In Canada, the minimum clear spacing between parallel bars in a beam is the largest of 1.4 bar diameters, 1.4 times the maximum coarse aggregate size, and 30 mm (CSA A23.1, Clause 6.6.5.2, which CSA A23.3 points to). With the common 20 mm aggregate, that is 30 mm for 10M to 20M bars and 1.4 bar diameters for 25M and larger. The same clear distance applies between a contact lap splice and the next bar or splice, and bars in an upper layer must sit directly above those in the layer below.
This post focuses on beams: how the rule plays out across a beam's width, how it compares with ACI 318 and Eurocode 2, and what to check on drawings. For spacing limits in slabs and walls, see maximum rebar spacing under CSA A23.3.
Why beams need a minimum gap
A beam's bottom layer is where the tension steel is packed most tightly, often between the legs of the stirrups. If the bars are too close:
- Concrete cannot flow between them. Coarse aggregate bridges the gap, leaving voids and honeycombing under and between bars.
- Bond is lost. A bar only works if concrete grips it all the way around. A bar with a void beside it develops less force over the same length.
- Vibrators cannot get through. Concrete placed from the top of a deep beam has to pass the top bars and settle around the bottom bars.
- Splitting cracks link up. Closely spaced bars under high stress can split the concrete along a plane through the whole layer.
That is why the rule has three parts: a bar-size term (bond and splitting), an aggregate term (concrete flow), and a fixed minimum (practical placing).
The CSA A23.1 rule in practice
For each bar size, the governing minimum is whichever of the three terms is largest. With 20 mm maximum aggregate (1.4 × 20 = 28 mm, so the 30 mm floor governs over the aggregate term):
| Bar | Nominal diameter | Minimum clear gap | Minimum centre to centre |
|---|---|---|---|
| 10M | 11.3 mm | 30 mm | about 41 mm |
| 15M | 16.0 mm | 30 mm | about 46 mm |
| 20M | 19.5 mm | 30 mm | about 50 mm |
| 25M | 25.2 mm | 35 mm | about 61 mm |
| 30M | 29.9 mm | 42 mm | about 72 mm |
| 35M | 35.7 mm | 50 mm | about 86 mm |
With larger aggregate, the aggregate term starts to govern for small bars: 28 mm aggregate gives a minimum of about 39 mm for any bar up to 25M.
Three points that often get missed:
- Bundled bars are treated as a single bar with a diameter derived from their total area, so a bundle needs a wider gap than its individual bars.
- Lap splices count. The minimum clear distance applies between a contact lap and the adjacent bars or laps, so a lapped pair needs the same gap to its neighbour as a single bar does. That is often where a beam that fits on paper becomes congested.
- Real bars are bigger than their nominal size. The deformations make the outside diameter larger than the nominal diameter, so leave some margin in tight layouts.
Minimum beam width for a row of bars
The practical question in a beam is how many bars fit in one layer. Here is the minimum beam width for a single layer, assuming 30 mm cover to the stirrups (CSA A23.1 class N, interior exposure), 10M stirrups and 20 mm aggregate. Widths are rounded up to the next 10 mm.
| Bars in layer | 15M | 20M | 25M | 30M | 35M |
|---|---|---|---|---|---|
| 2 | 150 mm | 160 mm | 170 mm | 190 mm | 210 mm |
| 3 | 200 mm | 210 mm | 230 mm | 260 mm | 290 mm |
| 4 | 240 mm | 260 mm | 290 mm | 330 mm | 380 mm |
| 5 | 290 mm | 310 mm | 350 mm | 400 mm | 470 mm |
| 6 | 330 mm | 350 mm | 420 mm | 480 mm | 550 mm |
How the numbers are built: beam width = 2 × cover + 2 × stirrup diameter + total bar diameters + gaps between bars. For four 20M bars: 60 + 22.6 + 78 + (3 × 30) = 250.6 mm, so 260 mm.
Adjust for your job:
- Exterior or corrosive exposure needs more cover. Moving from 30 mm to 40 mm adds 20 mm to every width. See concrete cover under CSA A23.1.
- 15M stirrups add about 9 mm.
- Corner bars sit inside the stirrup bend, which has a radius. Detailers check the real fit at the corners, which can need a little more width than the simple sum.
- Laps in the layer add a bar's width plus a gap for each lapped pair.
The RSIC Manual of Standard Practice has full minimum web width tables for combinations of bar sizes. Our post on choosing rebar sizes for beams and columns works through the same check from the design side.
Two layers and the gap between them
When one layer will not fit, the designer adds a second. CSA A23.1 requires bars in the upper layer to be placed directly above the bars in the bottom layer, so the bars form vertical columns with clear paths between them for concrete and vibrators.
The vertical gap between layers is normally held with spacer bars: short bars laid across the beam between the layers at intervals. The size of the spacer sets the clear gap. Check the drawings for the spacer size and the gap the designer wants, and show them on the bar list so they are actually supplied.
A second layer has a structural cost. The centroid of the steel moves up, reducing the effective depth, and the designer has to account for it. That is one reason designers often prefer a wider beam or larger bars to a second layer.
CSA vs ACI 318 vs Eurocode 2
The three codes approach minimum spacing the same way but set different numbers.
| CSA A23.1 (Canada) | ACI 318-19 (US) | EN 1992-1-1 (Eurocode 2) | |
|---|---|---|---|
| Bar-size term | 1.4 × bar diameter | 1.0 × bar diameter | k1 × bar diameter (recommended k1 = 1) |
| Fixed minimum | 30 mm | 1 in. (25 mm) | 20 mm |
| Aggregate term | 1.4 × max aggregate size | Also includes a term based on aggregate size | Max aggregate size + k2 (recommended 5 mm) |
| Layers | Upper bars directly above lower | Upper bars directly above lower, with a minimum gap between layers | Bars in each layer vertically above each other, with space for vibrators |
| Columns | Same clear distance rule for parallel bars | Larger: 1.5 in. or 1.5 bar diameters | Same clause |
Eurocode's recommended values can be changed by each country's National Annex, so a UK, Irish or other European design uses the values in its own annex.
The practical takeaway: a beam laid out to ACI or Eurocode spacing may not meet CSA A23.1. The 1.4 bar diameter factor means larger Canadian bars need noticeably wider gaps. For a 35M bar, A23.1 needs 50 mm clear; a 1.0 bar diameter rule would give about 36 mm. If a design originates outside Canada, the bar layout has to be rechecked, not just the bar sizes converted. Our post on ACI vs CSA rebar detailing covers the other differences.
Checking spacing on drawings and on site
Before fabrication:
- Count bars against beam width using a table like the one above, including any laps that fall in the same section.
- Check beam-column joints, where beam bars from two directions cross the column bars. This is the most congested spot in most frames.
- Watch for added bars around openings, at supports and at cut-offs, which can push a layer over its limit.
- Ask early if bars do not fit. The fixes (wider beam, larger bars, bundles, two layers, mechanical splices instead of laps) are design decisions. See rebar detailing for beams and columns.
On site:
- Measure clear gaps, not centres, where bars look tight.
- Keep laps from drifting together. Staggered laps that end up side by side create a local pinch point.
- Do not push bars together to make room for a sleeve or embed. Ask the engineer.
Beam steel detailed to fit
We prepare bar lists and placing drawings from your structural drawings and flag beams where bars, laps and stirrups will not fit with the required clear spacing before anything is cut. Then we cut, bend, tag and bundle the bar, 10M to 55M, for delivery across the GTA and Ontario.
Send us your drawings for a free take-off and quote, or see our rebar detailing and fabrication services.
Frequently asked questions
What is the minimum clear spacing between rebar in a beam?
In Canada, CSA A23.1 sets the clear distance between parallel bars at no less than 1.4 bar diameters, 1.4 times the maximum coarse aggregate size, and 30 mm, whichever is largest. With 20 mm aggregate that is 30 mm for 10M to 20M bars and 1.4 bar diameters for 25M and larger.
How much space should be between rebar layers in a beam?
CSA A23.1 requires bars in an upper layer to sit directly above the bars in the layer below, so concrete and vibrators can pass down between the columns of bars. The gap between layers is set by the designer and is commonly held with spacer bars; confirm it on the drawings.
What is the minimum rebar spacing in Eurocode 2?
EN 1992-1-1 Clause 8.2 sets the clear distance at the largest of k1 times the bar diameter, the maximum aggregate size plus k2, and 20 mm. The recommended values are k1 = 1 and k2 = 5 mm, which each country's National Annex can change.
Is minimum rebar spacing measured centre to centre?
No. Code minimums are clear distances, from the edge of one bar to the edge of the next. Add one bar diameter to get the centre-to-centre spacing.
