Rebar chairs in Canada are bought by type and height, and the height is set by the concrete cover on your drawings. For a bottom mat, the chair height equals the cover (for example 20 mm for an interior slab or 75 mm for concrete cast against earth under CSA A23.1). For a top mat, it is the slab thickness minus the top cover minus the bar layers. Chairs are sold by the bag, box or bundle, and what you pay depends on material, height, load rating, coating and quantity, so the best way to price them is as a line item on your rebar order.
This guide covers the chair types you will see in Canada, how to pick the right height, how to estimate quantities and what drives the cost.
Why chairs matter
Rebar only works if it ends up in the right place in the concrete. Chairs and other bar supports hold the steel at the height the designer intended, so it has the specified cover and sits at the right depth to resist bending and control cracking.
Bars laid on the ground and "hooked up" during the pour rarely end up where they belong. That is one of the most common reinforcing mistakes on residential slabs, as our guide to rebar spacing in concrete slabs explains. CSA A23.1 requires reinforcement to be supported and secured so it stays in position during placing, and specifications commonly require bar supports to meet A23.1 and be accepted by the engineer. Industry practice for supports is described in the RSIC Manual of Standard Practice.
Types of rebar chairs and supports
| Type | What it is | Where it is used |
|---|---|---|
| Individual high chair | A single chair, plastic or wire, that holds one bar or an intersection | Slab bottom and top mats, beams |
| Plastic rod chair | Moulded chair with a seat for the bar, often with more than one seat height | Slabs on grade, suspended slabs, light mats |
| Slab bolster | Continuous support, a long strip with legs, that carries several bars | Bottom mats on formwork, where many bars run the same way |
| Continuous high chair | A longer, taller continuous support for top mats | Top mats in thicker slabs and decks |
| Precast concrete block | Small concrete block, sometimes with a tie wire cast in (often called a dobie) | Footings and slabs on soil, where wire or narrow plastic legs would sink |
| Wheel or side spacer | Round plastic spacer that clips onto the bar | Walls, columns and beam sides, to hold side cover |
| Coated or plastic-tipped wire chair | Wire chair with plastic tips or full coating | Exposed soffits and epoxy-coated bar |
Material choices:
- Plastic does not rust and will not show as rust spots on exposed surfaces. Good for architectural concrete and coated bar.
- Bright (uncoated) wire is strong and common for heavy mats on forms, but the feet can rust and stain an exposed soffit.
- Plastic-tipped, galvanized or epoxy-coated wire reduces staining and suits corrosive exposures. Specifications for epoxy-coated bar usually call for coated or non-metallic supports.
- Precast concrete blocks are the usual choice on soil and for footings, because they bear on the ground without sinking.
For GFRP bar, use non-metallic supports. GFRP is lighter and more flexible than steel; one manufacturer recommends supports at two-thirds of the spacing used for steel.
How to choose the chair height
Bottom mats
The chair height is the specified cover from the drawings or CSA A23.1. Typical values from A23.1 Table 17 (see our concrete cover guide for the full table):
| Situation | Specified cover | About in inches |
|---|---|---|
| Interior slab or wall, Class N | 20 mm | 3/4 in. |
| Interior beam or column, Class N | 30 mm | 1-1/4 in. |
| Freeze-thaw or sulphate exposure (F and S classes) | 40 mm | 1-1/2 in. |
| Chloride exposure, such as parking decks (C classes) | 60 mm | 2-3/8 in. |
| Cast against and permanently exposed to earth | 75 mm | 3 in. |
Many chairs in Canada are still labelled in inches, so you will often be matching a metric cover to the nearest imperial chair. Check whether the stated size is the height to the underside of the bar, which is what controls cover. Do not round down below the specified cover. Cover for slabs on grade is usually set on the drawings to place the steel where it controls cracking, so take it from the drawings.
Top mats
Work down from the top of the slab. Example: a 200 mm suspended interior slab with 20 mm cover top and bottom, and two-way 15M mats top and bottom (15M is 16.0 mm nominal diameter):
- Top of the upper top bars: 200 − 20 = 180 mm above the form.
- The top mat is two layers of 15M: 2 × 16 = 32 mm.
- Underside of the top mat: 180 − 32 = 148 mm above the form.
- If the top chairs stand on the bottom mat instead (20 mm cover + 32 mm of bars = 52 mm), they need to be 148 − 52 = 96 mm tall.
Deformations make real bars slightly larger than nominal, so check the result against the chair sizes available and the tolerance on the drawings. Our Canadian rebar size chart gives the nominal diameters.
How many chairs do you need?
There is no universal count per square metre. The number depends on:
- Bar size and spacing. Small bars sag between supports and need them closer together. Larger bars span further.
- Mat type. A top mat that crews walk on needs stiffer, closer support than a bottom mat on forms.
- Support type. One continuous bolster replaces many individual chairs along its length.
- Specification or manufacturer guidance. Some specifications set spacing; the RSIC manual and chair manufacturers give guidance.
A practical way to estimate: lay out a support grid on a plan of the slab, count supports per bay, multiply by the number of bays, and add a margin for breakage and edges. On larger jobs, the rebar detailer can include supports in the take-off. Our article on rebar cost for a concrete slab shows where accessories fit in a slab estimate.
What drives the cost of rebar chairs
This guide does not list prices, because chair prices move with resin and steel costs and vary by product. These are the factors that set what you pay:
- Material. Plastic, bright wire, coated wire and precast concrete are priced differently.
- Height. Taller chairs use more material and usually cost more per piece.
- Load rating and design. Heavy-duty chairs for thick mats and walked-on top mats cost more than light rod chairs.
- Coating. Plastic tips, galvanizing or epoxy coating add cost on wire supports.
- Quantity and packaging. Full bags, boxes or bundles are cheaper per piece than loose counts.
- Delivery. Chairs are light but bulky. Adding them to a rebar delivery avoids a separate trip.
When comparing rebar quotes, check whether chairs and accessories are included. A quote with supports and one without are not the same scope.
Where to buy rebar chairs in Canada
Chairs are sold by concrete accessory suppliers, building supply and contractor yards, some home centres (usually a narrow range of plastic chairs), and rebar fabricators and distributors. For a one-off residential pour, a contractor yard is usually the simplest. For a structural job with a bar list, buying supports with the rebar keeps sizes matched to the cover and bar layers on the drawings.
We detail, cut, bend and deliver 10M to 55M rebar and GFRP bar across the GTA and Ontario. We don't supply chairs or accessories, so source those from a building supply yard. Send your drawings for a quote, or see our rebar products.
Frequently asked questions
What size rebar chair do I need?
For a bottom mat, the chair height equals the concrete cover on the drawings, for example 20 mm for an interior slab or 75 mm for concrete cast against earth under CSA A23.1. For a top mat, work down from the top of the slab: thickness minus top cover minus the bar layers.
How many rebar chairs come in a box?
It depends on the product and supplier. Plastic chairs are commonly packed in bags or boxes of a few hundred, with 500 per bag a common count in retail listings. Bolsters and continuous chairs are sold by length or by bundle.
How much do rebar chairs cost?
Cost depends on the material, the height, the load rating, any coating, the quantity and delivery. Taller and heavy-duty chairs cost more per piece, and buying with the rebar order usually saves a separate delivery. Ask your supplier to quote chairs as a line item.
Can I use bricks or rocks instead of rebar chairs?
Not on engineered work. Bricks and rocks can break, shift or leave weak spots and water paths in the concrete. Use purpose-made chairs, bolsters or precast concrete blocks sized to the specified cover.
Are plastic rebar chairs better than wire chairs?
Each has a place. Plastic chairs do not rust or show at the surface, so they suit exposed and architectural concrete. Wire chairs are stronger for heavy mats. On soil, use concrete blocks or chairs with wide bases or plates so they do not sink.
