An ordinary concrete slab on grade does not need rebar under Part 9 of the 2024 Ontario Building Code. Section 9.16, Floors-on-Ground, sets the granular base, a minimum slab thickness of 75 mm, the concrete strength and the finish, but it has no reinforcement requirement. The exception is a slab that supports loads from the structure above: under Article 9.16.1.2 it must be designed under Part 4, and the design sets the bar size, spacing and position.
So the honest answer to "what are the rebar requirements for a slab on grade?" is: none by default, and whatever the designer specifies once the slab does more than sit on the ground. Here's what the code does require, when a slab moves into design, and what reinforced slabs on grade typically look like.
What OBC Section 9.16 actually requires
Section 9.16 applies to floors supported on ground or granular fill that don't provide structural support for the building above (Article 9.16.1.1). For those floors, the 2024 OBC (O. Reg. 163/24, Division B) requires:
| Item | Requirement | Where |
|---|---|---|
| Granular base | At least 100 mm of coarse clean granular material with not more than 10% passing a 4 mm sieve | 9.16.2.1.(1) |
| Exceptions to the base | Not required under slabs in garages, carports or accessory buildings, and some industrial buildings | 9.16.2.1.(2) |
| Fill | No fill that changes volume with moisture or freezing; fill compacted unless it's clean coarse aggregate | 9.16.2.2 |
| Water | Ingress of water under the floor prevented by grading or drainage | 9.16.3.1 |
| Hydrostatic pressure | Where groundwater can push up on the floor, it must be a poured concrete slab designed to resist it | 9.16.3.2 |
| Thickness | At least 75 mm, not counting any topping | 9.16.4.3 |
| Bond break | Between the slab and footings or rock | 9.16.4.4 |
| Finish | Trowelled smooth and even, no dry cement added to soak up water | 9.16.4.1 |
The concrete itself comes from Section 9.3. For unreinforced concrete, Article 9.3.1.6 sets a minimum 28-day compressive strength of 20 MPa for interior floors and 32 MPa for garage floors, carport floors and all exterior flatwork, and Article 9.3.1.7 limits the water to cementing materials ratio to 0.65 and 0.45 respectively.
The thickness rule has a tolerance. The explanatory note to 9.16.4.3 allows the slab to be 10 mm thin in spots, provided it's never less than 65 mm and the mean thickness is 75 mm.
Notice what isn't in the list: reinforcement, mesh or fibres. Part 9 simply doesn't require them in a floor on ground.
When a slab on grade has to be designed
A slab moves out of Section 9.16 and into engineering when it does structural work. The common cases:
- It supports the structure. A slab that carries loadbearing walls, posts or columns, such as a thickened slab under an interior bearing wall, falls under Article 9.16.1.2 and must be designed under Part 4.
- Hydrostatic pressure. Where groundwater can push up under the floor, Article 9.16.3.2 requires a poured concrete slab designed to resist it.
- It spans. A slab that bridges between supports instead of resting on the ground is a structural slab. The cold room slab over a basement storage room is the one case Part 9 covers prescriptively (Section 9.40: 125 mm minimum, 10M at not more than 200 mm each way); anything else is designed.
- Heavier loads. Commercial and industrial floors, forklift traffic, racking legs and heavy equipment.
- Poor ground. Soft, expansive or variable soils, or deep fill, where the designer wants the slab to bridge local soft spots.
- The building isn't Part 9. Larger or non-residential buildings are designed under Part 4 from the start.
For a designed slab, the drawings govern. There is no single "minimum rebar for slab on grade" number to look up; the designer chooses bar size, spacing and position for the loads, joint layout and soil. Slab-on-ground design guides such as ACI 360R are commonly referenced for this, alongside the CSA concrete standards.
Why many Part 9 slabs are reinforced anyway
Plenty of garage floors, driveways, shop slabs and basement floors carry steel even though Section 9.16 doesn't ask for it. The usual reasons:
- Holding cracks tight. All concrete shrinks, and slabs crack. Reinforcement doesn't stop cracking; it keeps cracks from opening wide and keeps the pieces on either side level with each other.
- Wider joint spacing. Steel lets a designer space control joints farther apart than an unreinforced slab would tolerate.
- Vehicle loads. Garage floors and driveways see wheel loads at the edges and over the joints.
- Radiant heating. In-floor heating tubes are often tied to a rebar or mesh mat, which also controls cracking.
- Owner preference or local practice. Some builders reinforce every slab as a standard detail.
For basement floors specifically, our post on whether a basement floor slab needs rebar covers the details.
What a reinforced slab on grade looks like
When a slab on grade is reinforced, the details usually follow a familiar pattern:
- Bar size and spacing. Residential and light commercial slabs commonly use 10M or 15M in a single mat, written as something like 10M @ 400 EW (each way). Our guides to rebar spacing for concrete slabs and 10M vs 15M for a slab explain how those numbers are chosen and compared.
- Position. Steel for crack control in a thin slab is usually placed in the upper part of the slab, where shrinkage cracks start, and held there on chairs. The drawings set the exact cover.
- Edges and thickened areas. Thickened slab edges and thickenings under posts or walls get their own bars, often continuous bars along the edge.
- Dowels. Bars connecting the slab to foundation walls, or across construction joints, are detailed on the drawings.
- Openings and re-entrant corners. Trim bars around drains, sumps and inside corners help control the diagonal cracks that start there.
- Cover. Concrete cover depends on exposure and on whether the concrete is cast against earth. See our concrete cover guide.
Searches for a "rebar calculator for slab on grade" are usually after the quantity once size and spacing are known. The method is simple: count bars from the spacing, add laps, and multiply total metres by the weight per metre of the bar size.
Slabs on fill
A slab on fill sits on built-up material instead of undisturbed ground, common on sloping lots, raised garage floors and additions. Section 9.16 doesn't change the slab rules for fill, but it does control the fill: material that changes volume with moisture or freezing isn't allowed where it would damage the building, and fill has to be compacted unless it's clean coarse aggregate (9.16.2.2).
Rebar isn't a fix for poor fill. A slab over deep or loosely placed fill can settle away from its support, and when that's a risk the answer is proper compaction, or a slab designed to span, or both. If your slab is on more than a modest depth of fill, have the designer look at it before you choose the reinforcement.
Mesh, fibres or rebar?
Welded wire mesh and fibre-reinforced concrete both get used for crack control in slabs on grade, and neither is mentioned in Section 9.16. They can work in the right slab, but they aren't automatic substitutes when the drawings show rebar. See does fibre-reinforced concrete replace rebar.
Getting slab steel cut to your drawings
If your slab is reinforced, we can turn the drawings into a bar list, then cut and bundle the bar, edge bars and dowels to match. If you only have a sketch, get the bar size, spacing and cover confirmed by your designer first, especially for garage, structural or commercial slabs.
Send your slab drawings for a quote and take-off, or see the rebar products we supply.
Frequently asked questions
Is rebar required in a slab on grade in Ontario?
Not for an ordinary floor on ground under Part 9 of the 2024 Ontario Building Code. Section 9.16 sets the base, thickness and finish but has no reinforcement requirement. A slab that supports loads from the structure above has to be designed under Part 4, and that design sets the reinforcement.
Is rebar needed for a 4 inch slab?
Code doesn't require it for an ordinary Part 9 floor on ground, where the minimum thickness is 75 mm. Many 100 mm slabs are still reinforced or have fibres for crack control. If your slab carries loads, vehicles or sits on poor ground, have the designer specify it.
Does a 6 inch concrete slab need rebar?
Thickness alone doesn't decide it. A 150 mm garage or shop slab is often reinforced because of vehicle loads, joint spacing or soil, but the requirement comes from the design or the drawings, not from a code rule for 6 inch slabs.
What size rebar is used in a slab on grade?
Residential and light commercial slabs on grade that are reinforced usually use 10M or 15M bar in a single mat. The size and spacing come from the drawings; our 10M vs 15M guide explains how they compare.
