Positioning steel reinforcing mesh at the correct height inside a concrete slab is one of the most important steps in getting a strong, durable result. If the mesh sits too low or too high, it won't do its job — and the slab can crack or fail prematurely. The good news is that with the right tools and a basic understanding of concrete cover, most competent DIYers can get this right.
Why Mesh Height Matters
Steel reinforcing mesh works by resisting the tensile (stretching) forces that concrete alone can't handle. For it to be effective, it needs to sit within the correct zone of the slab — typically in the lower third for ground-bearing slabs, where tension forces are greatest. Just as importantly, the mesh must have adequate concrete cover on all sides to protect the steel from moisture and corrosion.
Too little cover and the steel can rust, expand, and cause the concrete surface to spall and crack. Too much cover and the mesh ends up in the compression zone where it adds little structural value.
Understanding Concrete Cover Requirements
Concrete cover is the distance between the outside face of the mesh and the nearest concrete surface. In New Zealand, cover requirements are set out in NZS 3101 (Concrete Structures Standard) and are referenced through the NZ Building Code.
As a general guide for residential ground-bearing slabs:
| Exposure condition | Minimum cover (typical) |
|---|---|
| Interior slab on ground (dry) | 40 mm from bottom |
| Exterior or garage slab | 50 mm from bottom |
| Coastal or aggressive environment | 65 mm or more |
Important: These are indicative figures only. Always confirm cover requirements with your structural engineer, building consent documents, or your local council. Conditions vary significantly across NZ — from the humid north to the freeze-thaw conditions of the South Island high country.
Tools and Materials You'll Need
- Bar chairs / mesh chairs (plastic or steel) — these are the key tool for holding mesh at the right height
- Wire ties and tie wire (to join mesh sheets)
- Tape measure
- Chalk or marker
- Safety gloves and boots — mesh edges are sharp
Step-by-Step: Setting Mesh at the Correct Height
1. Prepare Your Sub-base
Make sure your compacted sub-base (typically compacted hardfill or AP20 gravel) is level and firm. A poorly prepared base will cause chairs to sink unevenly.
2. Choose the Right Bar Chairs
Bar chairs (also called mesh chairs or slab bolsters) are purpose-made plastic or steel supports that hold the mesh at a set height off the sub-base. They come in various heights — commonly 25 mm, 40 mm, 50 mm, and 65 mm. Choose the height that gives you the correct bottom cover for your slab specification.
Bar chairs are widely available at Bunnings, Mitre 10, and PlaceMakers.
3. Lay Out the Chairs
Place bar chairs across the sub-base before laying the mesh. A common spacing is 800 mm to 1000 mm apart in a grid pattern, though follow your engineer's specification if you have one. Make sure chairs are positioned so they'll support the mesh at intersections or along wires — not in mid-air between wires.
4. Lay and Tie the Mesh
Roll or place the mesh sheets onto the chairs. Overlap adjacent sheets by at least one full square (typically 200 mm for standard mesh) and tie the laps together with wire ties. Keep the mesh flat — push down any curled edges and add extra chairs as needed.
5. Check Your Cover
Use a tape measure to confirm the mesh is sitting at the correct height from the sub-base. Also check that the top cover (distance from mesh to finished slab surface) will be adequate once concrete is poured. For a 100 mm slab with 40 mm bottom cover and SL72 mesh (confirm wire diameter with the manufacturer or your specification), you'll have roughly 52 mm of top cover — check this suits your specification.
6. Keep the Mesh in Place During the Pour
This is where many DIY slabs go wrong. As concreters walk on the mesh and concrete is placed, chairs can tip or mesh can be pushed down. Avoid walking directly on the mesh — use walk boards (planks) spread across the mesh to distribute your weight. Check mesh height periodically during the pour and lift it back into position if needed before the concrete sets.
Common Mistakes to Avoid
- Using bricks or stones as chairs — these can shift, crack, or create voids. Always use proper bar chairs.
- Placing mesh flat on the ground — mesh sitting on the sub-base provides almost no structural benefit.
- Forgetting edge cover — mesh should also be kept at least 40–50 mm from the slab edges and any penetrations.
- Not tying laps — untied mesh can shift during the pour.
When to Call a Professional
For any slab that requires a building consent — including most new home slabs, garage slabs over a certain size, or slabs supporting structures — you'll need to follow an engineer's specification and may need inspections. Reinforcing placement is often an inspection hold point, meaning a building inspector must check the mesh before concrete is poured.
If you're unsure whether your project needs consent, check with your local council or visit MBIE Building Performance for guidance on Schedule 1 exemptions. Always confirm with your council — exemptions have conditions.
For larger or more complex slabs, or if you simply want peace of mind, it's well worth engaging a qualified concreter. You can find experienced local concreters on tradedirectory.co.nz.
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