Rebar is the stronger choice and the norm for any slab that carries real loads or is engineered, while welded wire mesh is a lighter crack-control reinforcement for thin slabs on grade, and fibres are a mix additive that can do part of mesh's job. None of the three stops a slab from cracking altogether. What they do is hold cracks tight, and only rebar is routinely used to carry structural load.
The bigger issue on real jobs is not which product you buy but where it ends up in the slab. Reinforcement lying at the bottom of the pour is wasted. The sections below cover what each option does, the position problem, and which kinds of slabs tend to get which.
What each one actually does
Concrete is strong in compression and weak in tension. As it dries it shrinks, and as temperatures change it moves. Where that movement is restrained, the slab cracks. Reinforcement crosses those cracks and keeps them from opening wide.
Welded wire mesh (also called welded wire reinforcement or WWR) is a grid of cold-drawn steel wires welded at every crossing, supplied in sheets or rolls. It puts a modest amount of steel at a regular, close spacing. Its job in most slabs is crack control: keeping shrinkage and temperature cracks narrow so the slab stays together.
Rebar is deformed hot-rolled bar, in Canada usually 10M or 15M for slabs, laid out in a grid at a spacing set by the design and tied at the crossings. It can provide far more steel per metre than common mesh, and it is the reinforcement engineers use when the slab has to carry loads in bending: suspended slabs, slabs that bridge poor ground, thickened areas under walls or columns, and heavily loaded floors.
Fibre-reinforced concrete mixes short fibres into the concrete itself. Synthetic micro fibres mainly help with plastic shrinkage cracking while the concrete is still fresh. Steel fibres and synthetic macro fibres, used at higher dosages, can carry some load across a crack after it forms. Fibres are spread through the whole depth, so there is nothing to place or hold up. They are covered in more depth in our post on whether fibre-reinforced concrete replaces rebar.
Side by side
| Welded wire mesh | Rebar | Fibres | |
|---|---|---|---|
| Main role | Crack control in thin slabs | Crack control and structural strength | Crack control; some post-crack capacity with macro or steel fibres |
| Steel per metre | Light to moderate | Set by design, can be heavy | Not comparable (dosage by volume of concrete) |
| Position in slab | Depends on support during the pour | Held on chairs, stays put when properly tied | Spread through the full depth |
| Labour | Fast to roll out, slow to support well | More pieces to place and tie | No placing; changes finishing and pumping |
| Structural slabs | Rarely the main reinforcement | Standard | Only where the designer's method allows |
| Who decides | Drawings or builder for simple slabs | Drawings | Concrete mix design and drawings |
The position problem with mesh
The single biggest weakness of mesh in practice is that it ends up in the wrong place. Mesh is flexible. If it is laid flat on the vapour barrier and "hooked up" with a rake during the pour, most of it stays at the bottom, and crews and wheelbarrows walking on it push the rest back down.
Reinforcement that controls shrinkage cracks needs to be within the slab, typically in the upper half, so it crosses the cracks where they open widest. Mesh on the subgrade is under the slab, not in it.
Doing mesh properly means supporting it on chairs or bolsters at close enough centres that it does not sag, and keeping traffic off it. Once you are doing that, much of mesh's labour advantage over light rebar disappears. Rebar, especially 15M, is stiffer between supports and holds its height better under foot traffic, which is one reason many contractors prefer 10M or 15M bar even in slabs where mesh would be acceptable on paper. Our guide to rebar spacing for concrete slabs covers how a bar grid is laid out and kept in place.
Reading a mesh designation
Mesh is described by wire spacing and wire size. In metric, a sheet called 152x152 MW18.7xMW18.7 has wires every 152 mm in both directions, and each wire has a cross-sectional area of 18.7 mm². "MW" means a plain (smooth) wire; "MD" means a deformed wire. The imperial version of the same sheet is written 6x6 W2.9xW2.9, where the W number is the wire area in hundredths of a square inch.
That lets you compare mesh and rebar on the same basis, steel area per metre of slab width:
- 152x152 MW18.7 mesh: 18.7 mm² × 1000 ÷ 152 = about 123 mm² per metre each way.
- 10M rebar at 400 mm: 100 mm² × 1000 ÷ 400 = 250 mm² per metre each way.
- 10M rebar at 300 mm: about 333 mm² per metre each way.
This is arithmetic, not a design recommendation, but it shows why a light mesh and a rebar grid are not interchangeable. Swapping one for the other changes the amount of steel in the slab, which is a decision for the designer. For bar sizes, see what size rebar for a concrete slab.
Which slabs get which
These are general patterns. The drawings, an engineer, or your building department decide what your slab needs.
Basement floors in houses. Part 9 of the 2024 Ontario Building Code sets a minimum thickness for floors on ground but does not require them to be reinforced, so many basement floors are plain concrete or have light mesh for crack control. More on this in basement floor slab rebar: is it needed.
Garage floors and driveways. These see vehicle loads, freeze-thaw cycles and road salt. Mesh, rebar and fibre all get used, and the answer depends on the ground, the slab thickness and how much cracking the owner will tolerate. We cover them separately in wire mesh vs rebar for a garage floor and for a concrete driveway.
Patios, walkways and sheds. Thin, lightly loaded slabs on grade. Control joints at sensible spacing do more for appearance than reinforcement; mesh or fibre is common where some reinforcement is wanted.
Shop floors, warehouse slabs and slabs with point loads. Racking, forklifts and equipment loads mean an engineered slab, usually with rebar or a structural fibre design, and a specified thickness and joint layout.
Structural slabs. Suspended slabs, slabs that span between footings or grade beams, and slabs that support walls or columns are engineered and reinforced with rebar. Mesh is not a substitute here.
Slabs that already have rebar on the drawings. Do not swap to mesh or fibre without written approval from the designer. The slab-on-grade rebar requirements post covers how those designs are set.
Combining them
The options are not exclusive. Common combinations include:
- Rebar plus micro fibres: rebar does the structural work while fibres help with early plastic shrinkage cracking.
- Rebar in thickened areas, mesh or fibre elsewhere: heavier bar under walls, at slab edges or around column pads, with lighter reinforcement in the field.
- Dowels at joints: bars across construction joints to transfer load between pours, regardless of what reinforces the field of the slab.
Cost is about more than the material
Mesh can look cheaper at the supply counter, but the fair comparison includes chairs, labour to support it properly, laps between sheets, waste at edges and openings, and what happens if it ends up on the ground. Rebar involves more pieces to place and tie but is easier to get right. Fibre shifts the cost into the concrete order and can change finishing. Our separate post on wire mesh vs rebar cost walks through how to compare them on a real slab.
Getting reinforcement for your slab
If your drawings call for rebar, we supply it straight or cut to length, with bent edge bars, corner bars and dowels fabricated, tagged and bundled so your crew can place it without sorting. If you have a slab to price and are unsure what it needs, start with your designer or building department, then send us what they specify.
Send your drawings or slab details for a free take-off and quote, or see our rebar products.
Frequently asked questions
Is wire mesh as good as rebar?
For crack control in a thin slab on grade, properly supported mesh can do the job. For slabs that carry real loads, span between supports or are designed by an engineer, rebar is the norm because it puts more steel in a controlled position.
Should I use wire mesh or rebar in a concrete slab?
Follow the drawings. If you have none, the deciding questions are what the slab carries, whether it sits on good ground, and how much cracking you can accept. Plain floors on ground in houses often use mesh or nothing; garage, shop and structural slabs usually get rebar.
Does wire mesh need to be raised in a slab?
Yes. Mesh left lying on the subgrade or vapour barrier does almost nothing. It has to be supported on chairs or bolsters so it ends up within the slab, usually in the upper half for crack control.
Can fibre replace wire mesh or rebar?
Fibres can replace mesh in some slabs on grade where the goal is crack control, if the designer accepts it. They are not a general substitute for structural rebar.
