The minimum concrete cover for rebar in Canada comes from CSA A23.1, Table 17, and it depends on two things: what kind of member the bar is in, and which exposure class the concrete falls under. The headline numbers are 75 mm for concrete cast against and permanently exposed to earth, 20 to 60 mm for slabs and walls, and 30 to 60 mm for beams and columns, with extra minimums tied to bar size and aggregate size. The cover on your drawings is the number to build to, and it is often more than these minimums.
What concrete cover is and why it matters
Cover is the thickness of concrete between the surface of the member and the nearest surface of any reinforcement, including ties and stirrups. It is measured to the outside of the outermost bar, not to the centre of the main steel.
Cover does three jobs:
- Corrosion protection. Concrete is alkaline, which protects steel. Chlorides from road salt or seawater and carbonation from the air slowly work in from the surface. More cover, and denser concrete, buys more years before they reach the bar.
- Bond. A bar needs concrete around it to develop its strength. Too little cover and the concrete can split along the bar under load.
- Fire resistance. In buildings that need a fire rating, cover insulates the steel. Fire resistance requirements can call for more cover than durability alone.
Too much cover causes problems too. Bars pushed deeper than designed sit closer to the middle of the member, which reduces its strength and lets surface cracks open wider. That is why cover is specified as a target with tolerances, not as "as much as possible".
CSA A23.1 Table 17 cover values
The values below are the specified covers from Table 17 of CSA A23.1:19, the edition referenced by the 2024 Ontario Building Code. A newer edition (A23.1:24) exists, and your project specification will say which edition applies.
| Location | Class N | Classes F-1, F-2, S-1, S-2 | Classes C-XL, C-1, C-3, A-1, A-2, A-3 |
|---|---|---|---|
| Cast against and permanently exposed to earth | Not permitted | 75 mm | 75 mm |
| Beams, girders, columns and piles | 30 mm | 40 mm | 60 mm |
| Slabs, walls, joists, shells and folded plates | 20 mm | 40 mm | 60 mm |
| Ratio of cover to nominal bar diameter | 1.0 | 1.5 | 2.0 |
| Ratio of cover to nominal maximum aggregate size | 1.0 | 1.5 | 2.0 |
How to read it:
- Take the largest of the three checks. The cover must meet the value for the member type and also the bar diameter ratio and the aggregate ratio. For small bars the millimetre value governs. For large bars it often does not.
- Class N applies only to concrete that stays dry inside the conditioned space, meaning members entirely inside the vapour barrier. That is why it is "not permitted" for concrete against earth.
- Bundled bars are treated as a single bar with the same total area when checking the ratio.
- Screeded surfaces (slab tops finished with a screed) need cover of at least 1.5 times the nominal maximum aggregate size.
Example of the bar ratio governing: a 35M bar (35.7 mm diameter) in an interior Class N wall needs at least 1.0 × 35.7 = about 36 mm of cover, not 20 mm. The same bar in a Class C-1 parking structure needs 2.0 × 35.7 = about 71 mm, more than the 60 mm table value. Bar dimensions are in our Canadian rebar size chart.
Exposure classes in plain language
The exposure class is chosen by the designer from Table 1 of CSA A23.1. It sets the concrete quality as well as the cover. The classes you will meet on most Ontario jobs:
| Class | What it covers | Typical examples |
|---|---|---|
| N | Not exposed to chlorides, freezing and thawing or sulphates | Interior members inside the vapour barrier, many footings and walls |
| F-1 | Freezing and thawing while saturated, no chlorides | Patios, pool decks |
| F-2 | Freezing and thawing while unsaturated, no chlorides | Exterior walls and columns away from salt |
| C-1 | Reinforced concrete exposed to chlorides, with or without freeze-thaw | Parking decks and ramps, bridge decks |
| C-XL | Chloride exposure with higher durability expectations than C-1 | Long-life parking and bridge structures |
| C-2 | Plain concrete exposed to chlorides and freeze-thaw | Garage floors, sidewalks, curbs, steps |
| S-1 to S-3 | Sulphate exposure from soils or groundwater, from very severe to moderate | Foundations in sulphate soils |
| R-1 to R-3 | Residential classes for footings, foundation walls and interior slabs | Part 9 houses |
The A classes cover agricultural and sewage environments (manure pits, silos, wastewater), which matter for farm and municipal work.
Garage floors are a common point of confusion. Plain garage slabs are typically Class C-2, while a reinforced structural slab exposed to de-icing salts, such as a parking deck, is C-1 or C-XL and gets the 60 mm column. Where chloride exposure is severe, designers may also specify a corrosion-resistant bar. Our post on when epoxy-coated rebar is required explains those options.
Cover in footings, slabs and walls
Footings. The bottom and any faces poured directly against soil fall under "cast against and permanently exposed to earth", so 75 mm. Faces formed with forms and later backfilled are a separate case, and the engineer decides which row applies. Our footing rebar guide covers the rest of footing reinforcement.
Slabs. A suspended slab inside a heated building may only need 20 mm, while the same slab in a parking garage needs 60 mm. For slabs on grade, cover is usually set on the drawings to place the steel where it controls cracking.
Walls. Interior faces of a dry basement wall can be Class N, while the exterior face in contact with soil may need more. Many drawings simply call out cover separately for each face.
What the Ontario Building Code says
The 2024 Ontario Building Code (O. Reg. 163/24) handles cover in two ways:
- Engineered buildings (Part 4) are designed to CSA A23.3, and cover comes from CSA A23.1 as described above.
- Houses and small buildings (Part 9) point to CSA A23.1 for concrete, with Article 9.3.1.1 directing plain and nominally reinforced concrete to the residential "R" class requirements. Part 9 gives specific cover only in a few places. For example, rebar in flat insulating concrete form (ICF) foundation walls goes in the inside half of the wall with at least 30 mm of cover from the inside face (Article 9.15.4.5).
Part 9 does not give a cover number for ordinary footings or floors on ground. If your drawings are silent, ask the designer rather than guessing. Our overview of rebar code requirements in Ontario shows how A23.1, A23.3 and the OBC fit together, and the foundation wall rebar guide goes into ICF and poured walls.
How Canadian cover compares with ACI
Many online answers about "rebar clear cover" quote the American Concrete Institute. For reference, ACI 318-19 Table 20.5.1.3.1 gives these minimums for cast-in-place, non-prestressed concrete:
| Condition (ACI 318-19) | Minimum cover |
|---|---|
| Cast against and permanently in contact with ground | 3 in (about 75 mm) |
| Exposed to weather or ground, #6 to #18 bars | 2 in (about 50 mm) |
| Exposed to weather or ground, #5 bars and smaller | 1.5 in (about 40 mm) |
| Not exposed, slabs, joists and walls, #11 and smaller | 3/4 in (about 20 mm) |
| Not exposed, beams, columns and pedestals | 1.5 in (about 40 mm) |
The ground contact value lines up with CSA, but the rest of the structure is different: ACI does not use exposure classes for cover in this table, and the CSA values for chloride exposure are higher. On a Canadian project, use CSA A23.1 and the drawings, not the ACI table.
Getting the cover right on site
Most cover problems happen during placing and pouring, not design:
- Use proper supports. Chairs, bolsters and precast concrete blocks hold the steel at the right height. Industry practice for supports is laid out in the RSIC Manual of Standard Practice, and placing bars on fresh concrete or adjusting them mid-pour should not be allowed.
- Watch the far side. A bar cut or bent too long eats the cover at the opposite face. Fabricated bar lengths are worked out with the cover already deducted, which is one reason ordering from a detailed bar list pays off.
- Check before the pour. Tolerances on cover exist in CSA A23.1, so get the inspector or engineer to check placement, not just the bar count.
If you have drawings, send them over for a quote. We detail bar lists with cover built into every cut and bend, then supply the bar tagged to the drawings. See our rebar products for straight, fabricated and GFRP bar.
Frequently asked questions
How much concrete cover does rebar need?
In Canada, CSA A23.1 Table 17 sets it by member type and exposure class. Concrete cast against and permanently exposed to earth needs 75 mm. Slabs and walls range from 20 mm in dry interior conditions to 60 mm where chlorides are present, and beams and columns from 30 mm to 60 mm.
What is the concrete cover for a footing?
Where a footing is cast directly against the soil, CSA A23.1 Table 17 calls for 75 mm of cover to the reinforcement on that face. Confirm the cover for each face of your footing on the structural drawings.
What is the difference between cover and clear cover?
They usually mean the same thing: the distance from the concrete surface to the nearest surface of the reinforcement, including ties and stirrups. It is not measured to the centre of the main bar.
Is Canadian rebar cover the same as ACI?
No. ACI 318 uses different tables and inch values, and it does not use CSA exposure classes. On Canadian projects, cover comes from CSA A23.1 and the engineer's drawings.
