Sunken interlock: why pavers sink and how to fix them

Sunken interlock is almost never the pavers’ fault. They sit on about an inch of sand, over compacted gravel, over soil. When one of those layers settles, washes out or gets pushed around by Ontario frost, the pavers follow it down.

The fix is to find out which layer moved and why, rebuild it with compacted gravel, and put the same pavers back.

If you’d rather have someone else do it, this is what our interlock repair and paver leveling work covers.

Why does interlock sink?

Interlock sinks because something under it has lost volume or support. The Concrete Masonry & Hardscapes Association (CMHA) is the industry group that publishes the technical guides for paver surfaces.

Its maintenance guide lists the causes of depressions:

• settlement of the soil or gravel base, often from poor compaction
• a saturated base or soil
• bedding sand that was too thick
• sand washed out of the joints and bedding
• loose or inadequate edge restraints

Here is how those causes tend to show up on a GTA driveway, walkway or patio:

How deep the base should be

On driveways, CMHA ties ruts in the wheel paths to the base settling under repeated wheel loads. It says major rutting often points to a base that’s too thin.

Base depth matters more than most people expect, and the guidance varies by who’s writing it:

CMHA says bases need to be thicker in colder climates and on continually wet or weak soils. It adds that greater thicknesses are often used where there are lots of freeze-thaw cycles. Unilock says clay soil needs deeper digging than sandy soil.

Southern Ontario winters bring plenty of freezing and thawing, so a driveway built to the bare minimum here is more likely to rut.

Does frost make sunken pavers worse in Ontario?

Yes. Frost heave happens when water in fine-grained soil, such as silt or clay, freezes into layers of ice and lifts whatever sits on it. Clean gravel doesn’t do this, which is one more reason the base matters.

In spring the ground thaws from the top down. The meltwater can’t drain through the frozen layer underneath, so the soaked soil loses much of its strength (Pavement Interactive’s frost action reference covers both effects).

A weak spot gets pushed up in winter and settles back lower in spring. A small dip in the fall can be a puddle and a trip edge by April.

How do you fix sunken interlock properly?

You lift the sunken section, rebuild the base with compacted gravel, lay fresh bedding sand, relay the same pavers, compact them and refill the joints. The full sequence is in the step-by-step list below.

Three details decide whether the repair lasts.

Fix the base with gravel, not sand

CMHA’s guide specification says flatly not to use bedding sand to fill depressions in the base. A thick pocket of sand compacts more than the thin layer around it, and the dip slowly comes back.

Take up more than the dip

CMHA’s maintenance guide, written for utility cuts, says to remove pavers at least 18 inches (0.45 m) beyond the opening on each side. It also says to leave a 6 to 12 inch strip of undisturbed bedding sand as a height guide. A repair that stops right at the edge of the low spot leaves a ridge.

Compact the pavers, don’t just tap them

CMHA calls for at least two passes with a plate compactor of at least 5,000 lbf (22 kN), overlapping onto the pavers around the repair. Its construction guide says that compaction is what seats the pavers in the bedding sand and pushes sand up into the joints. A rubber mallet can’t do that evenly.

If the joints on the repaired section are polymeric sand, they’ll need new polymeric sand once the pavers are back. Our guide on how to apply polymeric sand covers that last step.

Our polymeric sand guide explains why cracked joints are often a sign of movement underneath rather than bad sand.

When is the best time to relevel pavers in the GTA?

Any time the ground isn’t frozen, which in the GTA usually means spring through late fall. CMHA says frozen base material shouldn’t be placed and no material should go over a frozen subgrade.

Polymeric sand adds its own limit. Techniseal asks for temperatures above 0°C for the 48 hours after installation and recommends planning for 24 hours without rain.

Spring repairs should wait until the soil has drained. CMHA notes that full compaction may not be achievable on saturated soil, and that saturated bedding sand shouldn’t be installed.

Common mistakes that make it sink again

Keeping pavers from sinking a second time mostly comes down to avoiding a few shortcuts. Each one goes against the CMHA or Unilock guidance above.

On slope, Unilock says every paver surface should slope away from the house. CMHA’s construction guide gives 1.5% as a minimum, roughly 3/16 inch per foot, and its maintenance guide recommends 2% (about 1/4 inch per foot) as a typical minimum.

Unilock also calls edge restraints critical to the durability of a paver surface.

When should you call a pro?

Call a pro when the base, not just the sand, has to be rebuilt under weight. A single sunken paver or a small dip in a walkway is a fair weekend job if you can rent a plate compactor and you’re patient with the levelling.

It’s worth getting help when:

• the dip is on a driveway that takes car weight
• it’s large or keeps coming back
• the section next to the house slopes toward the foundation
• roots or a drain are involved
• several pavers are cracked and need matching replacements

Lifting and levelling sunken pavers, and replacing damaged ones, is part of the interlock work we do across Markham and nearby towns like Richmond Hill and Stouffville. Quotes are free.

Have a look at our interlock repair service, call or text (437) 684-2535, or text us a photo of the low spot.

If the joints across the whole surface are also weedy or worn, our guide to weeds between pavers explains why it often makes sense to deal with both at once. The interlock cost guide covers what a full replacement involves if repair isn’t enough.