Do I need reinforcing mesh or bar in a concrete slab?
What reinforcement actually does, why its position in the pour matters more than its presence, and the domestic cases where fibres or a movement joint are the better answer.
- Difficulty
- intermediate
- Time
- 12 min
- Read
- 4 min
- Safety
- caution
Short answer
Concrete is strong in compression and weak in tension, and reinforcement takes the tension. For a light ground-bearing domestic base on a good sub-base, mesh is often not needed at all and movement joints matter more. Where mesh is used, it must sit within the concrete on chairs — mesh laid on the ground and poured over does nothing whatever.
Reinforcement gets added to domestic slabs as a general precaution, frequently in the wrong place, where it contributes nothing. The physics is simple: steel resists tension, and tension in a slab occurs where it is being bent. Steel that is not in the part of the section that goes into tension is inert, which is why the single most common reinforcement mistake — laying mesh flat on the membrane and pouring on top of it — buys nothing at all.
Safety
Step by step
- Understand what reinforcement is for.Concrete carries compression very well and tension very badly. Wherever a slab bends, one face goes into tension and will crack. Steel bedded in that tension zone carries the tension across the crack and holds the slab together.
- Decide whether your slab bends at all.A ground-bearing slab on a well-compacted, uniform sub-base sits on continuous support and barely bends. It is when the support is uneven — a soft spot, a filled trench, a service run — that it starts to span, and that is when reinforcement earns its place.
- Hand structural cases to a designer.A slab carrying a building or a wall, a suspended slab, a raft, a lintel, a beam, or anything retaining ground. The bar size, spacing, cover and position are all calculated. This is not a place for judgement.
- Choose the reinforcement for the case.A-series mesh sheets in various weights for slabs. Steel bar where a specification calls for it. Polypropylene fibres mixed through the concrete where the concern is shrinkage cracking rather than load. They are not alternatives to each other — fibres do not carry load and mesh does not stop plastic shrinkage cracking.
- Get the position right, which is the whole point.Mesh must sit inside the concrete with a proper depth of cover to every face — normally in the upper part of a ground-bearing slab where the tension is, supported on plastic or concrete chairs so it stays there while the concrete goes round it.
- Do not let the mesh be lifted into position during the pour.Pulling mesh up with a hook as the concrete goes in is a common site trick and it produces mesh at an unknown and variable depth. Use chairs and place the concrete around them.
- Lap the sheets properly.Mesh sheets are lapped by a specified number of squares — normally at least a full mesh plus a bit — and tied. Butted sheets are a line of weakness exactly where the slab is most likely to crack.
- Keep the steel away from the edges and the membrane.Steel with insufficient cover rusts, expands and blows the concrete off the face. It also must not be resting on the damp proof membrane, both because it does nothing there and because it may puncture it.
- Consider whether joints are the better answer.Reinforcement does not stop concrete cracking; it controls how the crack behaves. On a plain domestic slab, well-placed movement joints and a properly compacted base prevent more visible cracking than mesh does.
- Keep chlorides away from it.Any admixture containing calcium chloride corrodes embedded steel. Check that any accelerator you use is chloride-free.
Alternatives
- Steel mesh on chairs: The standard for slabs that need it. Positioned within the concrete with correct cover, lapped and tied. Effective where the slab may span a soft spot or a service trench, or where a designer has specified it.
- Polypropylene fibres: Mixed into the concrete at the batching plant or added at the mixer. Controls plastic shrinkage cracking in the first hours, is cheap, and needs no positioning. Carries no structural load and does not replace designed reinforcement.
- Movement joints: Not reinforcement at all, but often the more effective answer on a plain domestic slab. Gives shrinkage a controlled line to crack along rather than letting it choose one.
- A better sub-base: The genuine alternative. Most domestic slab cracking is differential settlement, and money spent on excavating properly and compacting the sub-base in layers does more than money spent on steel.
Questions people ask
Do I need mesh in a shed base?
Usually not, if the sub-base is properly excavated and compacted and the slab has movement joints where it needs them. Mesh is worth including where the ground is variable, where a trench has been backfilled under the slab, or where the base is large.
Where in the slab should the mesh sit?
Within the concrete, on chairs, with proper cover to every face — commonly in the upper part of a ground-bearing slab. Never on the membrane at the bottom with concrete poured over it, which achieves nothing.
Are fibres as good as mesh?
They do a different job. Fibres control early shrinkage cracking; mesh carries tension. For a plain domestic base fibres are often the more useful of the two, and for anything designed they are not a substitute.
Can I use old bar or bedframes as reinforcement?
No. Reinforcement bar is a specified grade with a ribbed profile designed to bond to concrete, and it needs correct cover to avoid corroding. Random steel at random depths rusts, expands and breaks the slab up from inside.