Picture a February evening. You pull in, and within an hour a 40-centimetre ridge of slush has dropped off the wheel wells and melted into a grey, salty puddle. Where does that water go? If the answer is "toward the door to the house" or "into a crack along the wall", the slab was planned wrong. Once concrete is poured, there's no fixing it cheaply.
A garage slab looks like the simplest part of a new home, which is exactly why it gets the least attention. Here is a stage-by-stage checklist to go through with your designer and builder, from permit drawings to the day of the pour.
Stage 1: Find out what your municipality allows for drainage
- Floor drain permission: ask the municipal building or engineering department whether a garage floor drain is allowed at all. Rules vary widely across the country. Some municipalities accept one connected to the sanitary sewer. Others prohibit any garage drain because it can carry fuel and oil into the system.
- Oil interceptor: where a drain is permitted, find out whether it must run through an oil and grit interceptor first. Ask who inspects it and how often it has to be cleaned out.
- Storm connection: never assume a garage drain can go to the storm sewer or a ditch. Contaminated water from a garage is usually treated very differently from roof runoff.
- Septic systems: on an acreage with a septic field, check with the provincial or regional authority that regulates onsite systems. Garage water containing oil, antifreeze and salt is generally unwelcome in a septic tank.
- Sump option: if a drain isn't allowed, ask whether a sealed holding pit or a slab sloped to the door is the accepted alternative.
Get the answer in writing, or at least note the name of the person you spoke to. Rules change, and a drain shown on a plan from another town may not pass in yours. In 2025, an interceptor and its piping commonly added roughly $1,500 to $4,000 to a build, depending on the site. That cost is worth knowing before you fall in love with the idea of hosing out the garage.
Stage 2: Choose between slope, a trench drain or both
- Slope to the overhead door: the simplest approach, and the one most codes accept. A fall of about 1/8 to 1/4 inch per foot moves meltwater out under the door.
- Direction of slope: the slab must fall away from the house entry and away from any wall with wood framing. Water should never pond against the door to the mudroom.
- Trench drain at the door: a linear drain just inside the overhead door catches water before it reaches the driveway. It only makes sense if it has a legal outlet and is protected from freezing.
- Centre drain with four-way slope: this keeps the floor drier under parked cars. It also creates a low point where salt and sand collect, and it needs the most permissions.
- Raised entry step: the code typically requires the door into the house to sit above the garage floor. Confirm the height on the drawings, because it protects against both water and fumes.
For most Canadian homes, a slab sloped to the door with a well-detailed apron is the most reliable choice. A trench drain earns its place when the driveway slopes toward the house, as it often does on a sloped lot, or in a wide garage where water would otherwise travel 30 feet or more. A three-bay layout like this one-storey ranch with a side-load garage has enough floor area that it's worth discussing a drain line or a second slope plane with the builder.
Stage 3: Specify concrete that survives salt and freeze-thaw
- Air entrainment: garage floors in Canada are generally required to use air-entrained concrete of around 32 MPa, with roughly 5 to 8 percent air. Ask the builder to put the class on the ready-mix order.
- Water-cement ratio: no extra water added on site to make placing easier. Retempering at the truck is a major cause of scaling in the first winter.
- Finish: a broom or light trowel finish. Hard steel-trowelling of air-entrained concrete can trap a weak surface layer that flakes off.
- Thickness and base: a 4-inch slab, or more under heavy vehicles, on well-compacted granular fill. A poor base is the main reason garage slabs crack and settle.
- Curing: wet curing or a curing compound for the first days, and protection from frost if the pour happens in late fall.
- Sealer: a penetrating sealer applied once the slab has cured, usually after the first month. Plan to reapply it every few years.
Road salt is hard on young concrete. If the pour happens in autumn, the slab sees its first chlorides only weeks later. That's why curing and sealing matter more here than almost anywhere else in the house. Many concrete suppliers recommend keeping de-icing salt off new concrete for the whole first winter. Brush slush out rather than letting it sit.
Stage 4: Plan the joints, insulation and frost protection
- Control joints: saw-cut or tooled joints at about 24 to 30 times the slab thickness, so panels of roughly 8 to 12 feet for a 4-inch slab. Cut them to about a quarter of the depth.
- Joint layout: aim for near-square panels. Add a joint at every re-entrant corner, such as where a garage steps around a stair or a mechanical room.
- Isolation joints: a compressible strip where the slab meets foundation walls and columns, so the slab can move independently.
- Frost walls: the garage foundation must reach below local frost depth. That's often about 4 feet in southern regions and considerably more farther north. Check the figure for your municipality.
- Under-slab insulation: in an unheated garage, rigid insulation is optional. In a heated garage, or one with in-floor heating, plan continuous rigid foam under the slab and at its edges.
- Shared wall with the house: the wall and ceiling between the garage and living space need proper air sealing and fire separation. If there's a basement or living space below, expect an engineered structural slab rather than a slab on grade.
Garages attached to homes with a full basement deserve extra care. Backfill along the foundation wall is deep and often settles for years, and a slab poured over it can crack and tilt toward the house. Compare a compact Craftsman bungalow with a full basement to a larger five-bedroom family home that also sits on a basement. In both, the garage slab bears on fill next to a deep excavation, so ask for compaction in lifts or structural support at that edge. You can browse other house plans with a basement with the same question in mind.
Stage 5: Hand the builder a checklist before the pour
- Slope and elevations: high point, low point and fall written on the drawing, with the step height at the house door.
- Drain details: drain type, location, outlet and interceptor (if required), with the permit reference noted.
- Concrete ticket: strength, air content and exposure class stated. Ask to see the delivery ticket on the day.
- Joint plan: a simple sketch of control and isolation joints and when the saw cuts will be made.
- Insulation and vapour barrier: type and thickness of rigid foam, poly under the slab, and edge insulation if the garage will ever be heated.
- Conduit and sleeves: any pipes for in-floor heating, an electric vehicle charger, or a future drain line placed before the concrete goes down.
- Apron and driveway: the transition at the overhead door detailed so the driveway falls away from the garage.
The planning matters most on larger homes. A contemporary house with four garage bays or a modern mountain design with a three-car garage creates a lot of concrete and a lot of meltwater, and small slope errors add up across that width. If you're still choosing a design, look at plans with an attached garage and check where the house entry sits relative to the overhead doors. An entry on the high side of the slab makes drainage much easier.
The most common mistakes
- Installing a floor drain without checking the municipal rules, then having to cap it at inspection.
- Pouring a flat slab, or one that drains toward the house door.
- Ordering concrete without air entrainment, or adding water at the truck.
- Skipping control joints or cutting them too late, so random cracks form first.
- Putting salt on the slab in its first winter, before it has cured and been sealed.
- Pouring a garage slab over uncompacted basement backfill.
- Deciding to heat the garage later, when there's no insulation under the concrete.
