Installation
Concrete wire mesh chairs: spacing, height and support types
Support welded wire mesh on dedicated plastic chairs or concrete dobies spaced 600 to 900 mm (24 to 36 in) on centre. For a standard 100 to 150 mm (4 to 6 in) concrete slab, use 38 to 50 mm (1.5 to 2 in) chairs with wide base sand plates so they do not sink into gravel or puncture vapour barriers. One standard 8 × 20 ft sheet requires approximately 18 to 24 chairs.
Why dedicated chairs are mandatory
Welded wire mesh controls drying shrinkage cracking only when held securely in the upper third of the slab depth. If mesh rests on the subbase, it provides zero structural restraint against top-surface cracks.
According to WRI Tech Fact TF 702-R-21, mechanical bar supports (chairs or bolsters) are mandatory for slabs on ground. Field surveys of concrete contractors show that slabs poured without chairs leave over 85% of the mesh sitting within the bottom inch of the slab (WRI World of Concrete Survey Results, 2022).
Three common chair types for wire mesh
| Chair Type | Material & Design | Best Base Conditions | Typical Spacing | Key Job-Site Advantage |
|---|---|---|---|---|
| Plastic sand-plate chairs | High-density polyethylene with integral flat base plate (sand plate) | Compacted gravel, sand, or over poly vapour barriers | 600 to 750 mm (24 to 30 in) | Wide foot prevents sinking; does not corrode or puncture poly |
| Continuous slab bolsters | Heavy plastic or wire runners with legs spaced along a 5 ft (1.52 m) rail | Firm compacted granular base or smooth mud slabs | Parallel lines spaced 900 to 1,200 mm apart | Quick continuous placement; supports multiple wire intersections |
| Concrete block dobies | High-strength precast concrete cubes with embedded wire ties | Heavy industrial slabs, unformed rocky ground | 750 to 900 mm (30 to 36 in) | Matches concrete thermal expansion and compressive strength |
Chair spacing and count per 8 × 20 ft sheet
To prevent the mesh from sagging when workers walk on it during placement, chairs must be laid out in a uniform grid:
| Mesh Designation | Wire Diameter | Recommended Maximum Chair Spacing | Required Chairs per 8 × 20 ft Sheet (160 ft²) | Recommended Chair Grid Layout |
|---|---|---|---|---|
| 6×6 W1.4/W1.4 (10 ga) | 3.43 mm (0.135 in) | 450 to 600 mm (18 to 24 in) | 28 to 36 chairs | 4 rows of 7 to 9 chairs |
| 6×6 W2.9/W2.9 (6 ga) | 4.88 mm (0.192 in) | 600 to 750 mm (24 to 30 in) | 20 to 24 chairs | 4 rows of 5 to 6 chairs |
| 6×6 W4.0/W4.0 (4 ga) | 5.72 mm (0.225 in) | 750 to 900 mm (30 to 36 in) | 16 to 20 chairs | 3 to 4 rows of 5 chairs |
For standard 6 gauge 8 × 20 ft sheets, budget approximately 20 chairs per sheet. For a double garage requiring five sheets, plan for 100 chairs.
Why broken red clay bricks and wooden blocks fail
On residential jobs, crews sometimes support mesh on broken red clay bricks, landscape paver chunks, or scrap dimensional lumber.
This practice violates CSA A23.1 and concrete engineering standards:
- Porous clay bricks wick moisture: Common red clay brick absorbs water rapidly from wet concrete paste, weakening the concrete matrix directly adjacent to the steel. During Canadian winters, water trapped in porous brick freezes, spalling the bottom of the slab.
- Wood decomposes: Wood blocks encased in concrete absorb water, swell, and eventually rot away, leaving hollow voids and pathways for radon, soil moisture, and insects.
- No positive wire attachment: Loose rubble lacks clips or wire ties. As concrete flows from the pump hose, rubble shifts and tips over, allowing the mesh to drop to the ground.
Always use factory-certified plastic chairs, wire bolsters, or precast concrete dobies with minimum 30 MPa compressive strength.
- 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
- Neuber (2006), Support requirements for welded-wire reinforcement in slabs, Concrete International. Link
- CSA A23.1, Concrete materials and methods of concrete construction. CSA Group.
Questions
How far apart should chairs be for wire mesh?
Space chairs 600 to 750 mm (24 to 30 in) on centre for standard 6 gauge (6×6 W2.9/W2.9) mesh, and 750 to 900 mm (30 to 36 in) for heavier 4 gauge mesh.
How high off the ground should wire mesh be in a 4 inch slab?
Use 38 to 50 mm (1.5 to 2 in) chairs. This places the mesh in the upper third of the slab depth while maintaining at least 38 to 50 mm of top cover.
What are sand-plate chairs?
They are plastic chairs with a wide circular or square flat bottom plate. The flat plate spreads weight over a wider surface area, preventing chair legs from sinking into gravel or puncturing polyethylene vapour barriers.
Can I use concrete dobies instead of plastic chairs?
Yes. Concrete dobies (precast concrete spacer blocks with embedded tie wires) are an excellent choice, especially on industrial floors, because they bond seamlessly with the poured concrete.