The end bar everyone forgets
Counting bars from a spacing is the classic fencepost problem, and it catches people constantly. A 4 metre width at 200 mm centres does not take 20 bars. Both edges carry a bar, so the count is:
bars = floor(width / spacing) + 1
That is 4.0 / 0.2 = 20 gaps, and 21 bars. Miss the plus one and you are a bar short on every run, on every mat, on every pour. On a large slab with mats both ways that is a delivery you have to chase.
One-way and two-way mats
A one-way mat runs bars in a single direction, usually across the short span, and relies on the concrete for everything else. A two-way mat runs a second set at right angles.
The pairing is where estimates go wrong. Bars running the **length** of a slab are spaced out across its **width**, and each one is a full slab-length long. The second set is the same relationship the other way round. Get that backwards and the total length is wrong by whatever the difference between the two dimensions is.
For a 6 m × 4 m slab at 200 mm centres, two-way:
- Bars running lengthwise: spaced across the 4 m width, so 21 bars, each 6 m long = 126 m
- Bars running crosswise: spaced along the 6 m length, so 31 bars, each 4 m long = 124 m
- Total: 52 bars, 250 metres of steel
Laps: why the steel is longer than the slab
Reinforcement is not delivered in slab-length pieces. Where two bars meet they must overlap far enough for the bond with the concrete to transfer the force from one bar to the other. Too short a lap and the joint is the weak point.
Lap length depends on bar diameter, concrete strength and whether the bar is in tension or compression, so the specification governs. As a working figure for ordinary reinforced work, laps are commonly in the region of 40 bar diameters — around 400 mm for a 10 mm bar, 500 mm for a 12 mm.
Two practical consequences:
- Stagger the laps: Do not line every joint up in the same plane. Offset alternate bars so the splices are spread through the slab.
- Laps are extra steel: On a slab wider than a single bar length, the laps are real metres you are paying for. A waste factor of 5 to 10 per cent covers ordinary laps and offcuts; a heavily spliced job needs more.
Cover is not optional
Reinforcement works only while it is protected from moisture and air. That protection is the concrete cover between the steel and the surface, and it is the difference between a slab that lasts fifty years and one that spalls in ten.
Bars laid on the ground and pulled up "as the pour goes" end up on the bottom, where they do nothing and rust. Use spacers or chairs, and set them out before the concrete arrives, not during.
Typical cover for a ground-bearing domestic slab is 40 to 50 mm from the bottom face, more where the concrete is cast against earth. The specification for your job is the number that counts.
Ordering in whole bars
Suppliers cut to standard lengths — 6 m and 12 m are common — and they will not sell you part of one. Whatever the calculation says, round the count up.
That rounding is not waste in the ordinary sense. It is the difference between the steel you need and the steel that exists in purchasable units, and it is worth doing deliberately rather than discovering on delivery day.