Plumbing
It is a building problem, not a weather problem
Two identical houses on the same street go through the same night at minus ten. One is fine. The other has a split line in a wall. The weather was the same, so the weather is not the explanation.
What differs is where the pipe runs relative to the insulation and the air barrier. A supply line in an exterior wall cavity, outside the insulation, is in effect outdoors. It survives most winters and fails on the cold night with wind, because wind is what drives cold air through a leaky rim joist and into the cavity.
The four places it starts
- Exterior hose bibs with a hose still attached. The hose holds a column of water against the valve, the freeze expands back into the pipe inside the wall, and the split is indoors where you cannot see it.
- Crawlspace and rim joist runs. Uninsulated and often draughty. These fail first and they fail on the windy nights.
- Lines run through an unheated garage wall to serve something on the far side. The house is warm, the pipe is not.
- Houses on a deep setback, or empty. A thermostat at 50 degrees keeps the living space above freezing and does very little for a pipe in an exterior wall cavity.
What to do while it is still frozen
A pipe that is frozen but not yet split is a situation you can still manage, and the first move is counter-intuitive: open the tap that the frozen section feeds. A path for meltwater to escape relieves the pressure that does the actual damage, and running water tells you the moment it clears.
- Open the affected tap, hot and cold.
- Warm the pipe along its length, working from the tap back toward the frozen section, with a hair dryer or a heat gun on low, or towels soaked in hot water.
- Open cabinet doors under sinks on exterior walls so room air reaches the pipe.
- Keep the room warm and keep going. Thawing takes longer than people expect.
How to tell it has already split
This is the part people miss. Ice plugs the hole it made. A pipe can split under the pressure of the freeze and stay completely dry while the ice is still in it. Everything looks like a success right up until the thaw.
The signs worth acting on: a section of pipe that is visibly bulged or deformed, a seam that looks opened, frost or ice on the outside of the pipe in a spot where the rest of the run is dry, or a tap that produced nothing during the freeze and then runs at obviously reduced pressure afterwards. If you see any of those, shut the main before you finish thawing it and call.
What prevents the next one
The fixes are building fixes rather than plumbing ones, which is why they get skipped. Air-seal and insulate the rim joist. Insulate crawlspace runs properly rather than wrapping them loosely. Reroute the garage-wall line inside the heated envelope if the opportunity comes up. And replace an ordinary hose bib with a frost-free sillcock, which is a small job that removes the single most common version of this entirely.
Hose disconnection in autumn costs nothing and prevents the most common one. The rest of the short list is in plumbing maintenance that actually prevents something.
If it has already let go, what to do in the first ten minutes is the page you want. We cover Manchester for repiping, leak detection and faucets and fixtures, and our heating team handles the other half of a cold-weather failure.