What is the minimum compaction requirement for fill under a garage slab in NB?

Fill under garage slabs in New Brunswick must be compacted to a minimum of 95% Standard Proctor Density, with some municipalities and engineers requiring 98% compaction for optimal performance. This compaction standard ensures the fill won't settle over time, which would cause the concrete slab to crack, sink, or develop uneven areas that affect garage door operation and create trip hazards.

Proper compaction is critical in New Brunswick's freeze-thaw climate because poorly compacted fill allows water infiltration and frost penetration beneath the slab. When this moisture freezes, it expands and can heave sections of the garage floor. The combination of settlement from inadequate compaction and frost heave from moisture infiltration is responsible for most garage slab failures in the province.

The compaction process typically involves placing fill in 8-12 inch lifts and compacting each lift with a plate compactor or jumping jack tamper. Granular materials like crushed gravel (3/4-inch minus) or pit run gravel compact more reliably than clay-based soils. Many NB contractors prefer a 6-8 inch base of compacted granular material directly under the slab, even when the existing soil is suitable, because it provides better drainage and more predictable compaction results.

Testing compaction requires specialized equipment — a nuclear density gauge or sand cone test performed by a geotechnical technician. For residential garages, many contractors rely on visual inspection and "proof rolling" with heavy equipment to verify that the base doesn't deflect under load. However, if your garage will support heavy vehicles, workshop equipment, or vehicle lifts, formal compaction testing provides valuable insurance against future settlement problems.

The depth of fill affects compaction requirements. If you're building on undisturbed native soil with minimal fill (less than 12 inches), compaction is less critical. But if your garage site required significant cutting and filling to achieve proper grade and drainage, or if you're building on a sloped lot that required extensive grading, proper compaction becomes essential for long-term slab performance.

Moisture content during compaction is crucial — soil that's too wet or too dry won't achieve proper density. The optimal moisture content varies by soil type, but most granular fills compact best at 8-12% moisture content. In NB's wet climate, allowing fill to dry between rain events often produces better compaction than trying to work with saturated material.

When to Hire a Pro: Compaction testing requires specialized equipment and training that most homeowners don't have access to. If your garage project involves more than 2-3 feet of fill, or if you're building on a sloped site that required significant grading, hire a contractor experienced with proper fill placement and compaction. The cost of doing it right during construction is far less than repairing a settled slab later.

Find experienced garage contractors who understand NB soil conditions and compaction requirements through the New Brunswick Construction Network.