In a slab on grade, rebar for crack control belongs in the upper part of the slab, not at the bottom: the RSIC Manual of Standard Practice notes that slab-on-grade reinforcement is generally located about 50 mm below the top surface. In a suspended slab the answer flips: the main bars sit near the bottom at midspan and near the top over beams, walls and columns. In every case the exact position is set by the cover and depth on the drawings, and the bars are held there on chairs before the pour.
Here is why the position matters, how to work out the height, and how to keep the steel where it belongs.
Why position matters more than people think
Rebar only works where concrete is trying to crack. A bar in the wrong place is mostly dead weight.
- In a slab on grade, the main enemy is shrinkage and temperature movement. The top of the slab dries and cools faster than the bottom, so cracks start at the surface. Steel near the top holds those cracks tight. Steel lying at the bottom does little to keep a surface crack from opening.
- In a suspended slab, the slab bends under load. At midspan it sags, so the bottom stretches. Over a support it hogs, so the top stretches. The steel has to follow the tension, which is why the drawings show bottom bars and top bars in different zones.
- Cover protects the bar. Concrete between the bar and the surface protects steel from corrosion and fire and lets the bar bond. Too little cover and the bar rusts or the concrete spalls; too much and the bar sits closer to the middle, where it does less work.
Slabs on grade: steel in the upper part
For a typical garage floor, basement floor, shop slab or patio that is reinforced, the steel is there to control cracking, not to carry the slab between supports, because the ground supports it. That puts it in the upper part of the slab.
The RSIC Manual of Standard Practice (2020) says slab-on-grade steel, whether bars or welded wire fabric, is generally located 50 mm below the top surface, held on high chairs, single chairs or concrete blocks. Many designers put it in similar terms, as "upper third" or "upper half" of the slab, and some specify a cover dimension from the top. The drawing note is what you build to.
A few things change that default:
- Thickened edges and grade beams under walls or posts are structural, so they have their own bars, often near the bottom.
- Exposure to de-icing salt, as on driveways and garage aprons, can lead the designer to specify more top cover so the bars are protected. CSA A23.1 sets cover by exposure class; our concrete cover guide has the values.
- Thick industrial slabs may get two mats, one near the top and one near the bottom.
- Radiant heating tubes are often tied to the mat, so the tube depth and bar height have to be planned together.
Under Part 9 of the Ontario Building Code an ordinary floor on ground doesn't need reinforcement at all; when it is reinforced, the designer sets where the steel goes.
Suspended and structural slabs: bottom at midspan, top over supports
A slab that spans between beams, walls or columns is a structural slab, and the bars go where the bending puts the concrete in tension:
| Zone | Where the tension is | Where the bars go |
|---|---|---|
| Midspan | Bottom face | Bottom mat, at the specified bottom cover |
| Over interior supports | Top face | Top bars, held up on high chairs or bolsters |
| Discontinuous edges | Top face near the support | Top bars hooked or anchored into the wall or beam |
| Cantilevers | Top face | Top bars running out to the edge |
Bottom cover in a structural slab is usually small: CSA A23.1 Table 17 allows 20 mm for slabs in dry interior conditions (exposure class N), rising to 40 mm and 60 mm in harsher classes. The Ontario Building Code's one prescriptive suspended slab, the cold room slab in Section 9.40, puts its 10M bars at 30 mm clear cover from the bottom, with the second layer laid directly on the first in the other direction. That is a good illustration: a slab that spans needs its steel low.
For more on spanning slabs, see our post on rebar for suspended and elevated slabs.
Working out the bar height
The height of the support (the chair height) is not the same as the cover. Two layers of bars stack on top of each other, and the cover is measured to the outside of the outermost bar. A simple way to work it out:
Chair height = slab thickness − top cover − total bar diameters in the mat (for a mat measured from the top), or
Chair height = bottom cover (for a mat measured from the bottom, since the chair supports the lowest bar).
Example for a slab on grade, using a design that wants the top of the steel 50 mm below the surface:
- Slab thickness: 150 mm
- Top of bars: 50 mm below the surface
- Two layers of 10M (11.3 mm each): about 23 mm
- Bottom of the lower bar: 150 − 50 − 23 = about 77 mm above the subgrade
A support of roughly 75 mm, measured as the distance from its base to the bottom of the bar it carries, would put the mat close to the target. The RSIC manual defines chair height that way and allows a small tolerance on it. Run the same numbers for your own slab and bar size, using the bar diameters on our rebar products page. Our post on rebar chair heights for 4 and 6 inch slabs walks through more cases.
Check the result against the drawing note and the cover required for the exposure. If the design calls for a specific top cover instead of a depth, work from that.
How to raise rebar and keep it there
Getting the bars to the right height is easy. Keeping them there while a crew walks on the mat and concrete pours over it is the hard part.
- Use proper supports. Wire chairs and bolsters with sand plates, plastic chairs, or precast concrete blocks (often called dobies). They are made for the job and come in set heights. Bricks, stones and scraps of wood are not supports: they crack, tip, wick moisture or rot.
- Space them so bars don't sag. Supports need to be close enough that the mat doesn't dip between them under foot traffic. Smaller bars need closer supports.
- Tie the mat. Ties at intersections keep the bars from rolling off the chairs.
- Don't pull bars up during the pour. Hooking bars up from the ground as the concrete arrives is a common shortcut that leaves the steel at random heights. Our post on whether rebar should be off the ground covers why.
- Plan walkways. Planks or designated paths over the mat stop the finishers from pushing it down.
Who supplies supports is worth settling early. The RSIC manual's standard practice makes the placing contractor responsible for bar supports, except where rebar used as a support is shown on the structural drawings, in which case that steel is the fabricator's responsibility. If you are ordering bar for your own slab, include supports in your plan.
Checking position before the pour
A few minutes with a tape before the truck arrives saves arguments later:
- Measure from a string line or the top of the forms down to the top of the steel at several points, including midway between chairs.
- Check bottom cover on structural slabs by looking along the soffit form.
- Confirm top bars over supports are at the right height and extend the length shown.
- Look for spots where the mat has been trampled down or lifted at laps.
Placing tolerances on cover and bar position come from CSA A23.1 and the project specification. Our post on rebar placement tolerances explains how they work.
Getting slab steel cut to your drawings
The right position starts with bars that fit: correct lengths so laps land where they should, and bent top bars and edge bars made to the detail. We detail, cut and bend slab steel from your drawings, tag it by bar mark and deliver to sites across the GTA and Ontario.
Send your slab drawings for a free take-off and quote, or see the rebar products we supply.
Frequently asked questions
Should rebar be in the middle of a concrete slab?
Not as a rule. In a slab on grade the crack control steel is usually placed in the upper part of the slab, and the RSIC manual notes it is generally about 50 mm below the top surface. In a suspended slab the bars go near the bottom at midspan and near the top over supports.
How deep should rebar be in concrete?
Deep enough to meet the cover on the drawings, and no deeper than the design intends. CSA A23.1 sets minimum cover by exposure class, for example 20 mm for dry interior slabs and 75 mm where concrete is cast against earth.
Can rebar lie on the ground under a slab?
No. Bars lying on the subgrade end up at the bottom of the slab or in contact with the soil, where they do almost nothing for crack control and can corrode. Support them on chairs or blocks at the specified height.
Is it OK to pull rebar up into the concrete during the pour?
It is a poor practice. Bars hooked up from the ground during the pour rarely end up at a consistent height and often settle back down. Set them on supports before the concrete arrives.
