What is the best gravel base depth under a garage slab in Moncton NB clay soil?

For a garage slab in Moncton's clay soil, you need a minimum 6-inch compacted gravel base, though 8-12 inches is recommended for better long-term performance and drainage.

Moncton's heavy clay soil presents specific challenges for garage construction that make the gravel base absolutely critical. Clay soil retains water, expands when wet, shrinks when dry, and provides poor drainage — all of which can cause slab settlement, cracking, and moisture problems if not properly addressed with an adequate stone base.

The gravel base serves three essential functions in Moncton's clay conditions. First, it provides a stable, non-expansive platform that won't shift as the clay below moves through wet-dry cycles. Second, it creates a capillary break that prevents ground moisture from wicking up through the concrete slab. Third, it provides drainage so that water doesn't pool under the slab during Moncton's wet spring conditions or heavy Maritime rainfall.

Use 3/4-inch clear stone or pit run gravel — this is readily available from Moncton-area quarries and provides the right balance of drainage and compaction. The stone should be placed in 4-inch lifts and compacted with a plate compactor or jumping jack after each lift. Proper compaction is crucial — loose gravel will settle over time, causing the slab to crack or sink. Many contractors in the Moncton area use a combination of larger stone (2-3 inches) for the bottom half and 3/4-inch clear for the top half to maximize drainage while providing a smooth base for the concrete.

In areas with particularly problematic clay or poor drainage, consider increasing the gravel depth to 12-18 inches and installing weeping tile around the garage perimeter. Some Moncton contractors also recommend a layer of geotextile fabric between the clay subgrade and gravel base to prevent the clay from migrating up into the stone over time. This is especially important if your garage site tends to be wet or if you've noticed standing water in the area after heavy rain.

Don't forget the vapour barrier — place 6-mil polyethylene sheeting over the compacted gravel base before pouring concrete. This prevents moisture from moving up through the slab, which is particularly important in Moncton's humid Maritime climate. Overlap seams by 6 inches and seal with tape, and be careful not to puncture the poly during concrete placement.

When to Hire a Pro: Site preparation and concrete work should be handled by experienced contractors familiar with Moncton's soil conditions. Proper excavation depth, grading for drainage, gravel selection and compaction, and concrete placement all require equipment and expertise that ensure your garage foundation performs well for decades. A poorly prepared base is the most expensive garage problem to fix after construction.

Need help finding a professional garage builder familiar with Moncton clay soil conditions? New Brunswick Garages can match you with experienced local contractors for free estimates on your project.