Why Pipes Burst When They Freeze (It's Not Where You Think) — and Why the Flood Comes at Thaw

Water is one of very few substances that gets bigger when it freezes — about nine percent bigger. Everyone knows that part, and it produces a mental picture that is wrong: ice swelling inside a pipe until the pipe splits around it. If that were the mechanism, bursts would happen at the coldest point and there would be little anyone could do. The real mechanism is hydraulic, and it is why a trickle from a faucet can save a house.

What the Ice Actually Does

Ice forms first where the pipe is coldest — an exterior wall, a crawl space, a run through an unheated garage — and it grows along the pipe in both directions. Toward the street, the water it displaces simply pushes back into the main. Toward the house, it is pushing against a closed faucet. Water does not compress. Laboratory work at the University of Illinois measured the result: pressure in that sealed section climbing from ordinary household levels of around 50 psi into the thousands. Copper and PEX and CPVC all have a limit, and the pipe lets go wherever it is weakest — a fitting, a solder joint, a stretch thinned by age. That point is often several feet from the ice, inside the heated part of the house.

The same research gives the working threshold. Pipes do not freeze at 32°F; the building protects them for a while. For uninsulated pipes in unheated spaces, the alert point is an outdoor temperature around 20°F sustained for several hours — sooner with wind finding a gap in the wall, and sooner in southern houses, where pipes run through attics and exterior walls that northern builders learned long ago to avoid.

Before the Cold Arrives

1. 1Disconnect and drain hoses. A hose left on an outdoor faucet traps water in the faucet body and the pipe behind it, and defeats even a frost-proof sillcock. Take hoses off, and if the outdoor faucets have an indoor shutoff, close it and open the faucet to drain.
2. 2Open cabinets on exterior walls. Kitchen and bathroom sinks on outside walls sit in a closed box that the room's heat never reaches. Doors open, overnight, in a cold snap.
3. 3Set the protective drip. On the faucets served by vulnerable pipes — furthest from where water enters, or on exterior walls — a thin steady trickle, from both hot and cold. The point is not that moving water resists freezing. It is that an open faucet means there is no sealed section for pressure to build in.
4. 4Find and label the main shut-off. Everyone in the house should be able to reach it and turn it in the dark. The shut-off guide shows where it usually hides. This is the step that decides whether a burst costs hundreds or tens of thousands.

Add one more if you are leaving: heat on, set no lower than 55°F, and someone checking the house. Most catastrophic freeze losses are in empty buildings.

Which Pipes Go First

The frozen pipe prevention guide covers insulation, heat cable and sealing the air leaks that do the real damage.

A Faucet That Gives Nothing

Turn on a faucet on a bitter morning and get a dribble or nothing: that line is frozen. It has not necessarily burst, and what you do next matters.

• Leave the faucet open. As the plug melts, water needs somewhere to go.
• Warm from the faucet end back toward the frozen section, so melt water can drain out instead of being trapped between ice and ice. A hair dryer, a heating pad, towels soaked in hot water, a space heater at a safe distance warming the whole cabinet or room.
• Never an open flame. Torches on frozen pipes start house fires every winter, and boiling water inside a pipe can burst it by steam.
• Stand by the main. If the pipe has already split, you will find out when the ice lets go.

If you cannot find or reach the frozen section, a plumber can — and it is a far smaller bill before the burst than after.

Why the Flood Waits

A split pipe with an ice plug in it is sealed by the ice. It can sit that way, already broken, for a day or more. When the temperature climbs and the plug melts, the full pressure of the supply arrives at an open split, and a half-inch line can put out several gallons a minute for as long as nobody notices. This is why burst-pipe calls peak as the cold snap ends — often in the afternoon, often in a house where everyone is at work.

When It Has Already Let Go

Main off first, before anything else. Then the breaker for any room where water is near outlets or coming through a ceiling. Photograph everything before you move it. A plumber repairs the pipe; the water that escaped is a separate job, because it is now inside walls, ceilings and floors that will not dry on their own in winter air. (888) 217-6781 is answered 24/7 and connects you to a plumbing company covering your ZIP. Make that call as soon as the water is off. The burst pipe guide covers the order of calls in full.

On insurance: sudden freeze bursts are among the most commonly covered water losses. The usual condition is that you kept the heat on or drained the system — which matters most for vacant and seasonal homes. The burst pipe coverage guide has the detail.