Do I need concrete, or will a gravel pad do?

It depends on what sits on it. A light resin shed does fine on a compacted gravel pad, and we will tell you so at the site visit. Concrete is the right answer when the building is a workshop, holds heavy things, needs anchors against wind, or carries a hot tub or a car. If you are not sure, tell us what the pad is for and we will give you the honest answer before we quote.

Does my shed need a building permit in Ontario?

Under the Ontario Building Code, a shed needs no building permit when it is 15 square metres or less, one storey, not attached to another building, used only for storage that serves the house on the lot, and has no plumbing. If it is bigger, attached, or has plumbing, you need a permit. Your town's zoning rules, such as setbacks from the lot line, still apply either way, so check with your municipal building department before you pour.

What concrete does the Building Code ask for on a garage floor?

The Ontario Building Code sets a 28-day strength of at least 32 MPa for garage and carport floors and exterior flatwork, and 5 to 8% air entrainment for garage and carport floors. The air gives freezing water room to expand and helps the surface stand up to the salt that drips off a car in winter. Ask for this mix by name on any garage pad quote.

How long before I can use a new slab?

Keep vehicles off for at least 48 hours, and wait about 7 days before you frame a building on it. Concrete reaches its full design strength at 28 days, so heavy loads are better held off until then. Cold weather slows the cure, and we tell you the exact window for your pour.

Concrete Slabs and Garage Pads in Southern Ontario: The Complete Guide

A concrete slab or garage pad looks like the simplest pour there is. It is flat, it is square, and it holds still. This guide walks you through the choices in the order you should make them.

Step 1: Decide what the pad has to carry

Start with the load, because it decides almost everything else.

A light shed. A light resin shed does not need concrete. A compacted gravel pad does the job for less, and we will say so rather than sell you concrete you do not need.
A workshop shed, a gazebo or a heavy storage building. Concrete makes sense here. You get a floor you can sweep and a place to set anchors against wind. Light pads like these usually get welded wire mesh and are poured about 100 mm (4 in.) thick.
A hot tub. A filled tub with people in it weighs two to three tonnes, and the maker wants it dead level. The pad must be level, reinforced, and a little larger than the tub, so the steps and the service panel sit on concrete.
A garage or a shop. A floor that carries a car, a jack and jack stands is a different job. It is poured 125 to 150 mm (5 to 6 in.) thick with a 10M rebar grid, not light mesh.

If the plan for the building may change, pour for the bigger use. A shed pad can become a workshop floor later; a thin pad cannot become a garage floor.

Step 2: Check the permit and zoning before you book

Paperwork comes before the pour, because the slab is hard to move once it is in.

In Ontario, a shed needs no building permit when it is 15 square metres or less, one storey, not attached to another building, used only for storage that serves the house on the lot, and has no plumbing. Once a building is larger than that, attached, or plumbed, you need a permit. Most detached garages need one.

A permit exemption is not a zoning exemption. Your town’s setbacks from the lot line and its limits on lot coverage still apply to a small shed. The rules differ from town to town across Southern Ontario, so call your municipal building department with the size and the location before you book. When a project needs a permit, we flag it at the quote.

Step 3: Get the right concrete for our winters

Southern Ontario winters freeze and thaw a slab many times each year, and a garage floor also takes the road salt that melts off your car. The Ontario Building Code sets the floor for this:

Strength: at least 32 MPa at 28 days for garage and carport floors and for exterior flatwork.
Air entrainment: 5 to 8% for garage and carport floors. The tiny air bubbles give freezing water room to expand, so the surface does not pit and flake.

This is the same mix we use on outdoor pads and garage floors. If a quote does not name the mix, ask for it. A cheaper mix without air can look fine for a year and then start to flake after a few winters.

Step 4: Look under the slab, not only at it

The part of a pad you never see does most of the work.

Excavation. The topsoil and soft ground come out. A pad on scraped topsoil tilts or cracks when frost gets under it.
Base. We put 150 to 200 mm of Granular A under the pad and compact it in lifts. Where soft or disturbed soil comes out, the base goes deeper.
Edges. A detached garage pad is usually a floating slab, which moves with the frost as one piece. Its edges are thickened to about 300 mm so they can carry the walls.
Steel in the right place. Mesh or rebar only works when it sits in the slab, not on the ground under it.

Step 5: Plan the details you cannot add later

Some choices are easy on pour day and costly after it.

Anchor bolts. Bolts set into the wet concrete hold a garage’s bottom plate better than anchors drilled in later. Give us the plan, and we set them where the plates will land.
Slope to the door. A garage floor that slopes gently toward the door lets snowmelt from the car run out instead of pooling.
Size margin. Pour a hot tub pad a little larger than the tub, so the steps and the service panel land on concrete.
Old concrete. If an old pad is in the way, its removal and disposal should be a line on the quote, not a surprise.

Step 6: Plan the timing and the cure

Spring through fall is the main season. Fresh concrete needs protected time above freezing to gain strength, so a late-fall pour close to frost is a risk unless the conditions are right.

After the pour:

Keep vehicles off for at least 48 hours.
Wait about 7 days before you frame a building on it.
Concrete reaches full design strength at 28 days.

Step 7: Compare quotes on the spec, not the bottom line

The concrete is the small part of a slab’s cost; the base, the steel and the crew are the big part. Our slab and garage pad cost guide explains where the money goes. When you compare quotes, check that each one names:

the slab thickness and the thickened edges, if any;
the mix (32 MPa, air-entrained for a garage floor);
the reinforcement (mesh or rebar, and the grid);
the depth of excavation and the granular base;
anchors, slope and any removal of old concrete.

A quote that leaves these out is not cheaper; it is only less clear. We put all of them in writing after a free site visit, and the labour carries a lifetime warranty. When you are ready, ask for a free written quote.