How do I waterproof a garage that sits below grade in Moncton NB?

Waterproofing a below-grade garage in Moncton requires a multi-layered approach that addresses water from all directions — exterior foundation waterproofing, interior drainage management, proper grading, and a functioning weeping tile system — because Moncton's clay-heavy soils, high water table in many neighbourhoods, and significant annual precipitation of over 1,100 mm create persistent hydrostatic pressure against any below-grade structure. A garage that sits partially or fully below grade faces fundamentally different water challenges than a slab-on-grade garage, and half-measures will not solve the problem.

Exterior Waterproofing — The Primary Defence

The most effective and permanent waterproofing strategy is to address water intrusion from the outside. This means excavating down to the foundation footing on all below-grade walls, cleaning the concrete surface, and applying a proper waterproofing system. For a below-grade garage in Moncton, the recommended system is a rubberized asphalt waterproofing membrane (such as Blueskin or Bakor WP200) applied directly to the cleaned concrete wall, followed by a dimpled drainage membrane (such as Delta-MS or Platon) installed over the waterproofing to protect it from backfill damage and to channel water down to the weeping tile. This dual-layer system provides both a waterproof barrier and a drainage plane.

The weeping tile (perforated drain pipe) at the base of the footing is critical. If your below-grade garage does not have functioning weeping tile, or if the existing system is clogged with silt and roots — common in older Moncton properties — it must be replaced. The weeping tile should be 4-inch perforated PVC pipe wrapped in filter fabric, laid in a bed of 3/4-inch clear crushed stone at least 6 inches deep, sloped at a minimum of 1% toward a sump pit, storm sewer connection, or daylight outlet. The weeping tile must run along the entire perimeter of the below-grade walls and connect to a reliable discharge point. The cost for exterior excavation, waterproofing, and weeping tile replacement on a below-grade garage in Moncton typically runs $8,000-$20,000 depending on the depth and perimeter length.

Interior drainage serves as the secondary defence. A sump pit with a submersible sump pump is essential for any below-grade garage in Moncton. The sump pit should be at least 18 inches in diameter and 24 inches deep, with the pump discharging to the exterior at least 4-6 feet away from the foundation. Install a battery backup sump pump or water-powered backup — Moncton experiences power outages during ice storms and heavy rain events, and a flooded garage during a power outage is a common and preventable problem. A primary sump pump costs $200-$500, and a battery backup system adds another $300-$700.

If the garage floor itself is the source of moisture wicking up through the concrete, the slab was likely poured without a vapour barrier underneath. Short of demolishing and replacing the slab (which costs $3,000-$6,000 for a two-car garage), you can apply a penetrating concrete densifier and sealer to reduce moisture transmission through the slab, followed by an epoxy or polyaspartic floor coating that acts as a surface moisture barrier. This will not stop active water seepage through the slab, but it will address moisture wicking and dampness.

Grading around the exterior of the garage must slope away from the foundation at a minimum of 2% for the first 6 feet — roughly a 1-inch drop per foot. In Moncton's relatively flat terrain, particularly in neighbourhoods like Mapleton, Riverview adjacent areas, and lower-elevation parts of the city, poor grading is one of the most common causes of below-grade water problems. Ensure gutters and downspouts are installed on the garage roof and discharge water well away from the foundation via extensions or buried drain pipes.

This is a project that demands professional assessment and execution. A below-grade waterproofing failure leads to ongoing water damage, mould growth, structural deterioration, and loss of use of the space. Have a waterproofing contractor or foundation specialist inspect your garage before deciding on a repair strategy. Find contractors through the New Brunswick Construction Network at newbrunswickconstructionnetwork.com.