Rebar spacing: maximum spacing versus what gets tied on site

The right number of bars in the bay does not mean every gap is legal.

Setting out a mat

When a drawing says T16 at 200 mm centres, this is what happens on the deck:

Establish the first bar. Measure from the formwork edge or control line, allowing for cover plus half the bar diameter.
Mark the increments. Run a tape along the formwork or blinding and mark 200 mm steps with chalk, paint or a keel crayon.
Lay and align. Bars go down on the marks; runner bars or snapped chalk lines guide top mats and wall grids.
Tie. Usually 1.6 mm soft annealed wire, snap tie, figure-of-eight or saddle tie depending on where it is.

Checking it

Count the spaces, not the bars. Over 2,000 mm at 200 mm centres there must be exactly 10 spaces, which is 11 bars. Counting spaces catches the off-by-one that counting bars hides.
Measure across three to five consecutive bars, centre to centre, or consistently edge to edge.
Check individual gaps, not just the average.

Three different meanings of "spacing"

Design or nominal spacing: the target on the drawing.
Maximum allowable spacing: the code limit — ACI 318, BS 8110, Eurocode 2 — set to control crack widths, shrinkage and temperature cracking, and punching shear.
Achieved spacing: what is actually tied, measured at pre-pour inspection.

The gap between the second and third is where the problem lives. Layout errors accumulate along a long bay and get absorbed into the last space. A run that ends with one 260 mm gap has the correct total bar count and a location that does not comply, because that gap is a strip of effectively unreinforced concrete.

The opposite error also matters. Bars tied too close together — less than about 20 to 25 mm clear, or under the coarse aggregate size plus a margin — stop aggregate passing through. Concrete cannot flow, and you get honeycombing and voids under the mat, plus a bond that never fully forms because paste has not encapsulated the bar.

Cover

Cover is controlled with hardware, not with care:

Cover blocks under bottom mats and against vertical formwork, matched to the structural concrete strength and durability specification.
Chairs — fabricated bar chairs or continuous wire chairs — holding top mats up in slabs and rafts.
Wheel spacers clipped to vertical steel in columns and walls.

Before the pour, measure from the outer face of the steel, including links, to the shutter face or blinding. Check spacer frequency — around one to one and a half blocks per square metre is typical — and walk the top mat to confirm the chairs do not fold under a worker's weight. After the pour, a cover meter will scan the hardened concrete and confirm what was actually achieved.

Tolerances, and who sets them

Bar position: typically ±15 to ±25 mm from theoretical, provided the total number of bars is preserved.
Absolute rule: no individual space may exceed the designer's maximum allowable spacing, whatever the average is.
Cover up to 50 mm: commonly −5 mm to +10 mm (ACI 117). Over 50 mm: around −10 mm to +15 mm.
The negative side is capped tightly, because losing cover accelerates carbonation, chloride ingress and corrosion. Extra cover is a strength question; missing cover is a durability failure.

The structural engineer of record decides. A site engineer or QA/QC inspector cannot grant a relaxation beyond the code or the specification without written approval from the consultant's structural engineer, however reasonable it looks on the day.

What actually comes up at pre-pour inspection

Cover blocks crushed, displaced or too far apart, letting the cage sag onto the blinding or touch the shutter.
Short laps, or laps not staggered as the code requires — 500 mm provided where 800 mm was specified.
Missing secondary steel: trim bars around slab openings, shear link legs, or links bent to 90° where 135° hooks were required.
Spacing creep, ending with a 300 mm bay, or one bar simply missing from the schedule.
Top mats collapsing under foot traffic because chairs are missing, untied, or made from weak off-cuts.
Contaminated bar surfaces — release oil, mud, heavy rust scale or dried slurry from an earlier pour — which destroys bond.

None of these are exotic. They are what a careful walk of the deck with a tape and a torch finds, an hour before the concrete is due.

This guide describes general practice and typical figures. Your project specification, job mix formula and laboratory results govern the actual values on your job. Nothing here is a design or a substitute for the engineer of record.