Burst pipes: why the damage shows up at the thaw, not the freeze

All week it was bitterly cold and the cottage seemed to be holding up fine. Then a mild spell rolls in, and that’s when a neighbour notices water under the basement door.

That’s no coincidence. In many cases the pipe gave way during the cold, but the water only found its way out at the thaw. Understanding how that works tells you when to act.

What to do when a thaw follows a cold snap

1. Watch the forecast, not just the cold

People think about the cottage during a cold snap. Think about it during the mild spell that follows, too. Environment Canada’s forecasts and warnings for the area tell you when the weather is about to turn.

2. Get someone inside before or during the thaw

If nobody has been in since the cold set in, now’s the time. They check the thermostat and the thermometer in the coldest room, then the visible pipes and the basement floor.

3. At the first sign of a leak, close the main shut-off

Water on the floor, a fresh stain on the ceiling, a drip at a fitting, hissing in a wall: close the main valve. On a well, also switch off the pump breaker if it’s clearly labelled in the panel.

4. A tap has stopped running? Treat it as a frozen pipe

Never heat a pipe with a flame. The first-hour steps are in our guide frozen pipe at the cottage: what to do in the first hour.

5. Listen to the well pump

With every tap closed, a pump that keeps starting on its own means water is going somewhere. It’s often the first clue to a leak you can’t see yet.

6. Call an RBQ-licensed plumber

A split section gets replaced; it doesn’t get “fixed” with tape. Check the licence in the Régie du bâtiment du Québec registry. If water has run, take photos before moving anything and call your insurer or broker.

Why it happens at the thaw

The ice pushes the water, not the pipe

Water expands as it freezes. People often picture the ice bursting the pipe right where it forms. In practice, the most common failure happens somewhere else.

When an ice plug forms in a cold section, the water that’s still liquid gets trapped between that plug and a closed tap. As the ice grows, it pushes on that water, which has nowhere to go. Pressure builds in the trapped section, and that’s often where — sometimes well away from the ice — the pipe or a fitting gives.

During the cold, the ice plugs the leak

While it stays cold, the frozen section keeps water from reaching the crack. Nothing runs, or almost nothing. The cottage looks normal.

At the thaw, water arrives at full pressure

When the ice melts, water from the municipal line or the well starts moving again. It reaches the crack at full supply pressure, and it keeps running until someone closes the valve. That’s why so much damage gets discovered during a mild spell.

Copper, plastic and fittings

Copper pipe tolerates very little expansion. Cross-linked plastic (PEX) is often more forgiving, but its fittings, taps and the appliances connected to it are still vulnerable. No material makes plumbing freeze-proof.

Townships realities

Mid-winter thaws are common. In Brome-Missisquoi, a cold snap is often followed by a few mild days, sometimes with rain. Every cycle is another chance to freeze, then thaw.

Cottages have cold corners. Plenty of places in Sutton, Frelighsburg or Potton started out as three-season cottages. Pipes still run through a crawl space, an addition or a thinly insulated exterior wall, where the thermostat measures nothing.

The heat can stop with nobody around. A power outage, an empty propane tank, a tripped breaker: a few cold hours are enough for a section to freeze, even if the heat comes back on by itself afterwards.

Prevention: what you can do, and what belongs to a plumber

What an owner can do:

• Keep the heat at the setting your plumber or insurer recommends.
• Open cabinet doors under sinks against exterior walls.
• Find, label and test the main shut-off.
• Shut off and drain outdoor taps before the frosts.
• Leave a thermometer in the coldest room.
• Arrange a visit during or after every significant thaw.

What belongs to an RBQ-licensed contractor:

• Insulating or rerouting a pipe in an exterior wall or crawl space.
• Installing heat cable or frost-free outdoor taps.
• Replacing a split section or fitting.
• Draining the plumbing if the cottage is closed for winter.

In an empty house, how long before anyone notices?

At the thaw, a split pipe runs as long as the valve stays open. In an empty house, nobody hears it. If the next visit is two weeks away, the water has two weeks.

How often someone stops by sets the maximum length of any damage, and timing matters too: a visit shortly after a thaw sees what a visit during the cold couldn’t yet. On the insurance side, conditions for a home left empty vary from one insurer to the next: ask your broker.

What we look at on a visit

Every visit, we read the thermometer left in the coldest room, look at the incoming water line and its fittings (frost, condensation, drips), log the water meter or pressure gauge with every tap closed, listen for the well pump starting on its own, open the cabinets under the sinks, and run a flashlight over the basement floor and lower walls. Every finding is photographed and time-stamped in the report.

If something’s off, we call you the same day, with photos. If needed, we meet an RBQ-licensed plumber on site and document the work.