How deep should the gravel base be under a concrete driveway in Ottawa?

For a concrete driveway in Ottawa, you need a minimum of 450mm (18 inches) of compacted granular base — significantly deeper than most other Canadian cities due to Ottawa's extreme frost conditions.

The base should consist of two layers: 300mm of compacted Granular B sub-base (coarse crushed stone, 100mm to 0mm) followed by 150mm of compacted Granular A base (finer crushed stone, 25mm to 0mm). This depth ensures the base extends below Ottawa's frost line of 1.2 to 1.5 metres when combined with the concrete thickness and any topsoil removal.

Why Ottawa requires deeper base preparation than southern Ontario comes down to our brutal continental climate. With winter temperatures dropping to -25°C to -30°C and over 50 freeze-thaw cycles annually, frost penetrates much deeper into the ground. If your granular base doesn't extend below the frost line, frost heaving will push sections of your concrete driveway upward, creating cracks, trip hazards, and drainage problems that are expensive to repair.

Ottawa's clay soils make proper base depth even more critical. Clay is prevalent across Barrhaven, Orleans, Gloucester, Riverside South, and large sections of Kanata. Clay expands dramatically when wet and shrinks when dry, creating unstable subgrade conditions. Without adequate granular base depth and geotextile fabric between the clay subgrade and granular material, your concrete will move and crack regardless of how well it's poured and finished.

Each base layer must be compacted in 150mm lifts using a plate compactor or vibratory roller. Hand tamping or single-pass compaction is insufficient and leads to settling within the first winter. The Granular B provides structural support and frost protection, while the Granular A creates a smooth, stable surface for concrete placement.

For driveways with heavy vehicle traffic (large trucks, RVs, commercial vehicles), consider increasing the total base depth to 600mm. Sandy soils in areas closer to the Canadian Shield (west Kanata, Stittsville, Carleton Place) may allow slightly reduced depths but still require geotextile fabric to prevent base migration.

When to hire a professional: Base preparation requires precise excavation, proper material selection, and systematic compaction — this is not DIY work. Inadequate base depth saves a few hundred dollars upfront but costs thousands in premature replacement when the concrete cracks and heaves within 3 to 5 Ottawa winters.

Need help finding a qualified concrete contractor? Ottawa Driveways can match you with experienced professionals who understand Ottawa's specific base requirements and frost conditions.

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Find a Driveway & Paving Contractor

Ottawa Driveways connects you with experienced driveway and paving contractors through the Ottawa Construction Network:

• The Egress Group Inc ✓
• Procore Foundation Repair ✓
• Innovation Concrete Grind & Polish ✓