Laying and tying 150×150 reinforcing mesh

Concrete is very strong in compression but weak in tension. As a slab dries, it shrinks. When a car, a shelving unit or frost underneath loads it, tension builds up inside. Without reinforcement this shows as cracks that slowly widen. Mesh spreads those stresses over a larger area, so cracks stay fine and the slab holds together.

150×150 mesh typically goes into:

• ground-bearing floor slabs and blinding layers in houses,
• floors of garages, workshops and outbuildings,
• concrete bases for patios, paths and hardstanding,
• raft foundations and suspended slabs, if the structural engineer designs them that way,
• screeds over underfloor heating, where the mesh also holds the pipes in place.

Keep two situations apart. In a ground-bearing floor or a patio, mesh mainly controls cracking. In a suspended slab, a raft foundation or a loadbearing wall, the steel is part of the structure, and its bar size, position, laps and cover come from the engineer's design. On those jobs nobody on site should change the reinforcement on their own, and nobody should add a different mesh "just to be safe".

150×150 mesh

The wires sit 150 mm apart in both directions. Wire diameters usually range from roughly 4 to 8 mm, and 6 mm is the usual choice for house and garage floors. Sheet sizes depend on the supplier and country. Around 2×3 m is common in some markets, and larger sheets such as 2.4×4.8 m in others. This mesh suits floors, slabs and external hardstanding under ordinary loads. It goes down quickly because each square metre needs fewer sheets and the laps are easy to arrange.

100×100 mesh

A tighter grid suits thinner layers and places where fine cracking matters more. Typical examples are thinner screeds, underfloor heating with closely spaced pipes, and decorative or exposed concrete surfaces. The wire is often thinner. Laying takes a little longer and the mesh is harder to cut. Concrete with coarse aggregate also flows through it less easily, so check the aggregate size with the supplier and compact the concrete thoroughly.

Tying loose bar reinforcement

Strip footings, ring beams, lintels, columns and suspended slabs often use individual bars tied on site to a reinforcement drawing. The bars go down at the specified spacing, and the crossings are tied with wire. This takes more work than mesh because the crew handles different bar sizes, links, bends and anchorage lengths. In return, it puts steel exactly where the structure is most stressed. Both methods are often used on the same site: mesh over the main slab area, bars added at edges and openings.

In practice people sometimes swap 150×150 mesh for 100×100 mesh with thinner wire, thinking a tighter grid must be stronger. It need not be. What counts is the cross-sectional area of steel per metre width, so the combination of wire diameter and spacing. If an engineer designed the reinforcement, always agree any substitution with them first.

Mesh goes onto a finished base. If the crew turns up and the base isn't done, time is lost and the result usually suffers. Before laying starts, the following should be in place:

• compacted hardcore or blinding concrete at the correct level,
• insulation and damp-proof membrane or a separating sheet, if they are part of the build-up, with taped joints,
• edge formwork around the perimeter and at construction and movement joints,
• all ducts and pipes passing through the slab: drainage, water, electrics and radon or ground gas measures where needed,
• the top-of-concrete level marked, for example on the formwork or with a laser.

Access matters too. A 2×3 m sheet with 6 mm wire usually weighs around 20 kg, and heavier meshes weigh more. If every sheet has to be carried through a garden or up stairs, the job takes much longer. Store the mesh on timber bearers, not in the mud, and keep it away from oil and grease.

For a house you need the drawings with the reinforcement details. Agree in advance who will inspect the steel before the pour. That may be the structural engineer, a site supervisor or building control, depending on the project. For smaller structures such as garages or large patios, check with your local building authority whether they need approval. In a flat or shared property, also check with the building management or owners' association.

Setting out and cutting

The crew first plans the layout to keep waste low and get sensible laps. Sheets go down in rows, usually with staggered joints, so four corners never meet at one point. Where needed, sheets are cut with bolt croppers or an angle grinder. Grinder sparks can burn through insulation and membrane, so cut away from the prepared base or shield it.

Laps

Neighbouring sheets must overlap. For ground-bearing floors the lap is usually at least one grid, often two. With 150×150 mesh that means roughly 150 to 300 mm. In structural elements the lap comes from the design and is usually longer. The cross wires of both sheets must overlap, not just the loose wire ends. A short lap is one of the most common reasons for a crack running exactly along the line where two sheets meet.

Spacers and cover

The mesh must not lie on the ground, the membrane or the insulation. Mesh lying at the bottom does almost nothing in the concrete, and it rusts as well. It sits on plastic spacers, often called chairs, or on concrete spacer blocks. The spacer height sets the concrete cover. In floors that is usually roughly 20 to 40 mm, and more outdoors or in harsher conditions. Spacers go in a regular grid, roughly every 60 to 100 cm, so the mesh doesn't sag between them. Stones, bricks and offcuts of timber are not suitable. They shift, and under the concrete they leave weak spots.

Tying

Laps are tied with annealed tying wire, by hand with pliers or a twisting hook, or with a rebar tying gun on larger areas. The ties have to keep the sheets in place while people walk on them and pour concrete. At laps that usually means a tie every few tens of centimetres, and always at the ends. Turn the wire tails inwards, away from the surface, so they don't poke out of the concrete and rust. Welding mesh on site is normally not allowed, because the heat can weaken the steel.

Double mesh and extra bars

Thicker slabs often have a bottom and a top mesh. The top layer sits on continuous chairs or steel supports that must carry the weight of workers during the pour. Around openings, at the corners of openings and at re-entrant corners of the slab, short diagonal bars are added, because cracks start there most often. At movement joints the mesh stops. Otherwise the joint cannot work.

The amount of work per square metre depends on how demanding the area is. The job goes faster with:

• a rectangular area without openings, where hardly any cutting is needed,
• a single layer of mesh on a level, firm base,
• delivery by lorry or crane right next to the area,
• base and formwork finished when the crew arrives.

The work increases with:

• an irregular plan, curves, angled walls and many corners,
• lots of pipes, gullies and channels that the mesh must be cut and reinforced around,
• double mesh on supports, or mesh combined with loose bars,
• 100×100 mesh or thicker wire, which is harder to cut and bend,
• work on a suspended slab or in an awkward spot where sheets are carried by hand,
• fixing underfloor heating pipes to the mesh, if the same crew does it.

Check whether a quote covers materials, spacers and tying wire, or labour only. On NUMIQ the labour and material prices are shown separately above this text.

Reinforcement sits between two stages that depend on each other. Before it, the plumber and electrician must finish everything that runs in or under the slab. Cutting openings into a finished slab later means cutting through the steel. In a structural slab that is not acceptable without the engineer's agreement.

If the floor will have underfloor heating, agree beforehand whether the pipes are tied directly to the mesh or laid on a system board with studs. If they go on the mesh, the mesh must be flat and well supported. Otherwise the pipes end up at different heights and the screed above them is too thin in places.

After the steel come the inspection and the pour. The concrete gang should know where spacers are sparse and walk on boards or over the spacer points. On larger projects the inspection is recorded in the site records. Don't book the concrete until the steel has been checked. With a mixer waiting, nobody fixes mistakes any more.

How long does it take? On a simple garage floor or patio base, a two-person crew usually lays and ties the mesh in a few hours. A full ground floor of a house with openings and extra bars usually takes a day, and double mesh or a suspended slab takes longer. Steel has no drying time. It shouldn't wait long for the pour, though, because it gets dirty and trodden on, and the spacers shift. Ideally, fix the steel the day before the pour or early on the same morning.

• Mesh lying at the bottom or trodden down during the pour: the slab cracks as if it had no steel, and on a damp base the steel corrodes,
• mesh too close to the surface: thin cover, rust stains and spalling, especially outdoors where frost and de-icing salt act,
• short laps or none at all: straight cracks at regular intervals, exactly where the sheets meet,
• no extra bars at openings and corners: diagonal cracks running from the corners of openings,
• mesh running through a movement joint: the slab cracks next to the joint instead, where nobody wants it,
• greasy, muddy or heavily corroded mesh: the concrete bonds poorly, although light surface rust does no harm,
• wire tails pointing upwards: rusty dots on the finished floor or patio,
• membrane damaged by a grinder or sharp mesh ends: the damp problem only appears years later.

Most of these mistakes are visible during the check before the pour. After the pour, you can only spot them through cracks, and the repair is difficult.

A capable homeowner can lay mesh in a ground-bearing slab, for example under a garden shed, a patio or a garage floor. Do not reinforce suspended slabs, house foundations, stairs or any other structural element yourself. Those follow the engineer's drawings and need a professional inspection. Wear gloves and safety glasses, because the wire ends are sharp.

1. Check that the base is compacted and level and that the membrane or insulation is intact.
2. Plan the sheet layout with laps of at least one to two grids and staggered joints between rows.
3. Cut the sheets with bolt croppers, or with a grinder away from the base so sparks don't damage the membrane.
4. Place spacers in a grid of roughly 60 to 100 cm, with a height that gives the required cover.
5. Lay the sheets on the spacers and keep at least the cover distance from the edges.
6. Tie the laps with tying wire at regular intervals and at the ends, and turn the tails downwards.
7. Add short diagonal bars at corners and around openings.
8. Stop the mesh where a movement joint is planned.
9. Walk the whole area and check that the mesh doesn't touch the base anywhere and doesn't rock.
10. Walk carefully during the pour and hook any mesh that gets pushed down back into position.

Walk the whole area before the mixer arrives. On a house, the engineer or site supervisor should inspect the reinforcement. Check in particular:

• Mesh type, wire diameter and grid size match the drawings or the agreement
• Sheets sit on spacers and touch the base nowhere
• Cover is maintained at the formwork and around the perimeter
• Laps have the required length and are tied
• Extra bars are in place at openings and corners
• Mesh stops at movement joints
• With double mesh, the top layer is firmly supported and doesn't rock
• Steel is clean, free of mud, oil and flaking rust
• Wire tails don't point to the surface and the membrane under the mesh is undamaged
• All ducts and pipes are in place and secured against movement
• On a house, the inspection is recorded in the site records