal for thin slabs and spot placement. A slab bolster is a continuous wire support with a corrugated top that cradles an entire line of bars along its length. Use bar chairs for single bars and bolsters to carry a whole bottom mat efficiently.
Why use plastic-tipped rebar chairs?
Plastic-tipped chairs give you the load capacity of steel wire without rust stains on the finished surface. The plastic cap on each foot breaks contact between the steel leg and the concrete face, so no rust bleeds through. They are the practical choice for exposed, architectural, or sealed concrete where appearance and durability both matter.
How far apart should rebar supports be spaced?
Space supports on roughly a 3 to 4 ft (1.0 to 1.2 m) grid for typical slab mats, tightening to about 3 ft for small #3 and #4 bars and loosening to around 5 ft for #7 and larger. Closer spacing prevents sagging between supports. Always follow the project specification or CRSI placing guidance when it is stricter than the rule of thumb.
When should I use high chairs or continuous high chairs?
Use high chairs and continuous high chairs to support the top mat in a two-mat slab, elevated deck, or beam. They range from about 2 in up to 15 in (380 mm) tall and hold upper steel at the correct depth. Continuous high chairs span across form joists to carry a full line of top bars and speed placement.
Rebar supports are the cheapest insurance on any reinforced pour, and choosing the right height, type, and finish protects the steel for the life of the structure. Tell us your cover, mat layout, and exposure, and we will match wire gauge, height, and coating to your bar schedule. Request a quote and we will help you specify supports that pass inspection and last.
