Pipes do not always burst when they freeze, but the risk goes up a lot once water in the pipe has turned to ice and is trapped between closed valves or other blockages. Whether a given frozen pipe actually bursts depends on temperature, how long it stays below freezing, pipe material, and whether pressure can relieve back toward the supply.
Why frozen pipes burst
When water freezes inside a pipe it expands and can create a solid ice plug. If more water behind that plug keeps freezing or can’t move, pressure builds between the ice and any closed faucet or valve until the pipe ruptures at its weakest point.
Why some frozen pipes don’t burst
If the freezing is partial or the system is open so water can move away, pressure may never rise enough to break the pipe. Better insulation, more forgiving materials like some plastics, and shorter exposure to severe cold also reduce the chance that a frozen section actually fails.
Temperatures and risk
Household water pipes start to be at risk of freezing once air temperatures drop below about 32°F (0°C), and problems become much more common around 20°F (about −7°C) and below, especially over many hours. Pipes in unheated spaces or against exterior walls are the most likely to freeze and then burst under those conditions.
Simple prevention steps
To lower the odds that freezing leads to a burst:
- Keep vulnerable areas warmer or add pipe insulation so sections don’t reach freezing as quickly.
- During very cold snaps, let faucets on at‑risk lines drip slightly so pressure can’t build behind any ice plug that forms.
What to do if pipes freeze
If a pipe is suspected frozen but not yet burst, shut off the water supply to that line or the whole house, then gently warm the area (for example with room heat or a hair dryer on a low setting, kept away from combustibles). If a line has already burst or you see leaks when it thaws, keep the water off and contact a plumber promptly to prevent further water damage.
