Should I use crushed limestone for my Ottawa interlock base?

Crushed limestone can work for interlock base in Ottawa, but it's not the optimal choice compared to standard Granular A and Granular B materials specified by Ontario standards.

Crushed limestone (typically 3/4" minus or similar gradations) lacks the precise gradation curve of Granular A base material, which is specifically engineered to compact to maximum density and stability. Granular A contains a carefully balanced mix of stone sizes from 25mm down to fine particles that lock together when compacted, creating an extremely stable platform. Pure crushed limestone often has too uniform a gradation — mostly larger stones with insufficient fine material to fill voids and achieve proper compaction.

Ottawa's extreme freeze-thaw conditions demand the most stable base possible. With over 50 freeze-thaw cycles per winter and frost penetration to 1.2-1.5 metres depth, your interlock base must be rock-solid to prevent heaving and settling. The standard specification for Ottawa interlock driveways is 300mm of compacted Granular B sub-base (100mm to 0mm gradation) followed by 150mm of compacted Granular A base (25mm to 0mm gradation). This two-layer system with proper gradations provides superior stability compared to single-material limestone bases.

Crushed limestone also presents drainage concerns in Ottawa's clay soil areas (Barrhaven, Orleans, Gloucester, Riverside South). Limestone can become more plastic when wet compared to harder granite or trap rock aggregates commonly used in Granular A. In areas with Ottawa River valley clay subgrade, you need maximum drainage and stability — Granular A over Granular B with geotextile fabric separation provides better long-term performance.

Cost-wise, crushed limestone might seem attractive at $18-25 per tonne versus $22-35 for Granular A, but the savings are minimal on a typical driveway project. The risk of premature settling or heaving that requires lifting and releveling your entire interlock surface far outweighs any upfront material savings.

When to Hire a Pro: Base preparation is the foundation of interlock longevity. A professional paving contractor will excavate to proper depth, install geotextile fabric on clay soils, place and compact granular materials in proper lifts, and ensure correct drainage slopes. Improper base preparation is invisible once pavers are laid but becomes catastrophically expensive to fix when settling or heaving occurs.

Stick with MTO-specification Granular A and B materials for your Ottawa interlock base — the proven performance in our extreme climate is worth the modest additional cost.