Why interlock heaves in the GTA — and how to stop it

Your patio was dead level the day the crew packed up. Three winters later it rolls like a bad sidewalk, the steps have a lip you catch your toe on, and two pavers rock when you stand on them. Nothing hit it. Nothing was driven over it. So what happened?

Water, then ice, then a lever

Southern Ontario cycles above and below freezing dozens of times every winter — far more than a colder, more stable climate like Winnipeg. Each cycle is a chance for water sitting in the base material to freeze. Water expands roughly nine per cent when it turns to ice, and it expands in whatever direction meets least resistance, which is up. Under a patio, up means your stone.

One cycle moves nothing you would notice. Forty cycles a winter, five winters in a row, and the surface is a landscape of its own.

Why the base decides everything

A correctly built base does two things: it drains, and it stays put. Those are not the same job.

• Drains — clear angular granular material lets water pass through and away instead of sitting in pockets under the stone.
• Stays put — that material has to be compacted in thin layers, usually 100 mm at a time. Dump 300 mm in one go and run a plate compactor over the top and only the top 100 mm is actually compacted. The rest settles later, unevenly, exactly where you walk.

Depth matters too. A patio walked on needs less than a driveway carrying a vehicle, but both need to sit well below the depth at which the ground reliably heaves. Cheap quotes are almost always cheap here — 100 mm of base instead of 250, one lift instead of three.

The other three failure points

Edge restraint. Pavers push outward every time something crosses them. Without a restraint spiked into the base along every open edge, the field slowly spreads and the joints open.

Joint sand. Plain sand washes out and lets weeds and ants in. Polymeric sand sets firm, holds the pavers as a single surface and keeps the seams closed.

Grade. A patio should fall away from the house — roughly 1 to 2 cm per metre. It is invisible to the eye, and without it you get meltwater running back at the foundation every March.

What this means for your quote

Ask every contractor two questions: how deep do you excavate, and how many lifts do you compact? The answers will separate the quotes faster than the price does. Include us in that.

A patio built properly in the GTA is not a cheap thing. It is, however, a thing you buy once. A patio built on 100 mm of poorly compacted fill costs less in year one and gets lifted and relaid in year five — at which point you have paid twice and lived with a bad surface in between.

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