Why Pipes Burst During Winter

The common assumption is that expanding ice splits the pipe open from the inside. That is not usually what happens, and understanding the real mechanism changes what you do about it.

When water freezes in a pipe it forms a plug. Water continues to be pushed against that plug from the supply side, and because the other side is a closed faucet, the pressure has nowhere to go. It builds in the section between the ice and the fixture until the pipe wall fails — which is why the break is frequently found downstream of the ice, in a section that never froze at all.

This is also why the standard advice to leave a faucet dripping works. The drip is not there to keep the water warm. It is there to keep the line from being a sealed system, so pressure has somewhere to escape.

Which pipes are most at risk

• Pipes running through unheated cellars, crawl spaces, and the unheated ends of basements
• Pipes in exterior walls, especially in older buildings with little or no wall insulation
• Pipes near foundation vents, coal chutes, cellar hatches, and other places outside air gets in
• Outdoor spigots and the supply lines feeding them
• Pipes in unheated additions, rear extensions, garages, and vacant units
• Anything in a garden-level apartment where the heat is off or turned well down

New York adds a few of its own. Rear extensions built onto rowhouses are often heated to a lower standard than the main building, and the supply lines feeding a kitchen at the back of the house run right through them. Air shafts between attached buildings funnel cold directly against walls that carry pipe. And in two- and three-family houses, an empty unit with the heat turned down to save money is a genuinely common source of the call we get three days later.

Prevention that actually works

Insulating exposed runs is the highest-value step, and it matters most where the pipe passes through or near an uninsulated exterior wall. Sealing drafts is nearly as important — a gap at a foundation vent or around a cellar hatch can drop the temperature of a nearby pipe run far below what the rest of the cellar reads.

During a hard cold snap, let a faucet fed by a vulnerable run drip. Open the cabinet doors under sinks on exterior walls so heated room air reaches the pipes behind them. And do not turn the heat off in a unit you are not using; set it low rather than off.

If the same section of pipe has frozen more than once, stop treating it seasonally. A run that freezes repeatedly is telling you it needs permanent insulation, heat tape, or in some cases rerouting. Repeating the workaround every winter is how a pipe eventually bursts on the one night nobody remembered to open the tap.

If a pipe freezes but has not burst

You will usually notice it as a faucet that produces nothing, or a trickle, while other fixtures run normally. Leave that faucet open — as the plug thaws, the open faucet gives melt water and pressure somewhere to go.

Warm the section gradually. A hair dryer, a heat gun on low, warm towels, or a space heater positioned safely nearby all work. Start at the fixture end and work back toward the frozen section, so melt water can escape rather than being trapped behind more ice.

Never use an open flame. A torch on a pipe is how frozen-pipe calls become house-fire calls, and on a soldered joint it will simply melt the joint apart. This is the single most important line in this article.

If a pipe has already burst

• Shut the main water valve immediately — know where it is before you need it, not during
• Cut power to the affected area at the breaker if water is anywhere near outlets, panels, or a boiler
• Open faucets to drain the remaining water in the system and relieve pressure
• Move what you can out of the way, then photograph the damage once it is safe, for insurance
• Call for emergency service — a burst supply line does not stop on its own

The reason the main shutoff is first on that list is that everything else is secondary to stopping the flow. A half-inch supply line moves a remarkable amount of water in the time it takes to find a bucket.

The repair is rarely just the pipe

When we repair a burst section we are also looking at why it burst there. A single failure in an otherwise sound copper system is a repair. A failure in a run of original galvanized that has been narrowing with scale for eighty years is a signal about the rest of the system, and we will say so — a patched section in a line that is failing generally is a temporary fix that buys a season.

What we will not do is quote a repipe on the night of the emergency. Stop the damage, restore water, then have the larger conversation in daylight with the whole system in view.