How do I slope a garage floor for drainage in the wet Maritime climate of NB?

A garage floor in New Brunswick should slope toward the overhead door at a grade of approximately 1/8 to 1/4 inch per foot, which translates to roughly 1/2 inch to 1 inch of total fall over a typical 24-foot-deep garage slab. This gentle slope allows snowmelt, rain tracked in on tires, and wash water to drain naturally toward the door opening rather than pooling in the middle of the floor or against the back wall.

In New Brunswick's wet Maritime climate, getting the floor slope right is more important than it might be in drier regions. Every time you pull a snow-covered vehicle into the garage between November and April, a significant volume of water melts off the undercarriage, wheel wells, and tires. Without proper slope, that water sits on the slab, creating slip hazards, accelerating concrete deterioration from road salt, and raising humidity levels inside the garage. Over time, standing water on an improperly sloped slab leads to efflorescence — those white mineral deposits that leach out of concrete — and can contribute to mould growth on stored items, drywall, and wood framing near the floor.

The slope is established during the concrete pour by setting the forms and screed rails at the correct heights before placement. The back wall of the garage is the high point, and the floor drops gradually toward the overhead door opening. Your concrete contractor will typically set the back of the slab about 3/4 inch to 1-1/2 inches higher than the front, depending on the depth of the garage. A 1/8-inch-per-foot slope is the minimum — it is subtle enough that you will not notice it when walking or working in the garage, but sufficient to move water. Many experienced NB contractors prefer 1/4 inch per foot for garages that will see heavy winter use, as this provides more positive drainage and accounts for minor settling over the years.

Drainage Details for NB Garages

Beyond the slab slope itself, several other drainage details matter in New Brunswick's climate. The apron — the concrete pad just outside the overhead door — should slope away from the garage at a steeper grade of about 1/4 inch per foot to prevent water from running back inside. The junction between the garage slab and the apron should be smooth and continuous, with no lip or raised threshold that traps water. If your garage is on a lot where the driveway slopes toward the building, a trench drain (also called a channel drain) installed across the garage entrance is essential to intercept water before it enters the garage. Trench drains cost $500 to $1,500 installed in NB, depending on length and material.

Underneath the slab, make sure your contractor installs a minimum 4-inch layer of compacted clear gravel (3/4-inch crushed stone) topped with 6-mil polyethylene vapour barrier. The gravel provides a capillary break that prevents ground moisture from wicking up into the concrete, and the poly keeps that moisture from reaching the slab surface. In NB's high water table areas — particularly in Moncton, Riverview, Dieppe, and many Saint John neighbourhoods — this detail is non-negotiable. Without it, your garage floor will be perpetually damp regardless of how well you slope the surface.

For homeowners considering a floor drain inside the garage, be aware that NB plumbing codes require floor drains to connect to an approved drainage system — typically a dry well, soak-away pit, or the municipal storm sewer where permitted. Floor drains cannot discharge to the sanitary sewer in most NB municipalities without an oil-water separator, and they must include a trap primer or be regularly filled with water to prevent sewer gas from entering the garage. A floor drain adds $500 to $1,200 to the project but is worth considering if your garage doubles as a workshop or if site grading makes it difficult to drain water out the front.

Sloping a garage floor correctly is entirely a professional task — the forming, grading, and finishing must be done during the original pour. Attempting to correct a flat or improperly sloped slab after the fact requires either a self-levelling overlay ($3 to $6 per square foot) or, in severe cases, breaking out and replacing the slab entirely. Getting it right during initial construction is far more cost-effective. If you are planning a new garage, discuss the slope and drainage strategy with your concrete contractor before any forms are set. Need help finding a professional garage builder? New Brunswick Garages can match you for free through the New Brunswick Construction Network.