What goes under interlock: paver base depth, gravel and bedding sand for GTA driveways and patios

Most of an interlock driveway or patio is underground. The pavers are usually 2⅜ to 3 inches thick, with roughly an inch of bedding sand under them. Under that is the paver base: a deep layer of crushed gravel, compacted in thin lifts over firm soil.

That base decides whether the surface stays flat. Below are the published depths, what the materials are called at a GTA supplier, and what to ask a contractor before any interlock installation.

What are the layers under interlock?

A standard sand-set paver surface has four layers, from the bottom up:

Edge restraints (plastic, aluminum, steel or concrete) hold the outer rows in place. The surface slopes away from the house.

CMHA (the industry group behind the concrete paver tech notes) gives 1.5% as the minimum slope. That’s about 3/16 inch per foot.

How deep should the paver base be?

Plan on 6 to 10 inches of compacted gravel under a patio or walkway and 10 to 14 inches under a driveway, going by Unilock’s figures. The numbers vary with the source and the soil:

Unilock’s reason for the subdivision row is that freshly graded lots are hard to judge, while older neighbourhoods have settled. Clay drains poorly, so it wants the extra depth there as a frost-resistant layer.

CMHA adds that colder climates and wet or weak soils need thicker bases. Treat 4 and 6 inches as a floor.

How deep is the dig?

Deeper than the base. Unilock’s homeowner chart for a driveway on clay adds up like this:

• 14 inches of compacted gravel
• 1 inch of bedding
• 3 inch pavers

That’s 18 inches, plus any loose subsoil that comes out. Unilock’s contractor guide also suggests digging 6 to 12 inches past the edges to make room for the edge restraint.

Granular A or clear stone: which gravel goes under pavers?

Either works if it’s angular crushed stone.

Granular A is the Ontario name for a dense-graded crushed aggregate, defined in the provincial spec OPSS.MUNI 1010 as base material for pavement. It’s a mix of stone and fines: everything passes a 26.5 mm sieve, and 2 to 8% passes the finest 75 micron sieve. The fines let it compact into a very tight, firm layer.

Clear stone has no fines. Unilock’s open-graded system uses 3/4 inch clear stone (ASTM No. 57) as the base and a smaller chip (ASTM No. 8) as bedding. It suggests the same build for ordinary, non-permeable paving where it suits the site.

Contractors quoted in that Unilock article say a dense-graded base holds water that then freezes. Unilock says clear stone drains much better and carries the load just as well, but it needs a proper edge restraint.

Unilock’s contractor guide says the sharp edges of crushed stone are what make it lock together. It doesn’t recommend softer or finer material like limestone screenings or stone dust as a base.

What goes on top of the gravel: sand, HPB or stone dust?

About 1 inch of coarse bedding, screeded flat and left loose until the pavers go down. CMHA specifies bedding sand that meets ASTM C33 or CSA A23.1, which is concrete sand.

Unilock’s homeowner guide also lists HPB or ASTM No. 9 chip stone in place of sand, paired with a clear stone base.

Keep the bedding thin. CMHA says not to use it to make up for an uneven base, because the dips eventually show through. That’s one of the causes covered in our guide on why interlock sinks and how to fix it.

Do you need landscape fabric under the base?

On clay or soil that stays wet, yes. CMHA lists geotextile as optional, but suggests it in three cases:

• where the soil is saturated part of the year
• where there’s freeze and thaw
• over clay and moist silty soils

It keeps soil from working up into the gravel under load, so there’s less rutting. CMHA says to turn it up the sides of the dig, lay it without wrinkles and overlap seams by at least 12 inches.

How does Ontario frost affect the base?

A paver base can’t go below the frost line here. For comparison, the Ontario Building Code puts footings for an attached deck 1.2 m (48 inches) down.

Interlock copes instead by being flexible, draining well and sitting on gravel that doesn’t heave.

Frost heave happens when water in soils with lots of fine particles, like silt and clay, freezes into ice layers. Clean sand and gravel aren’t frost susceptible.

In a spring thaw the ground mostly melts from the top down. The frozen layer below traps the meltwater, and the soaked soil loses bearing strength (Pavement Interactive covers both effects). A deeper, well-compacted base spreads the load over that weak spring soil.

CMHA also says frozen base material shouldn’t be installed, and nothing should go over a frozen subgrade. In the GTA that rules out base work in the coldest months.

How is a paver base built, step by step?

The order is the same for a walkway or a driveway; only the depths change.

1. Call for locates. Ontario Dig Authority (formerly Ontario One Call) says locates are free and required by law before you dig, and asks for at least 5 business days’ notice for a single address. Private lines, like a gas line to a barbecue, need a separate private locate.
2. Dig out the topsoil, roots and soft material to the depth you need, plus 6 to 12 inches past the edges.
3. Compact the exposed soil. CMHA calls for at least 98% of standard Proctor density.
4. Lay geotextile if the soil is clay or stays wet.
5. Add the gravel in lifts and compact each one. Unilock’s homeowner guide says 3 inch layers. CMHA allows 4 to 6 inch lifts with a heavy reversible plate compactor (7,000 lbf) and thinner ones with a 5,000 lbf machine. Shape the slope into the base, not the sand.
6. Install the edge restraint.
7. Screed about 1 inch of bedding sand or HPB. Don’t walk on it or pre-compact it.
8. Lay the pavers, compact them into the bedding, then fill and compact the joints.

Our polymeric sand guide explains what the binder does, and how to apply polymeric sand has the steps.

How much gravel and sand will you need?

Work it out by volume: area in square feet times depth in feet gives cubic feet, and 27 cubic feet make a cubic yard.

That’s why base depth shows up so clearly in a quote. Our interlock cost guide goes through the rest of the price.

What are the most common paver base mistakes?

• A patio-depth base under a driveway. It holds up for foot traffic, then ruts in the tire tracks.
• One thick dump of gravel compacted only on top. The bottom of the layer never gets dense.
• Stone dust or masonry sand as bedding, against CMHA’s guidance.
• Fixing dips with extra sand instead of more gravel.
• No fabric on wet clay, so soil works up into the base.
• Base work over frozen or soaked ground, or right after heavy rain.
• No slope, or slope toward the house, so water sits on the base.

When should you call a pro?

A small walkway is a fair DIY job if you can rent a plate compactor and move a lot of gravel.

A driveway means trucking away soil and compacting a deep base in lifts over a big area. Mistakes there often show up only after a freeze and thaw, and fixing them means lifting the surface.

When you compare quotes, ask each contractor:

• how deep the base is, and which material
• the lift thickness and the compactor
• whether fabric goes in
• what edge restraint they use
• whether removal and disposal are included

If a quote won’t say, ask why.

We put in interlock walkways, patios and driveways across Markham and nearby, including Richmond Hill and Stouffville. See our interlock installation service, call or text (437) 684-2535, or text us a photo of the area for a free quote.