Rebar vs Wire Mesh for a Concrete Slab

Welded wire mesh and rebar both put steel inside a concrete slab, but they come in different forms, carry different names, and go in differently. This guide covers what reinforcement does in a slab on ground, how to read a mesh style or bar size, where the steel sits, and how to count it. It doesn't design your slab: how much steel, and where, comes from your plans and local code. For the concrete itself, use the concrete calculator.

What reinforcement does in a slab

The Wire Reinforcement Institute (WRI) is blunt: “Concrete by its very nature tends to crack.” Its tech note on light construction puts drying shrinkage of plain concrete at 1/2 to 1 in per 100 ft, and notes that “thickening the slab does not change shrinkage and temperature contraction.”

So steel in a slab on ground isn't there to stop cracks. “The primary purpose of welded wire reinforcement in slabs is to control cracking and crack widths in both directions,” keeping “the cracked sections of a slab closely knit together so that the slab will act as a unit.” Tight cracks matter because the jagged faces interlock and pass load across the crack; WRI says this aggregate interlock “is usually ineffective when the crack width exceeds 1/16 in.” Reinforcement works with control joints, not instead of them: WRI notes joints can be formed or sawcut in reinforced slabs “for additional crack control.”

Welded wire mesh vs rebar

The Concrete Reinforcing Steel Institute (CRSI) defines welded wire reinforcement as “wire reinforcement manufactured by means of welding the intersecting wires, used for reinforcement in slabs, slabs on grade, and highway pavements.” Its glossary entry for mesh simply says “See WELDED WIRE REINFORCEMENT.” Rebar is the deformed bar, made “with deformations (protrusions) to provide a locking anchorage with the surrounding concrete.”

From WRI's Manual of Standard Practice and Tech Facts TF 202 and TF 702, and CRSI's terminology and bar-placing pages.

Mesh isn't automatically the light option: WRI makes wires “up to and including 5/8” diameter (equivalent to a #5 rebar).” It also says “the design process is relatively the same as designing with conventional rebar”: an engineer calculates the area of steel required, then converts it to bars or to the wire sizes and spacing of a welded wire sheet. That is the honest answer to “which is better”: the plans set the steel, and mesh and rebar are two ways to supply it. WRI's own introduction:

Reading the sizes: 6x6-W1.4xW1.4 and #4

Welded wire styles

WRI's Manual of Standard Practice: “The letter ‘W’ designates a plain wire and the letter ‘D’ denotes a deformed wire. The number following the letter gives the cross-sectional area in hundredths of a square inch.” A style gives the wire spacing in both directions, then the wire sizes. So 6x6-W1.4xW1.4 means plain wires 6 in apart each way, each 0.014 sq in. Gauge numbers were replaced in the 1970s but are still common; WRI pairs two widely used styles with their old gauges:

Wire area is the W-number ÷ 100, per WRI's Manual of Standard Practice. Weights as published by WRI.

Rebar sizes

CRSI defines the bar number as “approximately the bar diameter in eighths of an inch,” so #4 is about 4/8 = 1/2 in. The size is rolled onto every bar, after the producing mill's mark. Caltrans publishes the standard ASTM dimensions:

Caltrans's table, ASTM Standard Reinforcing Bar Data, continues to #18.

Where reinforcement sits in the slab

Steel only controls cracks if it ends up inside the slab, near enough to the top. IRC Section R506.2.4, as adopted in Seattle: “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.”

WRI's tech note on supports agrees: a single layer “should be located at or above the mid-depth of the slab,” and “in general, positioning at one-third the depth below the surface is sufficient.” Some designers require “2 inches below the top surface,” and in thicker slabs the steel must sit low enough not to interfere with saw cutting.

What doesn't work is laying mesh on the ground and pulling it up into the wet concrete (“hooking”) or pressing it in from above (“walking-in”). WRI says both are “not recommended” because “the resulting location of the reinforcing steel is approximate and can not be inspected for actual placement.”

Supports, ties, and laps

Supports for welded wire

CRSI calls chairs and blocks bar supports: “devices of formed wire, plastic or precast concrete, to support, hold, and space reinforcing bars.” From WRI:

• On soft bases such as loose sand, use supports “with base plates or with appreciable contact areas.”
• Over polyethylene sheeting, supports “must not puncture the sheeting.”
• A simple support is “a piece of concrete 2 or 3 inches thick and about 4 x 4 inches square.”
• “Common practice is to place supports 2 to 3 feet apart”; closer if the wire sags.

WRI also notes that ACI 301 “currently requires a 12″ support spacing for wire sizes smaller than D4.0 / W4.0,” while arguing spacing “should be derived on a case-by-case basis.” If your plans invoke ACI 301, the support count changes a lot, as the worked example shows.

Supports and ties for rebar

Bar supports “range from as simple as plain concrete blocks to all-plastic chairs, to wire bar supports,” says CRSI. A bar grid isn't welded: “Rebar is tied together, using wire, to hold it in place,” and the snap tie “is usually used for rebar in a flat horizontal position.”

Laps

CRSI defines a lap splice as the overlap of two bars, and “similarly the side and end overlap of sheets or rolls of welded wire reinforcement.” Bar laps come from your plans. For welded plain wire from W1.4 to W6.0 in a slab on ground, WRI's typical detail shows a 2 in minimum lap between wire centerlines, and says its use is at the discretion of a licensed design professional.

When plans or code call for it

IRC Section R506.1, as adopted in Seattle, requires slab-on-ground floors to be “a minimum 3 1/2 inches (89 mm) thick.” R506.2.4 begins “Where provided”: it says how reinforcement must be supported, not when a slab needs it.

Local codes can add a requirement. American Canyon, California, amended R506.2.4 so that slabs inside a building “shall be reinforced with not less than six inches by six inches ten-gauge welded wire mesh, or an approved alternate.” That is one city's amendment, not the model code, and it is why the building department is the first call. Beyond code, WRI lists “slab thickness, joint spacing, subgrade composition, and external loading” as the considerations in choosing a style, for the design professional to analyze using ACI 360.

Worked example: steel for a 12 × 20 ft slab

A 12 × 20 ft slab, 4 in thick, with two hypothetical plans: 6x6-W1.4xW1.4 welded wire, or #4 bars at 18 in each way, both 3 in from the edges. These are example inputs, not recommendations. Area and volume come from the concrete calculator's own formula.

Area     = 20 × 12 = 240 sq ft
Volume   = 240 × (4 ÷ 12) = 80 ft³ (2.96 yd³), before any allowance
Band     = 4 ÷ 3 = 1.33 in to 4 ÷ 2 = 2 in below the top

What the published numbers give per foot of width, each way. These are two plans worked through, not a pair to choose between; the steel a slab needs is the designer's call.

6x6-W1.4xW1.4: 12 ÷ 6 = 2 wires × 0.014 sq in = 0.028 sq in per ft
#4 at 18 in:   12 ÷ 18 = 0.67 bars × 0.20 sq in = 0.133 sq in per ft

Bars along 20 ft = 1 + 138 ÷ 18 rounded up = 9, each 19.5 ft (17.25 in apart)
Bars along 12 ft = 1 + 234 ÷ 18 rounded up = 14, each 11.5 ft
Length = 9 × 19.5 + 14 × 11.5 = 336.5 ft; weight = 336.5 × 0.668 lb per ft = 224.8 lb

Across = (138 − 2) ÷ (60 − 2) = 2.34 → 3; along = (234 − 2) ÷ (120 − 2) = 1.97 → 2
Sheets = 3 × 2 = 6 (300 sq ft); weight = 300 × 0.21 lb per sq ft = 63 lb

36 in (WRI's 2–3 ft): (1 + 138 ÷ 36) × (1 + 234 ÷ 36), rounded up = 5 × 8 = 40
12 in (ACI 301):      (1 + 138 ÷ 12) × (1 + 234 ÷ 12), rounded up = 13 × 21 = 273

Rebar: 336.5 ft × 0.20 sq in ÷ 144 = 0.47 ft³ = 0.58% of 80 ft³
Mesh:  2 × 0.028 sq in per ft × 300 sq ft ÷ 144 = 0.12 ft³ = 0.15% of 80 ft³

The sheet count treats each sheet's outermost wires as sitting at its edges; real sheets can have wire ends past the last cross wire, and your plans set the lap. Either way the steel displaces well under 1% of the slab, a small fraction of the calculator's default 10% allowance, so don't reduce the concrete order for it.

What to order

• Welded wire: the style on your plans. WRI says stock sheets are typically 5 to 10 ft wide and 10 to 20 ft long, rolls generally 5 to 7 ft wide and 150 to 200 ft long, and “sheets inherently give better placement control than rolls.” Count sheets with the laps; CRSI lists mesh cutters for cutting it.
• Rebar: the size, spacing, edge distance, and laps on your plans. Count each direction as spaces plus one, total the lengths, and multiply by the weight per foot above. Add tie wire.
• Supports: enough for the spacing your plans or specification require, with plates or a broad base on sand.
• Concrete: unchanged by the steel. For the pour, see how to pour a concrete slab, and near freezing or in heat, concrete in cold and hot weather.