Rebar and Wire Mesh for a Concrete Slab

What reinforcement does

Concrete is strong under compression but weak under tension, so it will crack when the ground shifts or loads flex it. Reinforcement does not stop cracks entirely. What it does is hold the slab together across a crack, keeping the surface level and the slab structurally sound. A reinforced slab cracks quietly and stays flat; an unreinforced one can crack and then step apart into a trip hazard.

Rebar vs wire mesh

The two common choices suit different slabs.

Wire mesh is enough for a foot-traffic patio or a shed floor. For anything carrying a vehicle or structure, a rebar grid is the stronger choice, and how thick the slab should be moves with the same load question. Fibers mixed into the concrete help control fine surface cracking and can be combined with either.

Be careful which mesh, though. Welded wire is specified by ASTM A1064 and the W number in a style like 6x6-W1.4 is the wire area in hundredths of a square inch. Run the arithmetic and the light roll mesh sold for residential slabs gives 0.028 square inches of steel per foot, about a third of the 0.0864 benchmark for a 4 inch slab and roughly a quarter of what #4 bar at 18 inches gives. It also gets walked flat during the pour and ends up near the bottom where it does least good. Wayne Walker, chair of ACI Committee 360, has gone as far as recommending that you design the bottom of the slab as unreinforced.

If you want mesh to do real work, buy sheets rather than rolls. ASCC Position Statement #2, "Location of Rolled Welded-Wire Fabric in Concrete", published in Concrete International in February 2003, recommends sheets of welded wire reinforcement at 12 inch wire spacing specifically so a crew can walk on it during the pour, and its underlying complaint is blunter than that: contractors cannot place rolled fabric so that it stays in position, and supporting it properly would add real cost. Sheet mesh at wider spacing is heavier wire, so it needs fewer supports and survives being stood on. The arithmetic below says how much heavier it has to be. And keep the expectation in proportion whichever you buy: reinforcement holds a crack tight once it forms. Nothing in a slab stops cracks forming, which is the mechanism the positioning section below sets out.

What size rebar to use

For most residential slabs, #3 (3/8 inch) or #4 (1/2 inch) rebar in a grid spaced 15 to 18 inches on center is the standard. Heavier slabs and thickened edges call for #4 or #5 bar. A garage slab and a metal building slab both sit at that end, and a shed slab does not.

Those two combinations are not arbitrary. ACI 318 sets a benchmark for shrinkage and temperature steel at 0.0018 of the gross concrete area, which for a 4 inch slab is 0.0864 square inches of steel per foot of width. #3 bar at 15 inches gives 0.088 and #4 at 18 inches gives 0.133, so both clear it. #3 at 18 inches, which also gets recommended a lot, gives 0.073 and does not. The rebar calculator checks your grid against that benchmark and counts the bars, laps and weight for your slab.

ACI also caps the spacing at the lesser of five times the slab thickness and 18 inches, and on a slab of 4 inches or less that cap is usually what decides the spacing rather than the area calculation.

Where in the slab the steel goes

Not the middle, which is what most guides say including an earlier version of this one. ACI 302.1R says shrinkage and temperature reinforcement in a slab on ground should be "positioned in the upper third of the slab thickness," or, quoting the Wire Reinforcement Institute, "2 in. below the slab surface or within the upper one-third of slab thickness, whichever is closer to the surface."

The reason is the shape of a crack. ACI 302.1R notes that cracks "originate at the surface of the slab and are wider at the surface, narrowing with depth." A crack is a V, widest where you can see it, so steel high in the slab is working where the crack is open and steel near the bottom is barely working at all. The middle third is the floor of what is acceptable rather than the target.

The code asks for less than ACI does, and the gap is worth knowing. IRC section R506.2.4, here in Seattle’s adopted 2021 edition, says only that “where provided in slabs-on-ground, reinforcement shall be supported to remain in place from the center to upper one-third of the slab for the duration of the concrete placement.” So the middle of the slab is legal and the upper third is what ACI wants. Two things follow. The code sets a band rather than a point, and the operative words are "remain in place for the duration of the concrete placement", which is a requirement about the supports rather than about the steel.

Either way the steel has to be held up there. Set it on chairs at 2 to 4 ft centers. Rebar lying on the base does almost nothing, and neither does mesh that got walked flat, which is the next problem.

Side and bottom cover matter too, from ACI 318 Table 20.5.1.3.1: 3 inches where concrete is cast against earth, such as a footing, 2 inches for #6 and larger formed but exposed to weather, and 1.5 inches for #5 and smaller. The IRC matches these in section R403.1.3.5.3.

Rebar chairs: the height nobody checks

Everything above depends on one cheap plastic part, and the sizes sold at the big-box stores do not all work. Two rules fix the target. ACI 302.1R wants the steel in the upper third, or 2 inches below the surface, whichever is closer to the surface. IRC R506.2.4 will accept anywhere from the center to the upper third. Chairs hold the bottom of the bar, so the height you need is the target depth of the bar centroid, less half the bar diameter.

Read the 4 inch row twice, because it is the common case and it has a trap in it. To land a #4 bar in the upper third of a 4 inch slab you need roughly a 2.4 inch chair, so the 2.5 inch size is the one to buy. The 2 inch chair that gets picked up most often puts the bar centroid 1.75 inches below the surface, which is the middle third: legal under R506.2.4, and short of what ACI asks for. A 1.5 inch chair does not even clear the code band, because it puts the centroid 2.25 inches down on a 4 inch slab, below the center line that R506.2.4 sets as the floor. If you buy chairs by eye in the aisle, that is the one to avoid.

Spacing is governed by the words in the code rather than by a published number. R506.2.4 asks that the steel "remain in place for the duration of the concrete placement", which makes spacing a question of whether the grid holds its height with a crew walking on it and a hose dragging over it. Common practice is 2 to 4 feet in each direction, closer on a wide mesh sheet or a light bar than on a stiff #5 grid, and closer still anywhere a wheelbarrow wheel will run. The test is not the measurement, it is whether you can stand on the grid between two chairs without the bar touching the base.

What not to use. Bricks, broken block and stones get used because they are free, and they fail on three counts: they are the wrong height, they do not hold the bar so it moves sideways, and they introduce a different material into the slab section. Pulling mesh up with a hook as the concrete goes in, the other common shortcut, puts the steel at whatever depth the puller guesses and holds it nowhere. The Concrete Reinforcing Steel Institute publishes the standards covering bar supports and their classes if you need to specify them rather than buy them off a shelf.

The rebar calculator counts the bars and laps for a grid; the chair count is roughly the grid area divided by the square of your chair spacing, so a 20 by 20 slab at 3 foot centers comes to about 45. Counting actual positions instead, seven across by seven along, gives 49. Either way it is a bag of chairs, not a handful.

Fiber mesh against steel, and the price everybody quotes wrong

Fibers go in the mix rather than on chairs, which makes them the one reinforcement option with no placement problem at all. They are also the option with the most confused advice attached, in both directions.

What fibers do. The Fiber Reinforced Concrete Association describes the mechanism as modifying "the cracking mechanism from macro-cracking to micro-cracking": instead of a few visible cracks you get many invisible ones. Their primary job in fresh concrete is to "provide early-age tensile resistance to the formation of plastic shrinkage cracks", the ones that open in the first hours while the slab is still soft, which is a job neither mesh nor bar does at all because neither is in tension yet. At an appropriate dosage the association says fibers can "hold a crack tight in the same capacity as conventional welded-wire reinforcement and light-gage rebar".

What they do not do. They are not a structural substitute on an unsupported slab, and they do nothing for freeze-thaw, which the association is explicit about: that "requires an adequate air-void system in the concrete matrix" regardless of fibers. It also notes that steel or synthetic macrofibers "can increase moment capacity of fully supported slabs", which may permit a thinner slab. Read the qualifier in that sentence rather than the promise: fully supported, and a reduction an engineer signs off, not one you decide in the driveway.

The dosage figure worth having is that under 5 lb per cubic yard of synthetic macrofiber needs no change to the mix proportions, so at normal residential doses you are not trading strength or workability for the fibers. Above that, and for steel fibers, the supplier sets the rate.

The cost claim in circulation is wrong by a factor of twenty-five, and the error is checkable. Angi prices fiber mesh at $2 to $3 per square foot. A contractor publishing his own supplier rate puts fiber at $7.50 to $8.00 per cubic yard at the manufacturer’s dosage of 1 to 1.5 lb per yard, and a cubic yard covers 81 square feet at 4 inches, so the real figure is about $0.09 to $0.10 per square foot. The tell is inside Angi’s own page: it prices rebar more cheaply than its own fiber figure, which is backwards, because fiber is the budget option. Concrete slab cost by size has the full working and the other reinforcement prices beside it.

So which, for a residential slab. Fibers for plastic shrinkage and surface crack control on a patio, shed floor or path, at almost no cost and with nothing to place wrong. A rebar grid on chairs where a vehicle or a structure is going on top, because that is the case where holding a crack tight across the full depth matters. Mesh in sheets if the specification calls for it. And fibers alongside either, since they address a different failure at a different age rather than competing for the same job.

What goes underneath

The layers below the steel have their own page, because the detail that matters most is not the one everyone argues about. The order from the bottom up is compacted subgrade, granular base, insulation if you are using it, vapor retarder, then the steel on chairs and the concrete, with the sheet in direct contact with the underside of the slab.

Two things worth knowing before you buy anything. IRC R506.2.1 asks for at least 4 inches of clean graded fill, free of organic material, and it is there for uniform support and drainage rather than for bearing. And the roll of 6 mil poly at the big box is probably not a vapor retarder: ACI's position is that conventional polyethylene at 6, 8 and 10 mil does not meet ASTM E1745 and should not be used for below-slab moisture protection. Under-slab insulation and vapor barrier has the ASTM classes, the R-values the energy code asks for by climate zone, and why ACI reversed its own advice on where the sheet goes.