A rebar lap splice in Canada is sized from the bar's development length under CSA A23.3, Clause 12. Most tension laps are Class B splices, 1.3 times the development length and never less than 300 mm. For common Grade 400 bar in 30 MPa concrete, that puts a standard lap at roughly 390 mm for 10M, 550 mm for 15M and 1,080 mm for 25M, with longer laps for top bars, epoxy-coated bars and tight layouts. The chart below shows the numbers and, more importantly, the assumptions behind them.
How CSA A23.3 sets lap length
A lap splice transfers force from one bar to the next through the concrete between them. The length needed depends on how much force the bar carries (its yield strength), how well the concrete grips it, and how much confinement is around it.
CSA A23.3 handles this in two steps:
- Development length (ld). The embedment a bar needs to develop its yield strength. The simplified method in Clause 12.2.3 is ld = 0.45 × k1 × k2 × k3 × k4 × (fy ÷ √f'c) × db, where fy is the bar's yield strength in MPa, f'c is the concrete strength in MPa (with √f'c capped at 8 MPa) and db is the bar diameter.
- Splice class. A tension lap is either Class A (1.0 × ld) or Class B (1.3 × ld), and never shorter than 300 mm.
The k factors adjust for real conditions:
| Factor | What it covers | Values |
|---|---|---|
| k1 (bar location) | Horizontal bars with more than 300 mm of fresh concrete cast below them ("top bars") | 1.3 for top bars, 1.0 otherwise |
| k2 (coating) | Epoxy-coated bars | 1.5 or 1.2 depending on cover and spacing, 1.0 for uncoated |
| k3 (concrete density) | Structural low-density or semi-low-density concrete | 1.3 or 1.2, 1.0 for normal density |
| k4 (bar size) | Smaller bars develop more efficiently | 0.8 for 20M and smaller, 1.0 for 25M and larger |
The product k1 × k2 does not need to exceed 1.7. These values are from CSA A23.3; the 2019 edition is the one referenced by the 2024 Ontario Building Code.
Class A or Class B?
Class B is the default. A Class A splice (1.0 × ld) is only permitted where both of these are true at the splice location:
- the area of steel provided is at least twice what the analysis requires, and
- less than half of the total reinforcement is spliced within the lap length.
In practice, most laps on drawings are Class B unless the engineer has checked those conditions. That is why the chart below shows Class B.
Lap splice length chart for 10M to 35M
Assumptions for every number in this table:
- CSA A23.3 simplified method (Clause 12.2.3) with the 0.45 coefficient. That method only applies where clear cover is at least one bar diameter and clear spacing is at least 1.4 bar diameters, and the member has minimum ties or stirrups through the lap, or is a slab or wall with clear spacing of at least 2 bar diameters between the bars being spliced.
- Grade 400 bar (fy = 400 MPa) to CSA G30.18, uncoated (k2 = 1.0), normal-density concrete (k3 = 1.0).
- Class B tension splice = 1.3 × ld, with ld not taken less than 300 mm. Values rounded up to the next 10 mm.
- "Top bars" means horizontal bars with more than 300 mm of fresh concrete below them (k1 = 1.3).
| Bar | 25 MPa bottom | 25 MPa top | 30 MPa bottom | 30 MPa top | 35 MPa bottom | 35 MPa top |
|---|---|---|---|---|---|---|
| 10M | 430 mm | 550 mm | 390 mm | 510 mm | 390 mm | 470 mm |
| 15M | 600 mm | 780 mm | 550 mm | 720 mm | 510 mm | 660 mm |
| 20M | 740 mm | 950 mm | 670 mm | 870 mm | 620 mm | 810 mm |
| 25M | 1,180 mm | 1,540 mm | 1,080 mm | 1,400 mm | 1,000 mm | 1,300 mm |
| 30M | 1,400 mm | 1,820 mm | 1,280 mm | 1,670 mm | 1,190 mm | 1,540 mm |
| 35M | 1,680 mm | 2,180 mm | 1,530 mm | 1,990 mm | 1,420 mm | 1,840 mm |
Note the jump between 20M and 25M. That is the k4 bar size factor changing from 0.8 to 1.0, not an error.
When this chart does not apply:
- "Other cases." Where the conditions above are not met (for example, beams without minimum stirrups through the splice, or bars at tight spacing), A23.3 uses a 0.6 coefficient instead of 0.45. Laps become one third longer than the table.
- Epoxy-coated bars. Multiply by 1.2 or 1.5 depending on cover and spacing, keeping k1 × k2 at 1.7 or less. See when epoxy-coated rebar is required.
- Higher-strength bar. 500W bar needs longer laps than 400W, and the newer A23.3:24 edition adds a separate factor for bar grades above 400 MPa.
- Bundled bars. Laps for bars in a bundle are increased by 10% for a 2-bar bundle, 20% for 3 bars and 33% for 4 bars.
- The detailed method. Engineers can use the full equation in Clause 12.2.2, which accounts for actual cover, spacing and transverse steel, and often gives shorter laps where cover and confinement are generous.
Compression laps
Column bars and other bars that are only ever in compression use a separate, simpler rule. For bar with fy of 400 MPa or less, A23.3 sets the compression lap at 0.073 × fy × db, which is about 29 bar diameters for Grade 400 bar, and not less than 300 mm:
| Bar | 10M | 15M | 20M | 25M | 30M | 35M |
|---|---|---|---|---|---|---|
| Compression lap, Grade 400 | 330 mm | 470 mm | 570 mm | 740 mm | 880 mm | 1,050 mm |
Higher-strength bars use a different expression. Most column dowels and vertical bars are detailed as compression or tension laps by the engineer depending on the load combinations, so do not assume a compression lap is acceptable without the drawings saying so.
Laps in the Ontario Building Code (Part 9)
Houses and small buildings designed under Part 9 of the 2024 Ontario Building Code do not use the A23.3 calculation in most places. For flat insulating concrete form (ICF) walls of up to 2 storeys with floor-to-floor heights of 3 m or less, Article 9.3.1.1 requires Grade 400 bar to CSA G30.18, lapped at least 450 mm for 10M bars and 650 mm for 15M bars. Part 9 also requires 15M dowels across cold joints in ICF foundation walls, embedded at least 300 mm on each side.
Other reinforced concrete that falls outside those Part 9 provisions is designed under Part 4, which brings you back to CSA A23.3. Our overview of rebar code requirements in Ontario shows how the pieces fit, and the foundation wall rebar guide covers ICF and poured walls in more detail.
Rules of thumb and US sizes
You will see "lap it 40 bar diameters" or "40 to 60 diameters" quoted on job sites and forums. That is folklore, not code. The calculated Class B laps in the chart work out to roughly 32 to 61 bar diameters, so a flat 40 diameters can be short for top bars, 25M and larger bars, or lower-strength concrete.
Searches for "#4 rebar lap splice length" are asking about US bar sized to ACI 318, which uses its own equations in inches and psi. A #4 bar (12.7 mm) sits between 10M and 15M, so neither row above is a direct match. If you are working from US details on a Canadian job, have the engineer restate the bars and laps in metric.
Laps on bar lists and placing drawings
Laps are where a lot of rebar orders go wrong. A few points that keep them right:
- Laps add steel. Every lap is extra length on the bar list, so a take-off that ignores laps comes in light. Long runs with many laps can add a noticeable amount of tonnage.
- Stagger where the drawings say. Engineers often require laps to be staggered so they do not all fall at the same section. That changes bar lengths, and the detailer works it out.
- No laps over 35M. CSA A23.3 does not allow lap splices for bars larger than 35M except in limited compression cases. 45M and 55M bars are joined with mechanical couplers or welded splices.
- Mind the cover. Lapped bars sit side by side, so check that cover and clear spacing still work at the lap. See our concrete cover guide.
A good rebar detailer takes the lap lengths from the structural drawings and builds them into every bar mark. Our post on rebar detailing standards in Canada explains how that works.
Getting laps right on your job
The lap lengths that govern your job are the ones on the structural drawings or in the engineer's general notes, not a generic chart. If the drawings say "lap per CSA A23.3" without numbers, ask the engineer to confirm the concrete strength and splice class, because those change the answer.
We prepare bar lists and placing drawings with laps worked out from your drawings, then cut, bend and tag the bar to match. Send us your drawings for a take-off and quote, or see the rebar products we supply.
Frequently asked questions
How long should a rebar lap splice be?
It depends on bar size, concrete strength, bar position and coating. Under CSA A23.3, a standard tension lap (Class B) is 1.3 times the development length and never less than 300 mm. For uncoated 15M bottom bars in 30 MPa concrete, that works out to about 550 mm using the simplified method.
Is there a code for rebar lapping in Canada?
Yes. CSA A23.3 Clause 12 sets development and splice lengths for engineered concrete. The Ontario Building Code also gives fixed laps for flat ICF walls in Part 9: 450 mm for 10M and 650 mm for 15M.
Is a lap splice 40 bar diameters?
40 bar diameters is a rule of thumb, not a CSA requirement. Calculated Class B laps for Grade 400 bar range from roughly 32 to over 60 bar diameters depending on concrete strength, bar size and position, so use the length on your drawings.
Can all rebar sizes be lap spliced?
No. CSA A23.3 does not allow lap splices for bars larger than 35M, apart from limited compression cases. 45M and 55M bars are joined with mechanical or welded splices instead.
What is the lap length for 10M rebar?
For uncoated 10M bottom bars in 25 to 35 MPa concrete, the CSA A23.3 simplified method gives a Class B lap of about 390 to 430 mm. In flat ICF walls under OBC Part 9, the code sets 450 mm.
