The pavers are the last thing that goes down on an interlocking project. Everything underneath them determines whether those pavers are still in place and level in 25 years or being pulled up and relaid in five.
The interlocking aggregate base in Ontario is where that outcome is decided. Most homeowners never see the base once it is buried. Most quotes do not explain what is going in. And some contractors count on that.
Key Takeaways
- ▶Granular A is the primary load-bearing base material for interlocking in Ontario, compacted in lifts directly above the subgrade
- ▶Granular B is a coarser sub-base material used beneath Granular A on heavy-load or poor-soil sites
- ▶Stone dust and limestone screenings are not acceptable bedding materials and are a known failure point across the GTA
- ▶High Performance Bedding (HPB) outperforms concrete sand as a bedding layer in Ontario’s freeze-thaw climate
- ▶A quote that does not specify base material by name and layer thickness should be treated with caution
Why Base Material Choice Matters in Ontario
Ontario’s freeze-thaw cycle is aggressive. The GTA experiences dozens of freeze-thaw transitions in a single winter. Moisture-saturated soil expands when frozen and contracts when it thaws. Whatever aggregate material sits in the base zone either resists that movement or contributes to it.
The wrong interlocking aggregate base material in Ontario does not just settle unevenly. It retains water, which worsens the freeze-thaw effect. It loses its structure under repeated load. It can allow fine clay particles from below to migrate upward, slowly contaminating the base. The result shows up in heaving, rocking pavers, separated joints, and surface drainage that no longer works as intended.
Getting the interlocking aggregate base specification right for an Ontario project is not a detail. It is the foundation of why base preparation determines long-term performance.
Understanding the Primary Base Materials
How the Layered Base System Works
A properly specified interlocking aggregate base in Ontario is not a single material. It is a system of layers, each serving a distinct structural function.
Subgrade Preparation
The native soil at the base of the excavation. On clay-heavy GTA sites, this is compacted and sometimes treated with geotextile fabric to prevent upward migration of fine particles into the aggregate above. This is not a material you add. It is what you prepare before any aggregate goes in.
Granular B Sub-Base (Where Required)
On high-load or poor-subgrade sites, Granular B goes in first at the base of the excavation. It provides volume, load distribution, and a stable platform for the Granular A above it. Not every project requires this layer. Driveways on clay, large commercial-adjacent residential applications, and sites with demonstrated drainage problems are the main candidates.
Granular A Base Course (Core Structural Layer)
This is the core structural layer on every interlocking project in Ontario. It goes in at 4-inch lifts, compacted between each lift. For a pedestrian patio, the Granular A course is typically 6 to 8 inches of compacted depth. For a driveway, 10 to 12 inches of compacted Granular A is standard.
The gradation of Granular A, that mix of angular chips and fines, is what produces a stable surface for the bedding layer to sit on. Without adequate Granular A depth and proper compaction, nothing else in the system performs correctly.
See how deep to excavate before laying the baseBedding Layer
This is the 1 to 1.5 inch layer that sits directly beneath the pavers, on top of the compacted Granular A base. It serves as the final levelling and cushioning layer. Two materials are used here: concrete sand and High Performance Bedding (HPB).
Concrete Sand vs HPB: Which Bedding Layer Is Right
This is where a significant portion of Ontario interlocking base failures originate, and it is also where the most contractor shortcuts occur.
The Shortcut That Causes Most Base Failures: Stone Dust
Stone dust, also called limestone screenings or crusher dust, is a byproduct of crushing operations. It is widely available, cheap, and still used by a portion of GTA contractors as a base or bedding material.
It should not be.
Stone dust is highly moisture-retentive. Its fine particle structure compacts densely and creates a nearly impermeable layer. In Ontario’s climate, it saturates and then freezes, expanding and lifting whatever is above it. The result is frost heave, surface unevenness, and joint failure.
Stone dust also breaks down structurally over time under repeated freeze-thaw cycling. It does not maintain its compacted density the way Granular A does. On clay sites, saturated stone dust can actually migrate and contaminate the subgrade.
The Ontario contractor community has largely moved away from stone dust as a bedding material over the past decade, following manufacturer guidelines from Unilock and others that specifically exclude it. But it is still in use. If a quote references limestone screenings, stone dust, or crusher dust as the bedding or base material, that is a red flag.
How Base Material Choice Differs by Project Type
The same layered system applies across project types, but the thickness of each layer and the need for Granular B varies.
For patios and walkways on average GTA soil, Granular A alone with a proper bedding layer is the standard. For driveways, particularly on the clay-heavy soil common across Markham, Richmond Hill, Vaughan, and much of North York, the decision about whether to include a Granular B course below the Granular A should be based on what is found at subgrade depth during excavation.
For driveway interlocking services, every project assessment includes a subgrade evaluation before base material quantities are specified.
Common Base Material Shortcuts and Why They Fail
Based on projects we assess across the GTA, these are the base material shortcuts that appear most often in failed installations.
Using Stone Dust as the Bedding Layer
Stone dust retains moisture, freezes, heaves, and degrades under Ontario conditions. It is the single most common material-related cause of premature interlocking failure in the GTA.
Skipping Granular B on Clay Sites
On heavy clay subgrade, placing Granular A directly on native clay without a Granular B course can result in clay migrating upward into the base over time, reducing load-bearing capacity as fine particles fill the voids in the aggregate.
Using Recycled Crush or Road Base of Unverified Gradation
Their gradation is inconsistent compared to specified Granular A. Variability in compaction performance transfers risk directly to the homeowner.
Under-Specifying the Bedding Layer Depth
A bedding layer that is thicker than 1.5 inches becomes unstable. One that is inconsistently screeded creates micro-variation in the finished surface that is not visible on installation day but becomes obvious over time. The bedding layer is not a tolerance zone. It is a precise 1 to 1.5 inch layer on top of a stable base.
How to Verify Base Material in a Written Quote
A professional quote should specify base materials explicitly. Before signing, confirm in writing:
If a quote uses generic terms like gravel or base material without specifying the grade, ask for clarification in writing. If the bedding material is stone dust or limestone screenings, that is your answer. A contractor who knows their specifications answers these questions directly.
For more on how compaction locks the base in place, that article covers what happens to Granular A under proper compaction and why lift sequencing matters.
The Straight Answer on Interlocking Base Material in Ontario
The correct interlocking aggregate base Ontario contractors should specify is Granular A as the primary structural course, Granular B as a sub-base where soil conditions or load require it, and either concrete sand or HPB as the 1 to 1.5 inch bedding layer directly beneath the pavers. Stone dust is not an acceptable material in any layer of this system.
A quote that specifies these materials by name and depth is a quote from a contractor who understands the system. One that does not is worth questioning before work begins. For driveway interlocking services in Richmond Hill, Vaughan, Toronto, and other GTA neighbourhoods, every project we quote specifies base material, layer depth, and bedding choice before a machine touches the ground.
Disclaimer: All information reflects our professional experience and industry research across the Greater Toronto Area. Base material requirements vary by site conditions, soil type, and surface usage. Consult a qualified interlocking contractor for a project-specific assessment before work begins.