A small shed slab in Ontario doesn't legally need rebar: a storage shed of 15 m² or less, one storey, detached and without plumbing is exempt from both the building permit and the Building Code. Most builders reinforce it anyway, usually with a light grid of 10M rebar or welded wire mesh in the upper part of the slab, to hold shrinkage and frost cracks tight. Larger sheds and other accessory buildings need a permit, and if the slab carries the walls, its edges and reinforcement come from a design or the municipality's standard detail.
This post covers when a shed slab is regulated, whether to use rebar or mesh, typical sizes and spacing, thickened edges, and a worked example of how much rebar a typical shed slab takes.
When the Building Code applies to a shed
Under Sentence 1.3.1.1.(6) of Division A of the 2024 Ontario Building Code, a shed is exempt from the requirement to get a building permit and from compliance with the Code if it:
- is not more than 15 m² in gross area,
- is not more than one storey,
- is not attached to a building or any other structure,
- is used only for storage ancillary to a main building on the lot, and
- has no plumbing.
For scale, a 3.0 m x 4.3 m shed (about 10 x 14 ft) is roughly 13 m² and fits under the limit. A 3.6 m x 4.9 m shed (about 12 x 16 ft) is close to 18 m² and doesn't. Municipal zoning bylaws on setbacks, height and lot coverage still apply to small sheds, so check those separately.
Once a shed or other accessory building needs a permit, Part 9 applies. Two rules matter most for the slab:
- Floors-on-ground (Section 9.16) set a minimum concrete slab thickness of 75 mm (Article 9.16.4.3). Granular fill under slabs isn't required in accessory buildings (Sentence 9.16.2.1.(2)), though most builders still use a compacted gravel base.
- Slabs that carry the building must be designed under Part 4 (Article 9.16.1.2). A shed slab with thickened edges that the walls sit on is doing the job of a footing, so its edge size and rebar come from a designer or a standard detail your municipality accepts. Our post on monolithic slab rebar and thickened edge layout covers how those are reinforced.
Why reinforce a shed slab at all
Concrete shrinks as it cures and moves with temperature and frost. Without reinforcement, a small slab will often crack, and the pieces can shift and settle apart over the years. Reinforcement doesn't stop cracks forming, but it holds them tight so the slab stays flat and in one piece.
For sheds there are a few specific reasons to do it:
- Floating slabs move with frost. Under Sentence 9.12.2.2.(6), small accessory buildings that aren't masonry (one storey, not more than 55 m² and close to the ground) are exempt from the minimum foundation depths, which is why most shed slabs sit on grade and ride up and down with the frost. A reinforced slab moves as a unit rather than breaking into pieces.
- Point loads inside. Garden tractors, ATVs, snowblowers and heavy shelving put concentrated loads on the slab.
- Wall anchors at the edge. Anchor bolts or straps for the walls are often set near the slab edge, the part most likely to crack.
Rebar or mesh for a shed slab
Both are common on shed slabs. The bigger question is keeping either one in the right place.
| Rebar grid (10M) | Welded wire mesh | |
|---|---|---|
| Handling | Bars cut to length and tied into a grid | Rolled or flat sheets, cut with bolt cutters |
| Stays in position | Stiff, sits well on chairs | Flexible, easily pushed down during the pour |
| Thickened edges | Easy to add continuous edge bars and corner bars | Edge bars still needed separately |
| Strength for crack control | More steel area per metre at typical spacing | Less steel area, fine for light slabs |
The common failure with mesh is that it ends up lying on the gravel at the bottom of the slab, where it does very little. Slab-on-grade reinforcement belongs in the upper part of the slab: the RSIC Manual of Standard Practice notes that it is generally located about 50 mm below the top surface. Our post on where rebar should sit in a concrete slab explains why, and wire mesh vs rebar for concrete slabs compares the two in more depth.
Typical rebar spacing and layout for a shed slab
For a permit-exempt shed, the layout is your choice. For a permitted building, it comes from the drawings. Typical small-slab details look like this:
- Bar size: 10M is the usual choice for a slab around 100 mm thick. Thicker slabs, or slabs carrying vehicles, may move to 15M. See 10M vs 15M for a concrete slab.
- Spacing: a grid somewhere in the 300 to 450 mm range each way is common on small residential slabs.
- Position: on chairs or concrete bricks, in the upper part of the slab, with bars kept back from the edges so they have concrete cover.
- Laps: where bars join, they're overlapped and tied. On a small shed slab you can often avoid laps by cutting bars to the slab dimensions.
- Thickened edges: where used, continuous bars run around the perimeter in the thickened part, with corner bars bent to tie the two directions together.
- Control joints: cut or tooled joints help cracks form in straight lines. On small slabs, one joint across the middle of a longer slab is common.
Worked example: how much rebar for a shed slab
Take a 3.0 m x 3.6 m slab (about 10 x 12 ft) with a grid of 10M bars at 400 mm each way, bars kept about 75 mm in from each edge.
| Direction | Width to cover | Bars | Length each | Total length |
|---|---|---|---|---|
| Bars running 3.6 m | 3.0 m minus 150 mm = 2.85 m, divided at 400 mm max | 9 bars | 3.45 m | 31.05 m |
| Bars running 3.0 m | 3.6 m minus 150 mm = 3.45 m, divided at 400 mm max | 10 bars | 2.85 m | 28.5 m |
| Total | 19 bars | about 59.6 m |
The bar count is the number of spaces (width divided by 400 mm, rounded up) plus one. At the nominal 0.785 kg/m for 10M bar, the grid weighs about 47 kg. A thickened edge would add perimeter bars: two continuous 10M or 15M bars around a 13.2 m perimeter, plus laps and corner bars.
Our rebar calculator and estimating guide walks through the same method for larger slabs.
Ordering rebar for a shed slab
A shed slab is a small order: a few bar lengths for the grid, plus edge and corner bars if the slab has thickened edges. Having bars cut to the slab dimensions saves cutting on site and avoids laps.
We cut and bend rebar from 10M to 55M, including straight bars cut to length and bent corner bars, tagged and bundled to your bar list, and deliver across the GTA and Ontario. Send us your slab size or drawings for a quote, or see our rebar products. For hot tub pads, AC pads and other small pours, see rebar for concrete pads.
Frequently asked questions
Do I need rebar for a shed concrete slab?
Not by code for a small shed. In Ontario, a storage shed of 15 m² or less, one storey, detached and without plumbing is exempt from a building permit and from the Building Code. Most builders still reinforce the slab with a light rebar grid or mesh to hold cracks tight and help the slab ride out frost movement.
What rebar spacing is used for a shed slab?
A light grid of 10M bars somewhere in the 300 to 450 mm range each way is common on small residential slabs, placed in the upper part of the slab on chairs. Larger sheds, slabs carrying vehicles, or slabs designed for a permit will have the spacing set on the drawings.
Is rebar or mesh better for a shed base?
Both work for crack control. Rebar is easier to keep at the right height because it is stiffer and sits well on chairs. Mesh is quicker to roll out but often ends up at the bottom of the slab if it isn't properly supported, where it does little.
How thick should a shed slab be?
The Ontario Building Code's minimum for floors-on-ground is 75 mm, and permit-exempt sheds aren't bound by it at all. Many shed slabs are poured around 100 mm, with thicker slabs or thickened edges where walls bear on the slab or a garden tractor or ATV will be parked inside.
