Keeping your reinforcing mesh at the correct height inside a concrete slab is one of the most important steps in getting a strong, durable result. Steel mesh chairs (also called bar chairs or mesh supports) are simple plastic or steel spacers that hold the mesh up off the ground so concrete can flow underneath and fully encase the steel. Without them, your reinforcing can end up sitting on the ground, leaving it unprotected and prone to corrosion — which weakens the slab over time.
Why the Correct Cover Matters
The concrete cover is the thickness of concrete between the reinforcing steel and the finished surface. The NZ Building Code and standard practice (referenced in NZS 3109 and related standards) specify minimum cover requirements to protect steel from moisture and corrosion. For a typical residential ground-bearing slab, 40 mm of cover is commonly required on the underside of the mesh (check your engineer's drawings or consent documents, as this can vary). In aggressive environments — coastal areas, for example — greater cover may be specified. Always confirm the required cover with your designer or engineer before you start.
Choosing the Right Mesh Chair
Mesh chairs come in a range of heights to suit different cover requirements. Common heights include 25 mm, 40 mm, 50 mm, and 65 mm. When selecting chairs:
- Match the chair height to your specified cover — if your drawings call for 40 mm cover, use 40 mm chairs.
- Plastic bar chairs are the most common for residential slabs. They resist corrosion and are easy to handle.
- Steel wire chairs are also available and can be more stable on soft ground.
- Check that the chair is rated to support the mesh without sinking into the substrate under the weight of concrete.
You can find a range of bar chairs at Bunnings, Mitre 10, and PlaceMakers.
How to Place and Attach Mesh Chairs
Step 1 – Prepare the Sub-base
Make sure your compacted sub-base or blinding layer is level and firm. If the ground is soft or loose, chairs can tip or sink, which defeats the purpose. A thin layer of compacted GAP metal or a polythene DPM (damp-proof membrane) is typically laid first.
Step 2 – Lay Out the Chairs
Place chairs across the area in a grid pattern before laying the mesh on top. A typical spacing is 800 mm to 1000 mm apart in each direction, but follow your engineer's specification if one is provided. Place extra chairs near edges and at any laps in the mesh.
Step 3 – Position the Mesh
Lift or roll out the reinforcing mesh (commonly F72 or F82 for residential slabs in New Zealand) and lay it on top of the chairs. Work carefully — mesh edges are sharp, so wear heavy gloves at all times.
Step 4 – Secure the Mesh to the Chairs
Most plastic bar chairs have a saddle or clip on top designed to grip the mesh wire. Press the mesh firmly down into the saddle so it sits securely. For chairs without a clip:
- Use tie wire (soft annealed wire) to lash the mesh to the chair where needed.
- Twist the wire tight with a pair of tie wire pliers or a simple hook tool.
- Trim any excess wire ends so they don't protrude toward the surface.
At mesh laps (where two sheets overlap), tie the sheets together with tie wire at roughly 300–400 mm intervals to keep them aligned.
Step 5 – Check Your Cover Before Pouring
Before the concrete truck arrives, do a final check:
- Measure the height of the mesh above the sub-base at several points — it should match your specified cover.
- Make sure no chairs have tipped over or shifted.
- Confirm the mesh is not touching the formwork edges (side cover is also important).
Spacing and Cover Quick Reference
| Application | Typical Cover (underside) | Suggested Chair Height |
|---|---|---|
| Residential ground slab (sheltered) | 40 mm | 40 mm |
| Exposed or coastal slab | 50–65 mm | 50 or 65 mm |
| Suspended slab (top steel) | Per engineer's spec | Per engineer's spec |
Always confirm with your engineer or consent documents — these are guides only.
Common Mistakes to Avoid
- Using too few chairs — mesh sags between supports and drops below the required cover.
- Wrong chair height — double-check before you order.
- Chairs sinking into soft ground — use a blinding layer or wider-base chairs.
- Walking on unsupported mesh — this bends it down. Use planks or walk on the chairs themselves.
- Forgetting edge cover — keep mesh at least 40 mm (or as specified) away from formwork sides too.
When to Call a Professional
For simple garden paths or small shed slabs, a competent DIYer can handle mesh placement. However, for structural slabs, driveways, retaining walls, or any work covered by a building consent, you should engage a qualified concreter or structural engineer. Reinforcing placement that doesn't meet the engineer's specification can result in a slab that fails — and may not comply with the NZ Building Code. If your project requires a consent, the work may need to be carried out or supervised by a Licensed Building Practitioner (LBP). You can find experienced local concreters on tradedirectory.co.nz.
Safety note: Reinforcing mesh has very sharp edges and cut ends. Always wear heavy-duty gloves, long sleeves, and steel-capped boots when handling mesh. Keep bystanders and children well clear of the work area.
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