Slabs and cracking
Where should wire mesh go in a concrete slab?
Position wire mesh in the upper third of the slab, approximately 50 mm (2 in) below the finished top surface for a standard 100 to 150 mm (4 to 6 in) slab. Steel must be elevated on chairs or continuous bolsters before the concrete truck arrives. Mesh lying on the vapour barrier or subgrade provides negligible restraint against drying shrinkage cracking.
Why vertical elevation decides performance
Drying shrinkage cracking starts at the top exposed surface of the concrete, which evaporates and dries faster than the bottom of the slab. To restrain crack widths effectively, reinforcing steel must sit close to the zone of highest tensile strain.
Standard engineering guidance from the Wire Reinforcement Institute and the American Concrete Institute places crack-control reinforcement within the upper third of the slab depth, maintaining at least 38 to 50 mm (1.5 to 2 in) of concrete cover to protect the steel from surface abrasion and moisture (WRI WWR-500-R-26; NRMCA CIP 19). Mesh should never sit below the mid-depth of a slab on ground.
Concrete engineering assessments emphasize that unsupported welded wire mesh inevitably ends up pressed into the subgrade: if reinforcement is not mechanically supported on chairs prior to placement, it cannot remain at its intended design elevation (Neuber, 2006).
Target depth and cover by slab thickness
| Slab Thickness | Target Mesh Depth Below Top Surface | Minimum Cover Under CSA A23.1 | Recommended Chair Height |
|---|---|---|---|
| 100 mm (4 in) | 35 to 50 mm (1.5 to 2 in) | 38 mm (1.5 in) for interior; 50 mm for C-2 salt exposure | 38 to 50 mm (1.5 to 2 in) sand-plate chairs |
| 125 mm (5 in) | 40 to 50 mm (1.5 to 2 in) | 50 mm (2 in) for exterior flatwork | 50 to 65 mm (2 to 2.5 in) chairs |
| 150 mm (6 in) | 50 mm (2 in) | 50 mm (2 in) for exterior flatwork | 65 to 75 mm (2.5 to 3 in) chairs or dobies |
| 200 mm (8 in) | 50 to 65 mm (2 to 2.5 in) | 50 to 75 mm per structural drawings | High chairs or continuous upper-deck bolsters |
Where structural drawings specify exact cover dimensions, the project drawings govern. In unbonded slabs cast directly against earth, CSA A23.1 Section 6.6.6 mandates a minimum bottom cover of 75 mm (3 in) when concrete is deposited against ground without formwork.
Chair spacing and count recommendations
To prevent welded wire mesh from sagging under foot traffic during concrete discharge and screeding, chairs must be spaced based on wire gauge per WRI Tech Fact TF 702-R-21:
| Wire Mesh Designation | Wire Diameter | Recommended Maximum Chair Spacing | Required Chairs per 8 × 20 ft Sheet (160 ft²) | Alternative Continuous Support |
|---|---|---|---|---|
| 6×6 W1.4/W1.4 (10 gauge) | 3.43 mm (0.135 in) | 450 to 600 mm (18 to 24 in) on centre | 28 to 36 chairs per sheet | 6 lines of continuous slab bolsters |
| 6×6 W2.9/W2.9 (6 gauge) | 4.88 mm (0.192 in) | 600 to 750 mm (24 to 30 in) on centre | 20 to 24 chairs per sheet | 4 to 5 lines of 5 ft slab bolsters |
| 6×6 W4.0/W4.0 (4 gauge) | 5.72 mm (0.225 in) | 750 to 900 mm (30 to 36 in) on centre | 16 to 20 chairs per sheet | 4 lines of heavy-duty beam bolsters |
A standard layout for a 6 gauge sheet uses 4 rows of 5 chairs spaced approximately 700 mm (28 in) apart across the 2.44 × 6.10 m (8 × 20 ft) sheet footprint.
Why the "hook and pull" method fails
A frequent site practice is to lay welded wire mesh flat on the subbase and attempt to hook or pull the mesh up into fresh concrete with rakes or pull hooks during placement.
Extensive job-site testing and industry surveys show that this method is fundamentally flawed:
- Inconsistent elevation: Hooking lifts local points around the hook while adjacent wire sections stay trapped under aggregate.
- Downward displacement: Once the finisher steps forward to screed, crew boots push the pulled mesh directly back to the subgrade.
- Survey validation: A survey of concrete flatwork contractors conducted at World of Concrete found that over 85% of sampled slabs placed using the "hook and pull" method had mesh resting within the bottom 25 mm (1 in) of the slab (WRI World of Concrete Survey Results, 2022).
Field evaluations comparing synthetic fibres and welded wire reinforcement confirm that mesh placed without mechanical chairs consistently finishes at the slab bottom, providing negligible flexural crack control (Bischoff, Valsangkar & Irving, 2003).
Job-site placement protocol
- Subgrade preparation: Compact base material thoroughly. On loose gravel or sand, use chairs equipped with wide base plates (sand plates) or concrete dobies to prevent supports from punching into the ground under weight.
- Sheet placement and laps: Lay flat sheets with overlaps of at least one mesh square plus 50 mm (2 in), yielding an 200 mm (8 in) lap on 6×6 mesh. Secure laps with wire ties at 1.0 to 1.5 m (3 to 5 ft) intervals.
- Establish crew walking lanes: Provide temporary wooden planks or walk boards for the placing crew so workers do not step directly on the elevated wire between chair supports.
- Final inspection: Conduct a pre-pour walk-through immediately before the concrete pump or truck discharges to verify that chairs remain upright and sheets remain at their design height.
- Neuber (2006), Support requirements for welded-wire reinforcement in slabs, Concrete International. Link
- Wire Reinforcement Institute (2026), Manual of Standard Practice: Structural Welded Wire Reinforcement (WWR-500-R-26). Link
- Wire Reinforcement Institute (2021), Tech Fact TF 702-R-21: Supports Are Needed for Long-Term Performance of Welded Wire Reinforcement in Slabs-on-Grade. Link
- Wire Reinforcement Institute (2022), World of Concrete Survey Results on WWR Slab Support Practices. Link
- Bischoff, Valsangkar & Irving (2003), Use of fibers and welded-wire reinforcement in construction of slabs on ground. Link
- NRMCA, CIP 19: Curling of concrete slabs. Link
- CSA A23.1, Concrete materials and methods of concrete construction. CSA Group.
Questions
Should wire mesh be on the bottom or top of concrete?
It belongs in the upper third of the slab, roughly 50 mm (2 in) below the finished surface. Cracks originate at the top surface where concrete dries first; steel near the bottom provides minimal crack width restraint.
How high off the ground should wire mesh be when pouring concrete?
For a standard 100 mm (4 in) slab, wire mesh should be elevated 50 mm (2 in) above the subgrade on chairs with base sand plates.
How far apart should chairs be for wire mesh?
Space chairs 600 to 900 mm (24 to 36 in) on centre. That translates to approximately 18 to 28 chairs per standard 8 × 20 ft sheet so the mesh does not sag under foot traffic.
Can you walk on wire mesh after placing it on chairs?
Yes, if chairs are spaced at 600 to 750 mm (24 to 30 in) and crew members step near wire intersections or walk on temporary timber distribution planks.