What is the difference between Granular A and Granular B base material for Ottawa driveways?

Granular A and Granular B are different grades of crushed stone base material, with Granular B serving as the deep structural foundation and Granular A providing the smooth, dense surface base directly under your driveway surface.

Granular B (Sub-Base Foundation) is a coarser crushed stone mix ranging from 100mm down to fine particles (100mm-0mm gradation). This material forms the primary structural foundation of your driveway and must extend below Ottawa's frost line of 1.2 to 1.5 metres. The larger stone sizes in Granular B create excellent load distribution and drainage, preventing the driveway from settling under vehicle weight or heaving during freeze-thaw cycles. In Ottawa, you need a minimum 300mm compacted depth of Granular B, though 450mm is better on clay soils common in Barrhaven, Orleans, and Gloucester. The coarse gradation allows water to drain through rather than pooling, which is critical given Ottawa's 50+ annual freeze-thaw cycles.

Granular A (Surface Base) is a finer crushed stone mix ranging from 25mm down to fine particles (25mm-0mm gradation). This material goes on top of the Granular B and provides a smooth, dense surface for asphalt paving or interlock installation. The smaller stone sizes in Granular A compact to a very tight, stable surface that won't shift under traffic. You need a minimum 150mm compacted depth of Granular A in Ottawa. When properly compacted with a vibratory roller, Granular A creates an almost concrete-like surface that provides excellent support for the driveway surface material.

Why Both Materials Matter in Ottawa's Climate is all about managing our extreme conditions. The deep Granular B layer prevents frost heaving by extending below the frost line and providing drainage for groundwater. The dense Granular A layer prevents surface water from penetrating down to the subgrade where it could freeze and cause heaving. Together, they create a stable platform that can handle temperature swings from -30°C to +35°C without moving.

Ottawa Pricing and Specifications: Granular B typically costs $20 to $30 per tonne delivered, while Granular A runs $22 to $35 per tonne due to the additional crushing required for the finer gradation. Both materials must meet MTO (Ministry of Transportation Ontario) specifications. A typical Ottawa driveway (50 square metres) requires about 15 to 20 tonnes of Granular B and 8 to 12 tonnes of Granular A. Each layer must be compacted in 150mm lifts with a plate compactor or vibratory roller - hand tamping is insufficient and leads to settling within the first winter.

When to Hire a Professional: Base preparation is the foundation of driveway longevity and requires proper excavation equipment, knowledge of soil conditions, and compaction equipment. A contractor cutting corners on base depth or using only one type of granular material is setting you up for expensive failure within 3 to 5 Ottawa winters. Quality base preparation costs more upfront but prevents the catastrophic cracking and heaving that requires complete driveway replacement.