For a 4 inch (100 mm) slab, a 10M grid centred in the slab sits on chairs about 39 mm tall (a 1-1/2 in chair). For a 6 inch (150 mm) slab, the same grid needs chairs about 64 mm tall (2-1/2 in). Those numbers come from one simple rule: the chair height is the distance from the bottom of the slab to the underside of the lowest bar, so you subtract the bars from the position you want. If your drawings specify a top cover instead of "mid-depth", work down from the top surface, as shown below.
This post works through the heights for the two most common residential slab thicknesses. For why steel belongs where it does in a slab, see where rebar should sit in a concrete slab.
How chair height is measured
The RSIC Manual of Standard Practice defines the nominal height of a bar support as the distance from the bottom of its leg, sand plate or runner wire to the bottom of the reinforcement. A small variation, about 3 mm, from the stated height is permitted.
Two things follow from that definition:
- The chair height is not the bar's centre height. It stops at the underside of the bar the chair carries.
- In a two-way grid, the chair carries the bottom layer. The second layer sits on top of the first, so the mat is two bar diameters thick.
Many chairs sold in Canada are labelled in inches. The conversions you need for thin slabs: 1-1/2 in is 38 mm, 1-3/4 in is 44 mm, 2 in is 51 mm, 2-1/4 in is 57 mm, 2-1/2 in is 64 mm, 2-3/4 in is 70 mm and 3 in is 76 mm.
Nominal bar diameters from CSA G30.18: 10M is 11.3 mm and 15M is 16.0 mm. Ribs make real bars slightly larger, so leave a little margin where cover is tight.
Two ways to work out chair height
Method 1: the drawings say "centre of slab" or "mid-depth".
- Single layer of bars: chair height = slab thickness ÷ 2, minus half a bar diameter.
- Two-way grid: chair height = slab thickness ÷ 2, minus one bar diameter (so the middle of the two-layer mat lands at mid-depth).
Method 2: the drawings give a top cover.
- Chair height = slab thickness, minus the top cover, minus the mat thickness (one bar diameter for a single layer, two for a grid).
- Round down to the nearest chair you can buy, so the top cover is never less than specified. Then check the bottom still has the cover the drawings ask for.
Method 2 is the more common case on engineered drawings, and it is the one that matters when the slab is exposed to weather or de-icing salt.
Rebar chair height for a 4 inch (100 mm) slab
| Layout in a 100 mm slab | Calculation | Chair height | Nearest imperial chair |
|---|---|---|---|
| 10M grid, centred | 50 minus 11.3 | about 39 mm | 1-1/2 in |
| 10M single layer, centred | 50 minus 5.65 | about 44 mm | 1-3/4 in |
| 10M grid, 40 mm top cover | 100 minus 40 minus 22.6 | about 37 mm max | under 1-1/2 in |
| 15M grid, centred | 50 minus 16 | about 34 mm | about 1-3/8 in |
Look at the third row. A 1-1/2 in chair (38 mm) is 1 mm too tall for 40 mm top cover with a 10M grid: the top of the steel would end up about 39 mm below the surface. That is the kind of rounding that matters when the cover is specified, and it is why you round down rather than to the nearest size. If no stock chair fits, ask the designer whether the cover or the bar layout should change.
A 100 mm slab leaves very little room. With a 10M grid centred, there is under 40 mm of concrete above and below the steel. 15M in a two-way grid takes up a third of the slab depth, which is one reason 4 inch slabs that get rebar at all usually get 10M. Our comparison of 10M vs 15M for slabs covers the choice.
Rebar chair height for a 6 inch (150 mm) slab
| Layout in a 150 mm slab | Calculation | Chair height | Nearest imperial chair |
|---|---|---|---|
| 10M grid, centred | 75 minus 11.3 | about 64 mm | 2-1/2 in |
| 10M single layer, centred | 75 minus 5.65 | about 69 mm | 2-3/4 in |
| 15M grid, centred | 75 minus 16 | about 59 mm | 2-1/4 in |
| 10M grid, 50 mm top cover | 150 minus 50 minus 22.6 | about 77 mm max | 3 in |
| 15M grid, 50 mm top cover | 150 minus 50 minus 32 | about 68 mm max | 2-1/2 in |
Check the fourth row: a 3 in chair is 76 mm, which leaves about 51 mm of top cover with a 10M grid. That works. For the 15M grid with 50 mm top cover, a 2-3/4 in chair (70 mm) would leave only 48 mm, so the 2-1/2 in chair is the right pick.
In a 6 inch slab, the choice between "centred" and "upper part of the slab" makes a real difference: about 64 mm versus 77 mm for a 10M grid. Take the position from the drawings or the designer, not from habit.
Picking chairs for slabs on grade
Most 4 and 6 inch slabs are slabs on grade: basement floors, garage floors, driveways and patios. The chairs sit on granular fill or a vapour barrier, not on a form, which changes what works.
- Use a wide base. Narrow wire legs push into fill and sink, and the steel goes down with them. Chairs with sand plates, plastic chairs with broad feet, or precast concrete blocks spread the load. RSIC lists sand-plate chairs and concrete blocks specifically for bearing on soil.
- Protect the vapour barrier. A base that cannot puncture the poly is worth the small difference.
- Match the material to the exposure. CSA A23.1 classes garage floors, porches, steps, sidewalks and pavements as C-2 exposure (concrete exposed to chlorides and freeze-thaw), while interior residential slabs on ground are R-3. In salt-exposed slabs, plastic chairs avoid putting bare steel feet near the surface. GFRP bar needs non-metallic supports in any exposure.
- Mesh needs support too. Welded wire mesh is placed on chairs in the same way. RSIC practice is that supports for temperature mesh are not supplied unless the drawings or specification call for them, so order them separately if your supplier is not including them.
Remember that a plain slab on grade in a house may not need rebar at all under Part 9 of the Ontario Building Code. Our post on basement floor slab rebar covers when it is required.
Mistakes that put the steel in the wrong place
- Using the bar's centre height as the chair height. The chair stops at the underside of the bar, so this puts the steel too high by half a bar.
- Forgetting the second layer. In a grid, the top layer adds a full bar diameter.
- Rounding chairs up. It eats the top cover. Round down and check.
- Pulling the mat up during the pour. Steel laid on the ground and "hooked up" while concrete is placed rarely ends up at the right height.
- Too few chairs. The right height means nothing if the bars sag between supports or crews flatten them. See rebar chair spacing and placement for how many to use.
- Chairs on soft ground. A chair that sinks 20 mm into fill turns a centred mat into a bottom mat.
Ordering chairs with your rebar
Chair height is one of the details that is easy to get right on paper and wrong on site. Ordering chairs at the same time as the bar keeps the heights matched to the bar sizes and cover on your drawings. Our guide to buying rebar chairs in Canada covers the types.
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.
Frequently asked questions
What height rebar chair do I need for a 4 inch slab?
To centre a 10M grid (bars both ways) in a 100 mm (4 in) slab, the chair height is about 39 mm, so a 1-1/2 in chair. For a single layer of 10M at mid-depth it is about 44 mm (1-3/4 in). If your drawings give a top cover, work down from the top surface instead.
What height rebar chair for a 6 inch slab?
To centre a 10M grid in a 150 mm (6 in) slab, use a chair of about 64 mm (2-1/2 in). For a 15M grid it is about 59 mm. Where the drawings set a top cover, subtract the cover and the two bar layers from the slab thickness.
How tall are rebar chairs?
Chairs come in a wide range of heights, from small slab bolsters around 20 to 50 mm up to high chairs of several hundred millimetres for top mats in thick slabs. Many are still labelled in inches. The height that matters is from the bottom of the leg to the underside of the bar.
Should rebar be in the middle of a 4 inch slab?
It should be where the drawings put it. Many residential details place light crack-control steel at or a little above mid-depth. What does not work is steel lying on the ground, where it does almost nothing.
