How to Prevent Frozen Pipes, and What to Do When One Freezes

Our winterizing guide covers the freeze alongside everything else a Canadian winter asks of a house. This article takes the frozen pipe apart: why it bursts, which runs in your house are at risk, what to do when a tap stops running, and the one piece of information to find before the first cold snap.

Why a frozen pipe bursts

Water expands as it freezes, but that is only half the mechanism. Ice rarely splits a pipe by pressing outward beside it. An ice plug forms at the coldest point, grows, and seals the pipe. The water trapped between that plug and a closed tap has nowhere to go, and pressure in that trapped column rises until something gives.

Two consequences surprise people. The split is often not where the ice is: it opens at the weakest fitting in the sealed-off section, which can be in another room. And the flood usually arrives on the thaw, because a split pipe leaks very little while the plug holds. Many homeowners find the burst on the mild afternoon after the cold snap, which is when they have stopped worrying.

Which runs are vulnerable

Freezing is less about the outdoor temperature than about whether one length of pipe sits in cold air with no heat reaching it. Walk your own house and look for these.

• Exterior taps and the pipe feeding them. The most common freeze in Canadian homes, and the easiest to prevent.
• Pipes in exterior walls, especially where insulation was compressed behind the pipe or stops short, and on the side the winter wind hits.
• Unheated and semi-heated spaces: an attached garage, a crawl space, a cold room, the floor cavity under a cantilevered bay window, and the rim joist.
• Supply lines under a sink on an outside wall, where a closed cabinet door keeps room heat out.
• Anywhere heat has been shut off, and anywhere a chase or penetration leaks air. A draft through a small gap can freeze one pipe while the rest of the house is comfortable, which is why a freeze in a mild winter is usually an air-leak problem rather than an insulation problem.

Prevention, before the cold arrives

• Shut off and drain the exterior taps and disconnect every hose, which otherwise holds water against the valve and defeats a frost-free tap. Our fall checklist puts this in the seasonal routine.
• Seal the air leak first, then insulate. Pipe insulation slows heat loss but does nothing against a draft blowing across the pipe.
• Insulate the rim joist and any exposed runs in the garage, crawl space or cold room.
• Let heat reach the pipes. Leave interior doors open, open cabinet doors under sinks on exterior walls during a cold snap, and do not close vents in rooms with plumbing in the walls.
• Keep the whole house above freezing, including the parts nobody uses. Before leaving for a stretch, read the vacancy and insurance section of the winterizing guide, because policies carry conditions.
• Let a tap drip during extreme cold if it is fed by a vulnerable run. Moving water is harder to freeze, and an open tap also relieves the pressure that does the damage.
• If you use electric heat cable, choose a product certified for that pipe and location, follow its instructions exactly, never overlap it on itself, and inspect it each fall. A badly installed cable is a fire risk.

Find your main shutoff before you need it

In a flood, the difference between a repair and a renovation is how many minutes pass before the water stops, and that depends on whether you already know where the valve is. In most houses it sits where the supply enters, commonly on the street-facing wall of the basement or crawl space, near the meter. In a condominium there is usually a valve for your unit in a closet or behind an access panel.

Exercise it once a year: turn it fully off, confirm the taps run dry, then turn it back on. A valve you have never touched may not close on the night you need it. Do not force one that has seized, and if it weeps afterwards, call a licensed plumber while it is still a scheduled job. Leave the buried curb stop at the property line alone. Then label the valve and tell everyone in the house where it is.

A pipe is frozen but has not burst

The usual signs are a tap that runs to a trickle or nothing while others work normally, or frost, sweating or a bulge on an exposed pipe.

1. Open the tap that pipe feeds, both hot and cold. This gives melt water somewhere to go and relieves the pressure that causes the split.
2. Go to the main shutoff and be ready to close it. If you can see a bulge, split or deformed section, close it now.
3. Warm the pipe gently, starting at the tap end and working back toward the cold spot so melt water escapes ahead of the ice. A hair dryer, a heat lamp, warm wet towels, or a portable heater kept well clear of anything flammable are all reasonable.
4. Never use an open flame or a torch. Pipes run through cavities full of wood, dust and insulation, and both solder joints and plastic pipe fail under direct flame. This is the shortcut that turns a plumbing problem into a house fire.
5. Do not leave a heat source unattended, and keep watching as flow returns, because a split often only shows itself once the ice clears.

Call a licensed plumber instead of persisting if the frozen section is inside a finished wall, ceiling or floor, if you cannot find it, if gentle heat has not worked, if the pipe looks damaged, or if the line serves a fire sprinkler system. Plumbers book up fast in a deep freeze, so call early.

If it has already burst

Close the main shutoff, then open taps so the remaining water leaves through a sink rather than the ceiling. Stay out of standing water near panels, outlets or submerged cords: cut power only if you can reach the panel from a dry position and are confident doing it, and otherwise call an electrician or your utility. If water has reached the furnace, water heater or any gas appliance, do not relight it, and have a licensed technician inspect it, because controls that have been wet get replaced rather than dried out.

Call a licensed plumber and your insurer early, and photograph everything before cleanup. Drying speed matters, because mould begins in days.

Nobody writes down which fixture froze, and that note is the most useful thing you can keep, because the same run freezes again. Record the location, the date and what the plumber changed, alongside where the shutoff is. A homeowner record that holds that history, as Habyn's does, turns an annual scare into a known weak point you watch on purpose.

Frequently asked questions

At what outdoor temperature do pipes freeze? There is no single number, because what matters is the temperature at the pipe. An exposed run in a draughty crawl space can freeze in a mild cold snap, while a well-insulated interior run is fine far below. Judge the pipe, not the forecast.

Should I leave a tap dripping all winter? No. Use it during extreme cold, on taps fed by runs you know are at risk. It is a cold-snap measure, not a season-long habit.

Why did my pipe burst in a different room from where it froze? The ice plug seals the pipe and pressure builds behind it, so the pipe fails at the weakest point in that sealed section, which can be well away from the ice.

Can I thaw a frozen pipe myself? Often yes, with gentle heat and the tap open, if the pipe is exposed and undamaged. Never with a flame. If it is inside a wall or looks damaged, call a licensed plumber.

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A frozen pipe is one of the few winter failures that is almost entirely preventable. Habyn keeps your home's systems and service history in one place. See how Habyn helps homeowners.