Rebar Chair Spacing and Placement

Rebar chairs are usually spaced no more than about 1,200 mm (4 ft) apart in each direction, and closer, around 800 mm (about 2 ft 7 in), for light 10M bars. That is the basis Canadian rebar suppliers use under the RSIC Manual of Standard Practice, which supplies slab supports so the average spacing does not exceed 1,200 mm each way. On a 1,200 mm grid you need about 0.7 chairs per square metre; on an 800 mm grid about 1.6. Your drawings or specification can call for closer spacing, and they override the general practice.

This guide gives a spacing chart, shows how to count chairs for a real slab, and covers how to place them. For chair heights, see our post on rebar chair height for 4 and 6 inch slabs.

What Canadian practice says about chair spacing

The RSIC Manual of Standard Practice (Fifth Canadian Edition, 2020) has a chapter on bar supports. The points that set spacing:

• Supports must be sufficient in number and strength to carry the reinforcing and keep it at the specified distance from the forms during construction.
• Slab top bar supports are supplied in quantities so as not to exceed a 1,200 mm average spacing in each direction, for slabs up to 450 mm thick.
• Light bars need closer support. In one-way slabs, where temperature bars double as support bars, RSIC shows high chairs at 1,200 mm maximum under 15M temperature bars but 800 mm maximum under 10M temperature bars, and rows of support at 800 mm under continuous 10M top bars.
• Continuous supports (slab bolsters and continuous high chairs) have their end legs lapped where runs meet, and no bar should sit more than 50 mm beyond the last leg at the end of a run.
• In two-way flat plates, bottom slab bolsters run in lines at 1,200 mm on centre, starting 300 mm from the column centreline, and top support bars sit on high chairs spaced at 800 mm maximum.

The logic is simple. Small bars are flexible and sag between supports, especially when someone steps on them. Larger bars span further. GFRP bars are lighter and more flexible than steel and often need closer support again; see how to install fiberglass rebar.

Rebar chair spacing chart

This chart converts grid spacing into chairs per area for a large slab, before edge rows are added.

Two cautions:

• Per-area figures undercount small slabs. Every slab needs a row of chairs along each edge, so a garage floor or patio uses more chairs per square metre than the chart shows.
• Chairs should land under bar intersections. If your bars are at 400 mm, an 800 mm or 1,200 mm chair grid lines up with every second or third bar. If they are at 300 mm, a 900 mm or 1,200 mm grid does the same.

How many rebar chairs do I need?

Count rows and columns for the actual slab:

1. Pick the chair spacing (from the drawings, the specification, or the practice above).
2. In each direction, take the slab dimension, subtract the distance from each edge to the first row, divide by the spacing and round up. Add one. That is the number of rows.
3. Multiply rows in one direction by rows in the other.
4. Add a few spares for breakage and for extra supports at laps, openings and edges.

Worked example: a 6.0 m by 9.0 m slab (about 20 by 30 ft) with a 10M grid at 400 mm each way. First row of chairs 150 mm in from each edge.

Chairs on an 800 mm grid (every second bar):

• Across the 6.0 m width: (6,000 minus 300) ÷ 800 = 7.1, round up to 8 spaces, so 9 rows.
• Along the 9.0 m length: (9,000 minus 300) ÷ 800 = 10.9, round up to 11 spaces, so 12 rows.
• Chairs: 9 × 12 = 108, or 2.0 per m² once the edge rows are counted.

Chairs on a 1,200 mm grid (every third bar):

• Across: 5,700 ÷ 1,200 = 4.75, round up to 5 spaces, so 6 rows.
• Along: 8,700 ÷ 1,200 = 7.25, round up to 8 spaces, so 9 rows.
• Chairs: 6 × 9 = 54, or 1.0 per m².

For a light 10M grid that people will walk on while placing and finishing, the 800 mm grid is the safer choice. If the specification gives a spacing, use that.

For the bar count in the same slab, see our rebar estimating guide.

Continuous supports instead of individual chairs

On formed slabs and on long runs, slab bolsters and continuous high chairs are often faster than individual chairs. They carry every bar that crosses them, so you count lines and metres instead of pieces.

• Lines: count them the same way as rows of chairs, at the line spacing you want (RSIC uses 1,200 mm for bottom bolsters in flat plates).
• Metres: number of lines × run length.
• Pieces: slab bolsters are commonly supplied in 1,500 mm lengths per RSIC, so divide the total metres by the piece length and allow for lapping end legs where pieces meet.
• Ends: no bar more than 50 mm past the last leg. Start and finish each line close to the bar ends.

On soil, continuous supports need runners or plates so they do not sink. Precast concrete blocks and sand-plate chairs are the usual answer for slabs on grade.

Top mats and two-layer slabs

Where a slab has top and bottom mats, the top mat needs its own supports, and these carry more load because crews stand on them.

• High chairs stand on the form or on the bottom mat and carry the top mat directly. Their height is worked down from the top of the slab; our chair buying guide has a worked example.
• Support bars are often used under top mats: rows of 15M bars on high chairs, with the top bars laid across them. In RSIC's flat-plate sequence, support bars run at up to 1,200 mm on centre with their high chairs at up to 800 mm.
• Standees (bent bars) can support top mats in thick slabs and footings. Supports for top bars in footings and mats thicker than 450 mm are special items in RSIC practice and are arranged by agreement.

How to place rebar chairs

A practical sequence for a slab:

1. Lay out the chair grid on the base or forms, starting near the edges and including extra supports where bars lap or stop at openings.
2. Set the chairs (or bolster lines) at the planned spacing. On fill or vapour barrier, use chairs with wide bases or concrete blocks.
3. Place the bottom layer of bars across the chairs, then the top layer of the grid across the bottom layer.
4. Shift chairs under intersections where needed, so each chair carries a crossing point rather than a single bar.
5. Tie the bars to each other and to the chairs. RSIC practice is that bars are wire tied at enough intersections to prevent displacement during concreting.
6. Check height and spacing before the pour. Look for chairs that have tipped over, sunk or been pushed aside.
7. Lay walk boards where crews, hoses and buggies will cross the mat. Bar supports are designed to carry the steel, not equipment; RSIC is explicit that supports failing under concrete buggies or runways is not a defect of the supports.

Getting supports on the order

Supports are easy to forget on a rebar order, and RSIC practice puts their supply with the placing contractor unless the contract says otherwise. Check whether your quote includes them. We detail, cut and deliver rebar from 10M to 55M 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.