Welded wire mesh has to sit on chairs or blocks at the right height, not on the ground. For light mesh, which includes common 152x152 construction mesh, Concrete Ontario's reference guide gives a support spacing of 0.8 m or less in each direction, widening to 0.9 to 1.2 m for medium mesh and 1.2 to 2.0 m for heavy mesh. Use chairs with a wide base on poly or loose granular, set the height so the mesh lands where the drawings (or RSIC practice) put it, and tie the sheets at the laps so the mat stays put during the pour.
Mesh is only as good as its position. This post covers how to support it so it ends up there. Whether a slab should use mesh at all is covered in wire mesh vs rebar vs fibre for concrete slabs.
Why mesh needs chairs
Mesh is flexible. Laid flat on the vapour barrier or base, it stays at the bottom of the slab where it does almost nothing for crack control, which starts at the top.
The common shortcut is to lay the mesh on the ground and "hook it up" with a rake or boot as the concrete goes in. It does not work reliably. Most of the mesh stays low, the parts that are lifted get pushed back down by the crew and the concrete, and nobody can see where it ended up. The RSIC Manual of Standard Practice is direct about it: welded wire fabric shall not be placed on the subgrade and pulled up during concrete placement. Placing it on appropriately spaced supports and tying it at the laps is what RSIC describes as adequate to hold it in position.
One more point catches buyers out: RSIC practice is that bar supports are not supplied for temperature mesh unless the drawings or specification call for them. If you are buying mesh, order the supports separately.
Where the mesh should sit
The drawings or the engineer's specification set the position. Where nothing is specified, RSIC practice for slabs on grade gives these defaults:
| Slab | Mesh position (RSIC default) |
|---|---|
| Less than 150 mm thick, one layer | Middle of the slab |
| 150 mm and thicker, one layer | Top cover equal to one third of the slab depth |
| Two layers (usually slabs over 200 mm) | Top layer 25 to 50 mm below the surface, below any saw cuts |
| Bottom layer of a two-layer slab | At least 40 mm cover on earth, 25 mm on a vapour barrier |
Saw cuts matter: mesh should sit below the depth of control joint cuts so the saw does not cut the wires.
Working out chair height
Chair height is measured from the base of the chair to the underside of the steel it carries. Mesh has two layers of wire, one crossing the other, so the mesh is about twice the wire diameter thick.
Wire diameters for common Canadian mesh, calculated from the wire area in the designation:
| Wire | Approx. diameter | Mesh thickness (two wires) |
|---|---|---|
| MW9.1 | 3.4 mm | about 7 mm |
| MW13.3 | 4.1 mm | about 8 mm |
| MW18.7 | 4.9 mm | about 10 mm |
| MW25.7 | 5.7 mm | about 11 mm |
Examples using the RSIC defaults (rounded, check against your drawings):
- 100 mm (4 in) slab, 152x152 MW18.7. Mid-depth is 50 mm. With the mesh centred there, its underside is about 45 mm up, so use a chair of roughly 40 to 45 mm.
- 125 mm (5 in) slab, same mesh. Mid-depth is about 63 mm, so a chair of roughly 55 to 60 mm.
- 150 mm (6 in) slab, same mesh. Top cover of one third the depth is 50 mm, so the top of the mesh is at 100 mm and its underside about 90 mm up. Use a chair of about 90 mm.
Chairs come in set heights, so pick the closest one that keeps the mesh within the position on the drawings. For rebar in the same slab thicknesses, see rebar chair height for 4 and 6 inch slabs.
Support spacing for mesh
Mesh sags between supports much more than rebar does, so it needs supports closer together. Concrete Ontario's reinforced concrete reference guide lists spacing for support accessories by mesh weight:
| Mesh type | Wire size | Support spacing |
|---|---|---|
| Light | MW25.8 or less | 0.8 m or less |
| Medium | MW32.3 to MW51.6 | 0.9 to 1.2 m |
| Heavy | MW58.1 and larger | 1.2 to 2.0 m |
Every common construction mesh, from 152x152 MW9.1 to MW25.7, is light mesh. Plan supports at 0.8 m or closer each way for those.
How many chairs? At 0.8 m each way, that is a little over 1.5 supports per square metre. For a 6 m × 12 m garage slab:
- Along 12 m: 12 ÷ 0.8 = 15 spaces, so 16 rows
- Along 6 m: 6 ÷ 0.8 = 7.5, round up to 8 spaces, so 9 rows
- Total: 16 × 9 = 144 supports
Add extra supports under laps, at slab edges and around openings, and wherever the crew will walk or run the pump line. For comparison, rebar chair spacing and counts are covered in rebar chair spacing and placement.
Which supports work for mesh
RSIC notes that support manufacturers make concrete blocks, steel chairs (coated and uncoated), plastic chairs, bolsters and accessories made specifically for single- or double-layer mesh. What matters is the base:
- On a vapour barrier or poly: chairs with a wide base or plate. Narrow legs punch through the barrier under load, which defeats the barrier and lets the chair sink.
- On compacted granular: plate-base chairs, plastic chairs with a broad footprint, or precast concrete blocks.
- On soft or loose base: concrete blocks or sand-plate chairs. Wire legs sink.
- For long runs: continuous supports (bolsters or slab bolsters) carry the mesh along a line instead of at points, which helps with light, floppy mesh.
- Mesh-specific chairs that clip onto the wire stop the mesh from sliding off the support while the crew walks on it.
Do not use bricks, stones, wood blocks or broken concrete. They shift, leave weak spots in the slab and are a common inspection rejection. Our post on whether rebar should be off the ground covers the same point for bars.
Laying, lapping and tying mesh on chairs
A few practices that keep the mesh where the chairs put it:
- Flatten roll mesh first. Mesh from a roll wants to curl back. Unroll it, flip it and work out the curve, or use flat sheets, which lie better. RSIC limits roll mesh to lighter wire sizes where the material must lie flat.
- Lap and tie sheets. Laps between sheets are part of the design and need ties so the sheets act together and do not slide apart. RSIC describes typical structural laps for mesh as a minimum of 150 mm plus overhangs for plain wire.
- Tie the mesh to the supports around the edges and wherever traffic is heavy.
- Keep traffic off the mesh. Supports are designed to carry the reinforcement, not wheelbarrows, buggies or pump lines. Lay planks or runways for heavy traffic, and check heights again before the pour.
- Watch the edges. Keep the mesh back from the forms by the specified edge cover.
When rebar is easier than mesh
Supported properly, mesh does its job. The catch is that doing it properly takes a lot of chairs, laps and care, which narrows the labour advantage mesh seems to have. Light rebar such as 10M or 15M on chairs is stiffer, needs fewer supports and holds its height better under foot traffic. That is why many contractors switch to bar for garage floors, driveways and shop slabs; see wire mesh vs rebar for a garage floor.
If you are weighing a switch, we supply straight rebar cut to length for slabs, detailed from your drawings and delivered across the GTA and Ontario. Send your slab drawings or dimensions for a free take-off and quote, or see our rebar products.
Frequently asked questions
How far apart should chairs be for wire mesh?
For light mesh (wires of MW25.8 or less, which covers common 152x152 construction mesh), Concrete Ontario's reference guide gives support spacing of 0.8 m or less. Medium mesh is listed at 0.9 to 1.2 m and heavy mesh at 1.2 to 2.0 m.
Do you need chairs for wire mesh?
Yes. RSIC standard practice is that mesh must not be laid on the subgrade and pulled up during the pour. It should be placed on supports at the right height and tied at the laps so it stays in position.
What height chair for wire mesh in a 4 inch slab?
Without other instructions, RSIC practice places a single layer of mesh at mid-depth in slabs on grade thinner than 150 mm. In a 100 mm slab that puts the mesh around 50 mm up, so a chair of roughly 40 to 50 mm, depending on the wire size.
Are rebar chairs and mesh chairs the same?
Many chairs work for both, but mesh is lighter and more flexible than rebar, so it needs closer support, and chairs with a wide base are preferred on vapour barriers and soft granular bases. Some supports are made specifically to hold mesh wires.
