Height is not a detail, it is the job
Concrete is very strong when it is squeezed and weak when it is pulled. A slab that loses support underneath, at a soft spot in the base or across an old ditch line, has to carry itself across that gap, and doing that pulls the concrete apart. Steel is in the slab to take that pulling, and it can only take it if it is sitting where the pulling happens. Down on the stone, it is buried in the part of the slab that is already fine.
So the useful way to think about rebar is that its position matters as much as its presence. A slab with a tied grid lying on the ground has all the cost of reinforcement and close to none of the benefit. That is not a rare mistake either. It is a common thing to find when a failed slab gets broken out: good steel, in the wrong place.
Held up properly, the same steel does two things. It carries the slab across a soft spot, and if a crack ever does open, it holds the two sides together and level instead of letting one side drop below the other. A tight crack you can barely feel with a boot is steel doing its job.
Chairs versus hooking the steel up as you pour
There are two ways to get reinforcement to the right height, and they are not equal. The first is to set it on chairs before the pour starts, so the height is fixed and nobody has to think about it again. The second is to lay the steel on the base and pull it up with a hook as the concrete comes in, which is the method most people have watched at some point.
Hooking it up genuinely works when it is genuinely done, along every bar, at spacing close enough that nothing sags back down. The problem is that it depends on somebody doing careful work at the busiest moment of the day, while concrete is arriving and finishers are waiting. Wherever attention slips, the steel stays on the stone, and there is no way to check afterward. The slab looks the same either way.
That is why chairs win on anything that matters. They cost very little, they take a fraction of the time on a quiet morning instead of on pour day, and the result is inspectable before the first wheelbarrow or the first hose comes through. On our own pours we would rather set the height once and then think about the concrete.
What people put under steel instead, and how each one behaves
Plenty of things get used as chairs on smaller jobs. Some are fine and some cause problems later.
- Plastic chairs and wire chairs: made for this, cheap, come in set heights, and stay where you put them.
- Solid concrete blocks cast for the purpose: fine, and they behave like the surrounding concrete because they are concrete.
- A common brick: soaks up water, sits at whatever height a brick happens to be, and creates a weak plane through a thin slab.
- A rock off the site: no set height, rolls when it is stepped on, and no two are the same.
- Scrap wood: the worst of them. It rots out and leaves a void right where the steel is supposed to be supported.
- Nothing at all, with a promise to pull it up later: the option that gets chosen most often and works least often.
Chairs and a vapor barrier
Where a vapor barrier goes down over the stone, the chairs sit on top of it, and the ones we use have feet broad enough to spread the load instead of punching through under a boot. A barrier full of small holes is a barrier that is doing part of a job. Whether a given slab needs sheeting at all is its own question and depends on the building, so ask it separately.
The same thinking applies to spacing. Chairs too far apart let the bar sag between them, and a bar that has dipped back to the stone in the middle of a bay is right back to where it started. They go in close enough to hold a line.
What we do and what it looks like on your estimate
On our own work, reinforcement is set and tied on a grid, up on supports, and checked before concrete is ordered for the day. Wire mesh gets the same rule as rebar. Mesh works, and it works only when it is up in the slab rather than flat on the base, which is the honest reason mesh has a mixed reputation.
Which one your slab wants is a real decision rather than an upsell. Mesh is common on residential flatwork with normal loads. A tied rebar grid is the upgrade, and it earns its place on long runs, on fill, on slopes, on anything carrying vehicles or equipment. We tell you which one we would use on your job and why, and the answer goes on the free written estimate in plain words along with the thickness and the joint plan.