Frozen pipes
While the line is still frozen, nothing happens: the ice is its own plug. The water arrives later, when the thaw comes through, very often on a mild afternoon with the house empty and nobody to hear it start.
- 1 The ceiling stain. It measures the ceiling, not the leak. Water travels along a joist before it drops.
- 2 A burst supply line. Pressurized, so it runs until somebody closes a valve, and it runs downward.
- 3 A frozen line. The exterior wall is where the pipe meets the cold, and where it splits.
- 4 The washer hose. Rubber under constant pressure, and a standpipe that overflows quietly.
- 5 The water heater. It goes at the bottom of the tank, usually while the house is empty.
- 6 Groundwater and backup. Water pressing on the wall, and a floor drain that reverses when the combined sewer fills.
Where they freeze here
Lines run through an unheated crawl space, along an exterior wall in an uninsulated older house, through a garage ceiling, or out to a hose bib that was never shut down for the season. In the pre-war housing common across the Baltimore rowhouse belt, supply lines were frequently run in exterior walls with little or no insulation behind them.
It is rarely the whole house. A hard night with the air moving against one elevation will take out the lines behind that wall and leave the rest of the plumbing perfectly fine, which is why the failure so often surprises people.
Thawing without causing a burst
Open the tap that line serves first. It gives the melt somewhere to go and it tells you by ear the moment flow comes back. Then warm the pipe gently, working from the open tap backwards toward the blockage: a hair dryer, a heat pad, warm towels. Never a torch and never any open flame.
Warm the middle of the blockage before the ends and you create melt water trapped between two plugs of ice, under pressure, with nowhere to go. That is the mechanism that splits copper.
Check the whole run once it flows
The split that matters is often invisible, a hairline that only opens under full pressure. Once flow returns, walk the whole visible run with a light and a dry hand, checking joints and the length of the tube. Where the line disappears into a wall or a ceiling, keep an eye on the surface below it for the next twenty-four hours.