Rebar in a concrete slab should overlap by the lap length on the structural drawings. On designed slabs in Canada that comes from CSA A23.3, and a standard Class B lap for uncoated Grade 400 bars in 30 MPa concrete works out to about 390 mm for 10M and 550 mm for 15M, never less than 300 mm. A small pad on grade with no engineered design has no code-set lap, so give it a full lap for the bar size or, better, order bars long enough that the pad needs no laps at all.
This post covers laps in slabs and pads specifically: why the light steel in a slab still needs a proper lap, how laps are arranged, and how to avoid them. The underlying calculation is in our lap splice length chart, and footing laps are covered in footing rebar laps and corner bars.
How much rebar should overlap in a slab
Which answer applies depends on whether the slab is designed.
Designed slabs. Suspended slabs, structural slabs on grade, slabs that carry walls or columns, and any slab an engineer has detailed use laps calculated under CSA A23.3. Slab bars normally sit with well under 300 mm of fresh concrete below them, so they count as bottom bars for lap purposes. The exception is the top mat of a thick slab or mat foundation with more than 300 mm of concrete cast below it, which takes the 1.3 top-bar factor. Using the simplified method, Class B, uncoated Grade 400 bar:
| Bar | 25 MPa | 30 MPa | 35 MPa | Top mat over 300 mm of concrete, 30 MPa |
|---|---|---|---|---|
| 10M | 430 mm | 390 mm | 390 mm | 510 mm |
| 15M | 600 mm | 550 mm | 510 mm | 720 mm |
| 20M | 740 mm | 670 mm | 620 mm | 870 mm |
These numbers assume the cover and spacing conditions the simplified method needs. Epoxy-coated bar, Grade 500 bar and tight layouts all need longer laps; the lap splice chart lists every assumption.
Plain slabs on grade and small pads. Section 9.16 of the 2024 Ontario Building Code covers ordinary floors on ground and does not require reinforcement, as our slab-on-grade rebar requirements post explains. With no required steel there is also no required lap. If you reinforce a pad anyway, for crack control or because it carries a hot tub or equipment, treat the bar the way a designer would and give it a full lap for its size.
Rules of thumb. "Lap it 40 bar diameters" or "lap a foot" are common on site. Forty diameters for 10M is about 450 mm, which is longer than the calculated Class B lap in 30 MPa concrete, so it is safe for 10M bottom bars but not a reliable rule for larger bars or top mats. A one-foot (300 mm) lap is the A23.3 absolute minimum and is short for most bars. Use the drawing value where there is one.
Why light slab steel still needs a full lap
It is tempting to think the steel in a slab is "just for cracking" and a short overlap will do. Two reasons it will not:
- Minimum slab steel is required to be developed. CSA A23.3 requires a minimum area of reinforcement in structural slabs in each direction, and it requires that minimum reinforcement to be developed in tension for its specified yield strength wherever it is needed (Clause 7.8 in the 2004 edition we checked; confirm against the current edition). A lap that cannot develop the bar does not meet that.
- Crack control happens at the weakest point. Shrinkage and temperature steel works by holding cracks closed. A short lap is exactly where the steel can slip, so a crack that forms there opens wider than anywhere else in the slab.
On a pad that nobody designed, the same logic applies. A bar that stops 150 mm past the next one is barely better than no bar at that spot.
Contact and non-contact laps in slabs
Lapped bars in a slab can sit side by side and be tied together (a contact lap) or lie a short distance apart (a non-contact lap).
- Contact laps are the usual choice. The crew wires the two bars together at each end of the lap, which keeps them from drifting during the pour.
- Non-contact laps are allowed by CSA A23.3 in flexural members as long as the gap between the bars is no more than one fifth of the required lap length or 150 mm, whichever is less. They happen naturally when bars are placed at their marks and the lap falls between two bars rather than beside one.
Either way, the full lap length applies. A non-contact lap is not a shortcut to a shorter one.
Lap location in the mat. In a single mat, the bar being lapped stays in its own layer, so lapped bars in the bottom layer remain on the chairs and laps in the cross bars sit on top of them. Check cover at the lap: two bars side by side take up more room, and in a thin slab that matters.
Staggering laps
If every bar in a slab is lapped on the same line, that line becomes a weak band across the slab. Staggering spreads the laps out.
- Drawings often specify a stagger, for example alternate bars lapped at different locations. Follow it.
- The Class A lap (1.0 × development length) is only allowed under CSA A23.3 where the steel provided is at least twice what is required and less than half of the bars are spliced within the lap length. Staggering is part of meeting the second condition. Without both, the lap is Class B, which is what most slab drawings show.
- Suspended slabs usually have lap locations set by the designer, placed away from the points of highest tension. Do not move them without approval.
- Slabs on grade are more forgiving, but a simple stagger (laps in alternate bars offset by at least a lap length) is still good practice.
Avoiding laps in small slabs and pads
The best lap on a small slab is the one that is not there. Canadian rebar is produced in mill lengths of 12 m for 10M and 18 m for 15M and larger under RSIC standard practice, and fabricators cut to whatever your bar list says. That means:
- A pad up to about 12 m in its long direction can usually be reinforced with single, unlapped bars cut to fit, minus the edge cover.
- Laps on a small slab are usually a handling choice, because the bars were bought in short pieces, not a structural one.
- Ordering bars cut to length removes the laps, the extra steel in them, the extra ties and the risk of a short lap.
Example. A 7.2 m × 7.2 m garage pad reinforced with 10M at 400 mm each way. With bars cut to about 7.1 m (allowing for edge cover), there are no laps. Built from 6 m pieces instead, every bar line needs one lap, each adding roughly 390 mm or more of extra bar, and every one of those laps has to be placed and tied correctly. For a broader look at small pads, see rebar for concrete pads.
Mesh is different. Welded wire mesh comes in sheets and rolls, so it always laps. The RSIC Manual of Standard Practice describes typical structural laps for mesh as a minimum of 150 mm plus the overhangs for plain wire, and 200 mm including overhangs for deformed wire, with cross wires inside the lap providing the anchorage.
Checking slab laps before the pour
A quick walk of the mat before the inspector arrives catches most lap problems:
- Measure a few laps with a tape. Short laps usually come from bars cut on site or slid during placing.
- Look for laps lined up across the slab where the drawings show a stagger.
- Check the ties at both ends of every contact lap.
- Check the gap on non-contact laps: no more than 150 mm, and less for short laps.
- Check the chairs under the lap. Laps add weight, and a lap that sags between chairs ends up low in the slab with too little cover underneath. Add a support under laps where needed.
The full placing sequence is in our step-by-step guide to rebar in a slab.
Getting slab bars cut to length
We prepare bar lists with laps and staggers worked out from your drawings, or with bars cut long enough that small slabs need no laps, then cut, tag and bundle the bar by mark and deliver it across the GTA and Ontario.
Send your slab drawings or dimensions for a free take-off and quote, or see the rebar products we supply, from 10M to 55M.
Frequently asked questions
How much should rebar overlap in a concrete slab?
By the lap length on the drawings. On designed slabs in Canada it comes from CSA A23.3. A standard Class B lap for uncoated Grade 400 bars in 30 MPa concrete works out to about 390 mm for 10M and 550 mm for 15M, and never less than 300 mm.
How much should rebar overlap in a concrete pad?
A small pad on grade with no engineered design has no code-set lap, because Part 9 of the Ontario Building Code does not require rebar in floors on ground. If the pad is reinforced, use a full lap for the bar size, such as the CSA A23.3 values for 10M or 15M, or better, order bars long enough to avoid laps.
Do lapped bars in a slab have to touch?
No. CSA A23.3 allows non-contact laps in flexural members as long as the bars are no farther apart than one fifth of the lap length or 150 mm, whichever is less. Most crews tie the lapped bars side by side because it is easier to keep them in place.
Should rebar laps in a slab be staggered?
Staggering is good practice and is often required by the drawings, so that not every bar is spliced on the same line. A23.3 also only allows the shorter Class A lap when less than half the steel is spliced within the lap length, along with a steel area condition.
