No, an ordinary basement floor slab in an Ontario house doesn't need rebar. Section 9.16. of the 2024 Ontario Building Code requires a concrete floor-on-ground at least 75 mm thick, on at least 100 mm of clean granular fill, with no reinforcement requirement. Rebar comes in when the slab does more than sit on the ground: carrying walls or posts, resisting groundwater pressure, or spanning over a space like a cold room. Those slabs are designed, and the designer specifies the bar.
Here's what the code actually requires, where reinforcement shows up in basements, and what to expect when it does.
What the Ontario Building Code requires for basement floors
Section 9.16. covers floors-on-ground that don't provide structural support for the building. For a concrete basement floor, the main rules are:
| Requirement | Rule | Reference |
|---|---|---|
| Granular fill | At least 100 mm of coarse, clean granular material, with no more than 10% passing a 4 mm sieve | 9.16.2.1. |
| Slab thickness | At least 75 mm, exclusive of any topping | 9.16.4.3. |
| Concrete strength | At least 20 MPa at 28 days for interior floors other than garages and carports | 9.3.1.6. |
| Bond break | Bond-breaking material 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. |
| Dampproofing | Where installed below the slab, polyethylene at least 0.15 mm with 100 mm laps, Type S roll roofing, or suitable rigid insulation | 9.13.2.6. |
| Soil gas | Floors in contact with the ground must resist soil gas leakage, and houses need a radon rough-in | 9.13.4. |
The code's note on thickness allows a tolerance of minus 10 mm as long as the slab is at least 65 mm at every point and averages 75 mm.
Nothing in that list mentions rebar or mesh. Article 9.3.1.6. sets strengths for unreinforced concrete, which tells you how Part 9 expects a basement floor to be built.
Why an ordinary basement floor doesn't need rebar
A basement floor-on-ground sits on compacted or clean granular fill and carries furniture, people and light partitions. The load goes straight down into the ground. The footings and foundation walls carry the house, and the slab is separated from the footings by a bond break so it can move on its own.
Unreinforced slabs do crack, mostly from drying shrinkage. Rebar doesn't stop those cracks from forming. What well-placed reinforcement does is hold cracks tight so they stay narrow. In a basement floor that will be covered with flooring, most owners and builders control shrinkage cracking with good subgrade preparation, a sensible water content in the concrete, proper curing and control joints instead.
When a basement slab does need reinforcement
Reinforcement comes in when the slab has a structural job. Common cases:
- The slab supports the structure. Article 9.16.1.2. says floors-on-ground that support loads from the superstructure must be designed under Part 4. A bearing wall or post landing on a thickened slab instead of a separate footing is the classic example.
- Groundwater pushes up. Where groundwater could cause hydrostatic pressure under the floor, Article 9.16.3.2. requires a poured concrete slab designed to resist it. These slabs are engineered and usually heavily reinforced.
- Underpinning and lowering a basement. Dig-downs and underpinning in older GTA houses involve new footings, new slab levels and sometimes a structural slab, all to an engineer's drawings.
- Poor or variable soils. Fill, soft clay or a site where part of the slab sits on rock and part on soil may lead the designer to reinforce the slab.
- In-floor heating. Rebar or mesh is often used as something to tie radiant tubing to. That's an owner and installer choice, not a code requirement.
The other basement concrete that does have prescriptive rebar is the slab over a cold room, usually the front porch slab spanning over the cold cellar. That isn't a floor-on-ground: it's a suspended slab, covered by Section 9.40. It must be at least 125 mm thick, reinforced with 10M bars at no more than 200 mm on centre in each direction with 30 mm clear cover to the bottom, and bear at least 75 mm on the foundation walls, anchored with 600 mm × 600 mm 10M bent dowels at no more than 600 mm on centre. It applies only where the clear span is no more than 2.5 m along the shortest dimension, and not to slabs that carry vehicles.
What size and spacing to expect
When a basement slab is reinforced, the drawings set the bar size, spacing, position and laps. Some general points:
- 10M is the usual bar for residential slabs. 15M shows up in structural slabs, thickened areas and heavier loads. Our article on what size rebar for a concrete slab compares the two.
- Spacing is a grid each way, often tighter at thickened edges and under loads. See rebar spacing for concrete slabs for how spacing callouts are read and limited.
- Thicker slabs. A 75 mm floor leaves little room to keep a bar at a controlled depth with concrete cover above and below. Reinforced slabs are normally designed thicker.
- Support matters. Bars or mesh lying on the poly do almost nothing. They need chairs or bolsters to hold them at the height the designer intended.
For the broader rules on slabs on grade beyond basements, including garages and exterior slabs, see slab on grade rebar requirements.
Rebar, wire mesh or fibre
If a basement floor isn't required to be reinforced but you want some crack control, there are three common routes:
| Option | What it does | Watch for |
|---|---|---|
| Rebar grid | Holds cracks tight; can be designed for structural loads | Needs chairs; adds cost and labour |
| Welded wire mesh | Light crack control over large areas | Often ends up at the bottom of the slab if not supported |
| Fibre-reinforced concrete | Reduces early shrinkage cracking throughout the slab | Doesn't replace designed structural reinforcement |
Our comparison of wire mesh vs rebar for concrete slabs goes into the trade-offs.
Replacing an existing basement floor
When an old basement floor is broken out and repoured, for a new drain, waterproofing work or simply because it's in poor shape, the same Section 9.16. rules apply to the new slab: granular fill, thickness, strength and a bond break at the footings. A few things are worth checking on these jobs:
- Are interior posts or walls bearing on the old slab? If a column sits on the slab rather than on a footing, the work is a chance to add a proper footing under it. Our guide to basement column rules explains what the code expects.
- Is the floor being lowered? Going deeper than the existing footings means underpinning or benching, which is engineered work with its own reinforcement.
- Is there water under the floor? Persistent water or hydrostatic pressure turns a simple floor into a designed slab.
Getting the slab right
Before you order anything for a basement floor:
- Check the drawings. If the slab is reinforced, the structural drawings show the bar, spacing, cover, thickened areas and laps.
- Confirm with the building department if you're doing a new floor in an existing basement, lowering a floor or underpinning. Most of that work needs a permit.
- Order bar cut and tagged to the drawings, especially for thickened edges, dowels into existing concrete and cold room slabs with bent dowels.
We detail, cut and bend rebar from 10M to 55M, including dowels and bent bars, and deliver tagged bundles across the GTA and Ontario. Send us your drawings for a free take-off and quote, or see our rebar products.
Frequently asked questions
Does a basement floor need rebar in Ontario?
Not for an ordinary house basement floor. Section 9.16 of the 2024 Ontario Building Code requires a concrete floor-on-ground at least 75 mm thick on at least 100 mm of granular fill, with no reinforcement requirement. Slabs that carry loads from the structure above must be designed under Part 4, and those usually are reinforced.
How thick does a basement slab have to be in Ontario?
At least 75 mm, not counting any topping (Article 9.16.4.3). The code's note allows a tolerance of minus 10 mm as long as the slab is never under 65 mm thick at any point and averages 75 mm.
What size rebar for a basement slab?
When a basement slab is reinforced, the designer specifies the bar size and spacing. 10M is the common choice for residential slabs, with 15M used for heavier loads or structural slabs. The code's prescriptive slab over a cold room uses 10M bars at no more than 200 mm each way.
Should I put wire mesh or rebar in my basement floor?
Neither is required by the code for a normal floor-on-ground. Some owners add reinforcement for crack control or to hold in-floor heating tubes. If you add it, it only helps when it is supported at the right height in the slab rather than lying on the ground.
