Three jobs, one set of fundamentals
A patio, a shed pad and a hot tub pad carry different loads, but they fail the same ways. They settle on soft ground, crack between joints that are too far apart, or scale where salt and meltwater sit on the surface. Getting the mix, the base and the joints right covers all three.
C-2 or F-1? It comes down to salt
The national standard, CSA A23.1, sorts concrete by exposure. Two classes matter for backyard slabs:
| Class C-2 | Class F-1 | |
|---|---|---|
| Exposure | Freeze-thaw and de-icing salt | Freeze-thaw while saturated, no salt |
| CSA examples | Garage floors, porches, steps, sidewalks | Pool decks, patios, tennis courts |
| Minimum strength at 28 days | 32 MPa | 30 MPa |
| Maximum water-cement ratio | 0.45 | 0.50 |
| Entrained air | 5–8 % (category 1) | 5–8 % (category 1) |
On paper, a patio that will never see salt could be F-1. In practice, we pour C-2 for every patio and pad, for three reasons. The Ontario Building Code sets 32 MPa and a 0.45 water-cement ratio for all exterior flatwork (OBC 9.3.1.6–9.3.1.7). Concrete Ontario’s residential guide puts patios in class C-2. And a slab outside a back door gets salted the first icy morning, whatever the plan was. The MPa guide explains the numbers.
Base, slope and joints
The Building Code doesn’t set a base depth or thickness for exterior flatwork, so we specify both in writing. We strip sod and topsoil, compact granular stone in layers, and slope patios to drain away from the house. Ottawa’s sensitive marine clays can keep settling and shrink as they dry, which is why soft ground comes out rather than getting covered over.
Control joints go in at 24 to 36 times the slab thickness, to a maximum of about 4.5 m. Panels should be close to square, and joints should be cut at least a quarter of the depth. A 100 mm (4 in) patio gets joints about every 3 m. An isolation joint separates the slab from the foundation and any steps.
Shed pads
A shed pad needs to be flat, level and sized to the shed’s footprint so water drips off the edges, not under the walls. The pad itself is usually landscaping. Under the Building Code Act, though, a new building with a building area over 10 m² (about 108 sq ft) needs a building permit, and zoning setbacks apply to every shed. Settle the shed size first.
Hot tub pads
A filled hot tub puts a heavy, concentrated load on a small area for years. The pad has to be level, sized to the tub maker’s requirements and built on a compacted base that won’t settle at one corner. We follow the manufacturer’s base instructions for your model. We also plan conduit for the electrical feed before the pour, because cutting a finished slab later is the expensive way to do it.
Permits, zoning and grading
The City lists landscaping among projects that don’t require a building permit, so a patio laid on the ground often needs none. That doesn’t make it rule-free:
- Exempt projects must still meet the Building Code and the Zoning By-law, and designated heritage properties aren’t exempt.
- In some residential zones, an at-grade projection can’t extend into the front yard’s required soft-landscaped area.
- You may not change your lot’s approved grading or send water onto a neighbour.
- The City requires hardscaping contractors who build patios to be licensed, so ask any contractor for theirs.
- Every dig starts with a free utility locate.
Finishes and cost
A broom-finished patio or pad runs about $10–$25 per sq ft in Ottawa. Exposed aggregate adds about $3–$5 per sq ft, and stamped concrete runs about $18–$45 per sq ft. A patio is also a good time to rebuild tired back steps. See the concrete cost guide, how concrete is poured and our process.